The present invention relates to SCSI (Small Computer Systems Interface) and Ethernet controllers used in personal computers which interface with the PCI (Peripheral Component Interconnect) Local Bus. We will describe a single chip to replace the known SCSI and Ethernet adapter boards.

The PCI Local Bus is a high performance, 32-bit or 64-bit bus with multiplexed address and data lines. As illustrated in Fig. 1, the PCI Local Bus 100 is intended for use as an interconnect mechanism between peripheral controller components, such as the SCSI adapter board 102, Ethernet adapter board 104, and processor/memory system 106. A PCI Local Bus Specification, Rev. 2.0, effective April 30, 1993 includes protocol, electrical, mechanical and configuration requirements for the PCI Local Bus components and expansion boards. Further information concerning the PCI Local Bus specification can be obtained from the PCI Special Interest Group, M/S HF3-15A, 5200 NE Elam Young Parkway, Hillsboro, OR 97124-6497.

Ethernet is a standard in personal computer networking. An Ethernet adapter board provides components for transmitting and receiving signals on a network allowing a persona computer in which it resides to be networked with other personal computers. A PCI bus interface unit on the Ethernet adapter board interfaces the adapter board with the PCI Local Bus on which a CPU resides. The PCI bus interface unit may provide digital signals to control the PCI Local Bus.

SCSI is a standard that allows users to easily add up to seven peripheral devices on a personal computer such as CD-ROM and high capacity disk drives. A SCSI adapter board in a personal computer provides digital address, data, and control signals to a SCSI bus on which peripheral devices reside. A PCI bus interface unit on the SCSI adapter board interfaces the adapter board with the PCI Local Bus on which a CPU resides. The PCI bus interface unit may provide digital signals to control the PCI bus.

Previously, manufacturers have provided Ethernet and SCSI components spaced apart on separate adapter boards. The high current requirements for digital signals transmitted and received on the PCI and SCSI buses means that the digital signals can generate significant noise. With sensitive analog components in the Ethernet controller, such as the phase lock loop (PLL) circuitry, such noise has prohibited integration of components of the Ethernet and SCSI adapter boards.

We will describe the integration of the SCSI and Ethernet adapter board components by reducing noise generated by the digital signals resulting in very stable analog circuitry.

The components of SCSI and Ethernet adapter boards are integrated onto a single chip to form an integrated SCSI-Ethernet controller for use on a PCI Local Bus. The integrated SCSI-Ethernet controller first reduces noise by reducing ground bounce on V_SS pins connected to digital output buffers of the integrated SCSI-Ethernet controller. Ground bounce on V_SS pins is first reduced by providing substantially more V_SS pins than VDD pins to support large output buffers which continually switch current, each V_SS pin supporting a limited number of buffers in a local area near the pin. The reduced current which each V_SS pin has to sink, as well as limited line lengths to the V_SS pin reduces inductance resulting in reduced ground bounce. To further reduce ground bounce, separate lines are provided from each output buffer to a V_SS pin. By using separate lines, ground bounce resulting when multiple buffers switch together is reduced.

The integrated controller further reduces noise by utilizing circuitry for digital output buffers which limits the change in current over time (di/dt) during a signal transition. By limiting di/dt, noise created in the analog circuitry due to inductance is likewise limited.

The integrated controller additionally reduces noise by topologically organizing the digital control circuitry so that current density increases in a direction away from the analog circuitry.

Finally, the integrated controller reduces noise by preventing unnecessary current flow between separate analog and digital power supplies which may transfer noise between digital and analog components. Such current flow between the analog and digital power supplies is prevented by utilizing silicon control rectifiers (SCRs). SCks are placed between power supplies. The SCRs enable current flow between the digital and analog supplies to prevent latch up should only one supply be turned on.

Further details of the present invention are explained with the help of the attached drawings in which:

• Fig. 1 illustrates an Ethernet adapter board, a SCSI adapter board, and a processor/memory system as connected to a PCI Local Bus;
• Fig. 2 shows a block diagram of components of a combined Ethernet-SCSI controller of the present invention;
• Fig. 3 shows a pin-out for a 132-pin package containing the combined Ethernet-SCSI controller of the present invention;
• Fig. 4 illustrates how individual lines are utilized to carry power from output buffers to VSS3B pins.
• Fig. 5 shows circuitry for an output buffer utilized in the present invention along with a logic diagram for the output buffer;
• Fig. 6 illustrates how a decrease in di/dt is achieved by the use of output buffer circuitry of Fig. 5;
• Fig. 7 illustrates how the digital control circuitry is organized so current density increases in a direction away from analog circuitry;
• Fig. 8 shows a scale layout of an integrated circuit chip containing the combined Ethernet-SCSI controller of the present invention;
• Fig. 9 shows the configuration of source power lines in the Ethernet analog and digital regions and illustrates the layout of portions of the digital I/O buffer circuitry;
• Fig. 10 illustrates the five separate power distribution networks of the present invention as separated by SCR switching devices;
• Fig. 11 illustrates a method for forming an SCRs between source power supply lines; and
• Fig. 12 shows a diagram representing how the PNPN sequence of Fig. 11 is equivalent to two transistors and further shows an equivalent circuit for the configuration of the transistors.

Fig. 2 shows a block diagram of components of a combined Ethernet-SCSI controller embodying the present invention as coupled to a PCI Local Bus. A SCSI portion of the combined Ethernet-SCSI controller contains a Fast SCSI-2 core 200, a bus master DMA engine 202, and a PCI bus interface unit 204, collectively referred to herein as a SCSI controller. The Fast SCSI-2 core 200 provides an 8-bit SCSI interface supporting single-ended SCSI with transfer rates of 10MB/sec. The bus master DMA engine contains a 96-byte FIFO for 32-bit transfers in burst mode across the PCI Local Bus at 133MB/sec speeds. The PCI bus interface unit 204 includes configuration space and a PCI master/slave interface and is a combined PCI controller utilized both by the SCSI portion and the Ethernet portions of the combined Ethernet-SCSI controller of the present invention. One method of combining separate SCSI and Ethernet PCI bus interface units into a single PCI bus interface unit 204 is described in U.S. Patent Application Serial No.08/184,295 entitled "Apparatus and Method For Integrating Bus Master Ownership of Local Bus Load By Plural Data Transceivers' as filed on January 21, 1994, incorporated herein by reference.

The 32-bit Ethernet portion utilizes the combined PCI bus interface unit 204 and further includes a DMA buffer management unit 210, an individual 136-byte transmit FIFO 212, a 128-byte receive FIFO 214, a FIFO controller 216, and an IEEE 802.3-defined MAC (Media Access Control) core 218 supporting an IEEE 802.3 defined AUI (Attachment Unit Interface) and a 10Base-T MAU (Media Attachment Unit), all collectively referred to herein as an Ethernet controller.

The combined Ethernet-SCSI controller of the present invention is integrated onto a chip which can be made available in a 132-pin Plastic Quad Flat Pack (PQFP). The combined Ethernet-SCSI controller chip is intended for use on the motherboard of a personal computer. The Ethernet-SCSI controller chip is directly installed on the motherboard and coupled with the PCI Local Bus, the SCSI bus, and an Ethernet transceiver. The SCSI CLK input to the chip is provided by a SCSI crystal also installed on the motherboard.

Fig. 3 shows a pin-out 300 for a 132-pin PQFP capable of containing the combined Ethernet-SCSI controller of the present invention. As illustrated, the pin connections are arranged so that layout of circuitry on the chip includes an analog portion 302 provided separately from digital control circuitry 304 and digital I/O buffer circuitry 306. Table A lists the pin names along with a brief description of the pin functions. The pin names are further are organized to indicate whether the pins are utilized for the PCI Bus Interface, the Ethernet interface, the SCSI interface, Power Supplies, or Miscellaneous functions in Table A which follows. A more detailed description of the pins listed in Table are included in Appendix A.

Even with the analog circuitry 302 of Fig. 3 separated from digital circuitry 304 and 306, current in the digital circuitry will generate significant noise in sensitive components of the analog circuitry 302, such as the phase lock loop (PLL) creating problems. Additional measures are therefore implemented to reduce noise in the analog circuitry 302 as described in the sections below.

A. Localized VSS For Digital Output Buffers

Noise in the analog circuitry 302 of Fig. 3 can be generated from current switching in the PCI and SCSI output buffers included in the digital I/O buffer circuitry 306. The PCI interface includes large output buffers connected to the AD[31:0], C/BE[3:0], and PAR pins, each pin connected to an output buffer switching current nearly every clock cycle, the pins carrying the maximum current required by the PCI Local Bus specification, Rev. 2.0. The SCSI interface also includes large output buffers connected to all pins listed in Table A under the SCSI Interface, each large output buffer switching current nearly every clock cycle and receiving up to a 48 milliamp signal.

With several of the large PCI or SCSI output buffers switching simultaneously, significant ground bounce can result which introduces noise current to the analog circuitry 302. The ground bounce results in part because of the limited ability of the source pins which would normally be utilized in an integrated circuit to effectively sink the current received.

To reduce ground bounce, the present invention first utilizes a number of VSSB and VSS3B pins supporting the large PCI and SCSI output buffers substantially greater than corresponding drain voltage pins VDDB and VDD3B. The VSS3B and VDD3B pin connections support only the large PCI output buffers connected to the AD[32:0], C/BE[3:0] and PAR pins. The VSSB and VDDB pin connections support only the large SCSI output buffers connected to the pins listed under the SCSI interface portion of Table A.

Ground bounce is further reduced by limiting the number of large output buffers supported by the additional VSSB and VSS3B pins. Blocks 311-318 and 321-323 of Fig. 3 show the VSSB and VSS3B pins along with the output buffer pins which they support. As shown by blocks 311-318, each VSS3B pin supports a maximum of five output buffers. As shown by blocks 321-323, each VSSB pin supports a maximum of six output buffers.

To further reduce ground bounce, as additionally illustrated by the blocks 311-318 and 321-323 of Fig. 3, the large output buffers supported by an individual VSSB or VSS3B pin are located in the local area surrounding the individual pin. By locating the output buffers close to their ground pins, line lengths which create inductances are reduced likewise limiting ground bounce.

Fig. 4 illustrates two additional ways in which ground bounce is reduced for the VSS3B pins connected to the large PCI output buffers. First, to further limit line length and reduce inductance, the VSS3B pin 420 is centrally located among the large output buffers 411-414 which it supports. By further reducing inductance in this manner, ground bounce will likewise be additionally reduced. Second, individual lines 401-405 are provided to carry power from individual output buffers 411-415 to the VSS3B pin 420. By using separate lines instead of a single power line, ground bounce resulting when multiple output buffers switch together will be reduced.

B. Limited di/dt in Digital Output Buffers

The present invention further provides circuitry for limiting the change in current (di/dt) sourced and sank by the PCI and SCSI output buffers to reduce noise affecting the analog circuitry 302 of Fig. 3. Fig. 5 shows circuitry for the output buffer 500 of the present invention along with a logic diagram 502 for the circuitry.

As shown in the logic diagram 502, the output buffer of the present invention receives a data signal (DIN) at the input of an inverter 504 and an enabling signal (EIN) at the input of an inverter 506. The output of inverter 504 forms the input of a tri-state buffer 508 which is enabled by a low signal from the output of inverter 506. The output of tri-state buffer 508 thus produces an output signal (OUT) corresponding to the DIN signal as enabled by the EIN signal.

To implement logic diagram 502 and provide circuitry for limiting di/dt, the circuitry 500 of the present invention includes three tri-state buffers 540, 550 and 560 which are driven by three delay sections 510, 520 and 530 respectively. Components of these sections and their operation are described below.

1. Delay Sections 510, 520 and 530

The DIN and EIN signals are received by first delay section 510. First delay section 510 delays the DIN signal utilizing delay elements 512, while the EIN signal is delayed utilizing elements 514. The delay elements 512 include two inverters 512a and 512b and a 200 ohm resistor 512c connected in series. The delay elements 514 are identical to elements 512.

Resistor 512c is utilized in series with the two inverters 512a and 512b to counteract processing variations to provide a smoother output di/dt. The processing variations cause variations in the thickness of the gate oxide layer for transistors in inverters 512a and 512b as well as transistors in the first tri-state buffer 540. A thinner oxide layer in the transistors of inverters 512a and 512b reduce capacitance, thus increasing speed, while a thicker oxide layer results in a reduced speed. Variations in delays of inverters 512a and 512b cause a potential increase in di/dt of the output buffer 500.

To counteract the process variations in the oxide layer, resister 512c is utilized in series with inverters 512a and 512b. In contrast to a thinner oxide layer decreasing capacitance and increasing speed of inverters 512a and 512b, a thinner oxide layer increases the parasitic capacitance at the input of first tri-state buffer 540. Resistor 512c acting in combination with the parasitic capacitance at the input of tri-state buffer 540 forms an RC time delay reducing speed, thus counteracting the speed increase of inverters 512a and 512b. With an increase in oxide thickness inverters 512a and 512b decrease speed, while the RC delay resulting from resistor 512c and the parasitic input capacitance of tri-state buffer 540 increases speed. Thus, by utilizing the resistor 512c in series with inverters 512a and 512b, an increased di/dt due to process variations is prevented.

The outputs of first delay section 510 are also fed to second delay section 520 which includes two sets of two inverters and a 200 ohm resistor in series, similar to the first delay section 510. Like the circuitry of first delay section 510, the second delay section 520 includes resistors in series with inverters to counteract processing variations in the gate oxide layer.

The outputs of second delay section 520 are also fed to third delay section 530, again including two sets of inverters and a 200 ohm resistor in series similar to the first and second delay sections 510 and 520 with resistors utilized to counteract process variations.

2. Tri-State Buffer Sections 540, 550 and 560

The outputs of first delay section 510 form the inputs to the first tri-state buffer 540. The first tri-state buffer 540 includes a p-channel pull up transistor 541 and an n-channel pull down transistor 542. The source of pull-up transistor 541 is connected to VDDB or VDD3B, while its drain is connected to the drain of transistor 542 forming the output (OUT) of the output buffer 500. The source of transistor 542 is connected to VSSB or VSS3B.

The gate of the pull up transistor 541 is connected to the output of a NAND gate 543 which has inputs connected to the outputs of first delay section 510. The gate of pull down transistor 542 is connected to the output of a NOR gate 544 which has inputs connected to the outputs of first delay section 510, the EIN output, however, being inverted by inverter 545.

Transistors 546 and 547 are provided to slow the current increase upon turn on or turn off of pull up and pull down transistors 541 and 547, thus reducing current spikes which increase di/dt on the output (OUT). Transistor 546 operates in conjunction with the pull up transistor of NAND gate 543, while transistor 547 operates in conjunction with the pull down transistor of NOR gate 547. The p-channel transistor 546 has a source connected to VDD3B or VDDB and a drain connected to the input of pull up transistor 541. The gate of transistor 546 is connected to the DIN output of first delay section 510. The n-channel transistor 547 has a source connected to VSS3B or VSSB and a drain connected to the input of pull down transistor 542. The gate of transistor 547 is connected to the DIN output of first delay section 510.

The outputs of second delay circuitry 520 are fed to the inputs of a second tri-state buffer 550. The second tri-state buffer 550 has circuit components similar to the first tri-state buffer 540 with inputs connected to the DIN and EIN outputs of second delay section 520 in the same manner that the first tri-state buffer 540 is connected to the first delay section 510, and an output connected to OUT in the same manner as the first tri-state buffer 540.

The outputs of third delay section 530 are fed to a third tri-state buffer 560. The third tri-state buffer 560 has circuit components similar to the first and second tri-state buffers 540 and 550 with inputs connected to the DIN and EIN outputs of third delay section 530 in the same manner as the first and second tri-state buffers 540 and 550 are connected to first and second delay sections 510 and 520, and an output connected to OUT in the same manner as the first and second tri-state buffers 540 and 550.

3. Operation With Output Buffer 500 Sourcing Current

In operation, we first look at the case where a HIGH OUT signal is provided. We, therefore, assume both the DIN and EIN signals are switched to HIGH. With DIN and EIN going HIGH, the outputs of first delay section 510 will go HIGH making the outputs of both the NAND gate 543 and NOR gate 544 LOW. Further with DIN going HIGH, transistor 546 will turn off, while transistor 547 turns on. With the output of NAND gate 543 LOW and transistor 546 off, pull up transistor 541 will turn on to pull up transistor 541 pulling the output (OUT) HIGH. With the output of NOR gate LOW and transistor 547 on, pull down transistor 542 will remain off.

After a short time delay through second delay section 520, the pull up transistor 551 of the second tri-state buffer 550 will turn on to additionally provide current to the output (OUT). Again, after another short delay through third delay section 530, the pull up transistor 561 of the third tri-state buffer 560 will turn on to provide additional current to the output (OUT).

Fig. 6 illustrates how a decrease in di/dt is achieved by the use of three separate tri-state buffers 540, 550 and 560 with delay sections 510, 520 and 530 as shown in Fig. 5. Curve 602 represents the change in current (I) vs. time (t) of an output buffer utilizing a single tri-state buffer designed to reach a current level A. By utilizing three separate tri-state buffers 540, 550 and 560 with outputs delayed by delay sections 520 and 530 as shown in Fig. 5, a curve 604 can be maintained to reach the same current level over a greater time at point B, thus decreasing di/dt. Note that transistor sizes are indicated for the output buffer circuitry 500 of Fig. 5, the sizes showing that the pull up transistors of the tri-state buffers 541, 551 and 561 gradually increase in size also allowing a gradually increasing current, also decreasing di/dt.

Current spikes which increase di/dt, as shown by the dashed lines in boxes 606 of Fig. 6, can be caused during turn on of the second and third tri-state buffers 550 and 560. The circuitry 500 of the present invention prevents the current spikes by utilizing the transistors such as 546 and 547 in each tri-state buffer which, as discussed above, slow the current increase of each tri-state buffer upon turn on.

4. Operation With Output Buffer 500 Sinking Current

We next look at the case where a LOW OUT signal is provided. We begin by assuming that the DIN signal switches to LOW while the EIN signal remains high. With DIN low and EIN high, delay section 510 will provide similar signals switching the outputs of both NAND gate 543 and NOR gate 544 to HIGH. Further, transistor 546 will turn on, while transistor 547 turns off. With the output of NOR gate HIGH and transistor 547 off, pull down transistor 542 will turn on, sinking current on the output (OUT). With the output of NAND gate 543 HIGH and transistor 546 on, pull-up transistor 546 will turn off. By providing three separate tri-state buffer sections with pull down transistors 542, 552 and 562 of increasing sizes turning on after respective delays, current sunk is gradually increased to a required amount, thus reducing di/dt and reducing ground bounce.

Transistors, such as 546 and 547, in each of the three tri-state buffers 540, 550 and 560 also enable a reduction in di/dt when additional tri-state buffer states switch to LOW, just as when they switch to HIGH as discussed above. A transistor, such as 547, operates in conjunction with the pull down transistor of a NOR gate, such as 544, to enable a reduction of current over time (di/dt) when a pull down transistor, such as 542, is turning on, thus reducing ground bounce and limiting noise in the analog section 302 of Fig. 3.

Transistors, such as 546 and 547, in each of tri-state buffers 540, 550 and 560 also prevent a crowbar effect when the output (OUT) transitions from HIGH to LOW or from LOW to HIGH. Because a second and third delay elements 520 and 530 delay the turn off of pull-up transistors 551 and 561 of the second and third tri-state buffers 550 and 560, a crowbar effect could occur when pull down transistor 542 attempts to sink current should pull up transistors 551 and 561 remain on. Transistors, such as transistor 546 and 547, are therefore sized as shown in Fig. 5 to delay turn on of transistor 542 to prevent such crowbarring. Transistors, such as 546 and 547 prevent such crowbarring both when the tri-state buffers outputs switch from HIGH to LOW as well as when they switch from LOW to HIGH.

Thus, by utilizing the output buffer circuitry 500 in the combined Ethernet-SCSI controller of the present invention the rate of change of current (di/dt) is controlled to reduce noise in current sourced, as well as to limit ground bounce on current sank.

Further information for enhancing the output buffer circuitry 500, including providing an mechanism for autosensing whether a 3.3 of 5.0 volt output is required, and providing an output in compliance with such autosensing is disclosed in U.S. Patent Application Serial No.08/185,137, entitled "Apparatus and Method For Automatic Sense And Establishment Of 5V And 3.3V Operation", by Wu et al. filed January 24, 1994, and incorporated herein by reference.

C. Topological Organization of Components

An additional source of noise created in the analog circuitry 302 of Fig. 3 is current flowing in the digital control circuitry 304. As illustrated in Fig. 7, to reduce such noise, the present invention topologically organizes components of the digital control circuitry 304 so that current density increases in a direction away from the analog circuitry 302 as illustrated by arrows 700. Additionally, the analog circuitry 304 is topologically organized to protect against noise from the digital circuitry. Organization of circuitry to reduce noise due to current flow in the digital circuitry is described in the sections below.

1. Overall Organization of Chip Circuitry

Fig. 8 shows a scale layout of the integrated circuit chip 800 containing the combined Ethernet-SCSI controller. Chip 800 is approximately 300 to 400 mils on each side and a 0.8 micron, double-metal process is used to define its circuitry. White bands are painted around regions 801, 803, 805 and 807 to better highlight them. The chip die lays circuit-side down when packaged in package 300. The pinout in Fig. 8 is therefore a mirror image of that shown in Fig. 3. The bonding pad for pin number 99, for example, near the top along the left edge of the layout shown in Fig. 8 while the corresponding bonding pad for pin number 99 (XTAL2) is positioned near the top along the right edge of the package pinout shown in Fig. 3.

The combination of the Ethernet analog region 801 and the Ethernet digital control region 803 generally defines a square shape in Fig. 8, with region 801 defining the top left quadrant of that square shape. Region 801 corresponds to the analog circuitry 302 of Fig. 3. The SCSI digital control region 805 is positioned as a rectangle having its longest side below and adjacent the bottom of the Ethernet digital control region 803. The PCI digital control region 807 is positioned as a rectangle having its long side extending to right and adjacent the right sides of both the Ethernet digital control region 803 and the SCSI digital control region 805. Digital I/O buffers 809, corresponding to the digital output buffers 306 of Fig. 3 are positioned about peripheral edges of digital control regions 805 to 807.

2. Routing Of VDD Lines

Fig. 9 shows the configuration of source power lines in the Ethernet Analog Region 801 and Ethernet digital region 803 as configured to reduce noise in analog region 801. As shown, separate DVDD pin pads 84 and 104 carry power to the ethernet digital region 803. Power is provided from DVDD pads 84 and 109 on power distribution line 902 along the perimeter of Ethernet digital region 803 fartherest away from analog region 801. Power is distributed from the power distribution line 902 to circuitry 906 as illustrated, enabling the maximum current density to be greatest farther away from analog region 801. By distributing power along dashed line 904 instead of power distribution line 902, current density would be greatest closer to the analog region 801.

Noise is generated in circuit components of analog region 801 by resistive coupling of current in the ethernet digital circuitry through the substrate to sensitive analog components. By having a larger current density, particularly with current carried by power distribution line 902, located next to the analog circuitry, significant noise can be created in the analog components due to coupling through the substrate. Thus, by locating power distribution line 902 near the perimeter of the Ethernet digital circuitry 803 away from the analog circuitry 801, noise is reduced.

Noise coupling through the substrate between the Ethernet digital region 803 and components in the analog region 801 is also reduced by the routing of an analog power distribution line 908. Analog power distribution line 908 is connected to AVDD pin pads 91, 96, 103 and 108 as shown. Analog power distribution line 908 is routed around the perimeter of the analog circuit 801 to provide a barrier to current coupling through the substrate from Ethernet digital section 803 to sensitive analog components located within.

3. Organization Of VSS Portions

Fig. 9 also illustrates the configuration of the digital I/O buffer section 809 so that layout for output buffers and their supporting VSSB and VSS3B connections reduces noise in Ethernet analog section 801. As shown in Fig. 9, isolated p wells 911-918 are provided in the digital I/O buffer section 809. Each of p wells 911-918 support connections for a VSS3B pin and portions of their corresponding output buffers, each p well 911-918 supporting structures connected to pins in corresponding sections 311-318 of Fig. 3, respectively. Another isolated p well 920 is provided to support the VSSB pins and portions of their corresponding SCSI output buffers, the p well 920 supporting structures connected to pins in sections 321, 322 and 323 of Fig. 3.

By providing the separate p-wells 911-918 and 920 for VSSB and VSS3B pins and components of the output buffers which they support, noise can be isolated. To further isolate noise, each of p wells 911-918 carry a VSS3B pin connection located in its center as illustrated by pad 930. Additionally, the n-channel transistors of output buffer circuits supported by the VSS3B pin at the center of the p well are provided in the p well region.

To further reduce noise, the p well is made as small as possible and located as close as possible to its VSS3B pin to minimize lead length and associated ground bounce. Further, remaining portions of output buffers supported by the p-well are positioned as close as possible to p well and supporting I/O pad and V_SS3B pin.

Note that the Ethernet interface pins of Fig. 8 are positioned about one corner of the square-shaped die and the PCI interface pins are positioned about a diagonally-opposed second corner and spaced apart from the Ethernet pins. This is done to limit noise form the usually-active PCI local bus from coupling to sensitive analog circuitry. Note in particular that the output buffers that are expected to generate the most switching noise, the large PCI output buffers connected to the AD[31:0], CB/E[3:0] and PAR pins, are arranged to be positioned as far away as possible from the analog circuitry 301.

The PCI output buffers create the most noise because they operate at the highest switching frequencies (e.g., 33MHz) and each such PCI output buffer both sources relatively large amounts of current (e.g., 44mA per pin) and sinks relatively large amounts of current. The SCSI output buffers do not source as much current as the PCI output buffers, and although the SCSI output buffers do sink relatively large amounts of current, they do not create as much noise as the PCI buffers because switching frequencies are lower. Hence p wells for the PCI output buffers 911-918 are provided separately, while a single p well 920 is provided for the SCSI output buffers.

D. Isolation of Power Supply Lines Utilizing SCRs

The layout of circuitry shown in Fig. 8 is unique in that, separate power supplies are provided for different sections. A first set of AVSS pins and AVDD pins are provided respectively for supplying the ground and power to the analog signal region 801. A second, separate, set of DVSS pins and DVDD pins are provided respectively for supplying the ground and power to the Ethernet digital portion 803. A third, separate, set of VSS pins and VDD pins are provided for supplying ground and power to the remainder of the digital regions 805 and 807. A fourth set of VSS3B pins and VDD3B pins are provided respectively for supplying the ground and positive potentials to output buffers of the PCI interface region. Finally, a fifth, separate, set of VSSB pins and VDDB pins are provided respectively for supplying ground and power to the output buffers of the SCSI interface region.

Thus, there are five relatively independent power distribution networks on the integrated circuit chip 800 having power source line connections to the VDD3B, VDDB, DVDD, VDD, and AVDD pins respectively. The five power distribution networks are represented by lines 1001-1005 in Fig. 10. If power is properly supplied to all lines 1001-1005, to reduce noise coupling from the digital to the analog power supplies, the present invention utilizes switching devices, illustrated by boxes labeled SCR in Fig. 10, to isolate the respective power distribution networks 1001-1005 from one another.

By isolating the power supplies, a new potential problem arises which is explained by way of example. Suppose, power is inadvertently applied to AVDD line 1001, but not to DVDD line 1002. This could happen, for instance when the power supply of one region is switched on late. By not powering up DVDD line 1002, circuitry in Ethernet analog region 801 of chip 800 might be damaged. Such damage may result because one or more PN junctions that is supposed to be reverse biased within the chip 800 will not be so biased because an N region which was supposed to receive power from a supply has not yet turned on and will not be at the required potential. Damage to the chip 800 may result because of excessive current flow through the PN junction which is not reverse biased.

To prevent such chip damage, the present invention, isolates the regions 1001-1005 utilizing back-to-back SCRs (silicon control rectifiers), as represented by the boxes labeled SCR formed between the positive power lines represented by lines 1001-1005. The SCRs assure that all PN junctions intended to be reverse biased during chip operation are so biased. The SCRs are structured to go into a latch-up, or a conductive state, when a voltage difference develops between any two of lines 1001-1005. This is done to assure that appropriate junction-reversing bias levels are maintained throughout chip 800 even in the case where one lines 1001-1005 receives power at the same time that another does not, for example if one power supply switches on late. No one region can be powered up without simultaneously applying some power to the other on-chip regions.

On the other hand, once power is appropriately supplied to all lines 1001-1005, the SCRs turn off, or do not latch-up. The SCRs remain off to provide isolation between the power distribution lines of the analog and digital circuit regions 1001-1005.

An area-efficient method for forming an SCR between source power supply lines is shown in Fig. 11. As is well known in the art, parasitic SCRs tend to form wherever a sequence of PNPN adjacent regions is found. The PNPN sequence is equivalent to a PNP transistor and an NPN transistor interlocked as shown in Fig. 12. catch-up is induced if enough stray current crosses the base-emitter junction of either of the PNP and NPN transistors. It is common practice to place shorting straps across base-emitter junctions of one or both of the PNP and NPN transistors where possible in order to avoid latch-up. And in places where this is not possible, the spacing D1 between the two N's of an NPN sequence and/or spacing D2 between the two P's of a PNP sequence are made sufficiently large, and the conductivities of the regions are adjusted, to minimize the risk of latch-up.

Each SCR in the sets of back-to-back SCRs of the present invention is formed by violating the traditional design rules. In Fig. 11, regions 1111, 1121 and 1112 define an NPN sequence implanted in the bulk 1130 of an N- bulk substrate. Regions 1121, 1112 and 1122 define a PNP sequence. P+ region 1122 is implanted in a P-well 1140 and strapped to positive power supply line Vdd2. N+ region 1111 is strapped to positive power supply line Vdd1. Note that P+ region is not shorted to Vdd1. Vdd1 and Vdd2 represent any two respective ones of voltages AVDD, DVDD, VDD, VDDB and VDD3B.

The spacing D1 between the two N regions, 1111 and 1112, of the NPN sequence 1111-1121-1112 and/or the spacing D2 between the two P regions, 1121 and 1122, of the PNP sequence 1121-1112-1122 are made sufficiently small, and the conductivities of the regions are appropriately adjusted, to assure that latch-up will occur when the difference between Vdd1 and Vdd2 exceeds a predefined threshold.

As seen in Fig. 12, respective distances D1 and D2 shown in Fig. 11 define equivalent, resistive paths R1 and R2 through the bulk substrate 1130. The resistance values of R1 and R2 may adjusted by lithography and/or selection of doping concentrations to set the trigger threshold of the SCR at a desired level.

Thus, the present invention provides a plurality of switching devices such as SCRs between otherwise isolated power distribution networks of chip 800 for sensing when an excessive difference develops between the voltages of two or more otherwise isolated networks and for forming a conductive path between the networks should an excessive voltage difference occur.

Although the invention has been described above with particularity, this was merely to teach one of ordinary skill in the art how to make and use the invention. Many modifications will fall within the scope of the invention, as that scope is defined by the claims which follow. For instance, although the integrated Ethernet-SCSI controller of the present invention is disclosed as interfacing with a processor/memory system through the PCI Local Bus, other bus structures may also be utilized for an interface to the processor/memory system. Additionally, although components and the organization of components in this invention described for reduction of noise are described for application with a combined Ethernet-SCSI controller, the components can be utilized in other devices.