Abstract:
A CMOS output cell with multiple output modes is disclosed. In one embodiment, the cell drives a differential output signal on two output pads in one mode and two single-ended output signals on the two output pads in another mode. Differential and single-ended driver transistors are included for this purpose. A logic circuit disables unused driver transistors, and supplies appropriate drive signals to those transistors for each mode. When disabled, the driver transistors serve an electrostatic discharge (ESD) protection function, at least partially alleviating the need for ESD-specific devices in the cell. The diminished need for ESD-specific devices allows the cell to offer a highly flexible chip interface, with little or no increase in circuit area over a conventional cell that offers only single-ended or differential output.

Description:
This application claims the benefit of Provisional application No. 60/292/182, filed May 18, 2001. 
    
    
     FIELD OF THE INVENTION 
     This present invention relates generally to CMOS (complementary metal-oxide semiconductor) integrated circuits, and more particularly to I/O (input/output) structures and methods for such circuits. 
     BACKGROUND OF THE INVENTION 
     Integrated circuits transmit and receive electrical signals to and from other circuitry using input and output “cells” designed for that purpose. The physical connection between each input or output cell and outside circuitry is conventionally made by bonding a small wire to a bonding “pad”, i.e., an extended and exposed conductive region located on one of the circuit&#39;s metal layers. For an input cell, receiving circuitry connects to the bonding pad. For an output cell, a transmitter or driver circuit connects to the bonding pad. Typically, both input and output cells also contain Electro-Static Discharge (ESD) protection circuitry that attempts to clamp large transient voltages (inadvertently applied to a bonding pad) before those voltages can damage a receiver or driver. 
     FIGS. 1,  2 , and  3  illustrate three aspects of a simple output cell  20 . Referring first to FIG. 1, P-channel MOS (PMOS) transistor  22  and N-channel (NMOS) transistor  24  operate as a complementary field-effect transistor (FET) pair signal driver. When signal IN is at a high voltage, transistor  22  is turned off and transistor  24  is turned on, pulling output pad  25  down towards Vss. Conversely, when signal IN is at a low voltage, transistor  24  is turned off and transistor  22  is turned on, pulling output pad  25  up towards Vdd. 
     PMOS transistor  26  and NMOS transistor  28  provide ESD protection for cell  20 . Note that the gate of transistor  26  is permanently connected to Vdd, and the gate of transistor  28  is permanently connected to Vss, ensuring that these transistors are permanently off. But as shown in FIG. 2, transistors  26  and  28  contain diode structures that provide protection against voltage spikes. PMOS transistor  26  protects the cell from pad voltages much greater than Vdd, and NMOS transistor  28  protects the cell from pad voltages much less than Vss. 
     FIG. 3 shows a cross-section of transistors  26  and  28 . Within PMOS transistor  26 , a diode junction exists between the P+ drain diffusion  36  (connected to output pad  25 ) and the N-well drain diffusion  32  (connected to Vdd). Thus when the voltage at output pad  25  is slightly higher than Vdd, this diode junction is forward biased and current can flow from the pad to the Vdd voltage rail, clamping the pad voltage to a safe level. 
     Similarly, within NMOS transistor  28 , a diode junction exists between the N+ drain diffusion  40  (connected to output pad  25 ) and the P-substrate  30  (connected to Vss). Thus when the voltage at output pad  25  is slightly lower than Vss, this diode junction is forward biased and current can flow from the Vss voltage rail to the pad, again clamping the pad voltage to a safe level. 
     SUMMARY OF THE INVENTION 
     Although transistors  26  and  28  in FIG. 1 are included for ESD protection, it is recognized herein that driver transistors  22  and  24  can have similar—albeit typically smaller due to smaller size—ESD benefits if their bodies are biased appropriately. The described embodiments make use of this observation in an output cell having no (or reduced-size) ESD-only devices, augmented with multiple sets of driver transistors. The output cell contains a multimode logic circuit that, in each mode, configures at least some sets of driver transistors in an “off” mode that provides ESD protection. 
     For instance in one embodiment, an input/output cell connects to two pads. The cell has one set of differential drivers that allows a signal to be driven differentially on the two pads in one mode. The cell also has a set of single-ended drivers that allow two different signals to be driven on the two pads in another mode. In still another mode, the cell accepts input signals on the two pads. A multimode logic circuit selects the appropriate drivers for each mode, and turns off the remaining drivers, essentially placing them in an ESD mode. 
     One beneficial use of such an embodiment is in providing a flexible interface for an integrated circuit. Historically, a designer had to choose an interface type for each pad from a library of standard input and output cells. If two customers desired two different interface types, the circuit designer had to either design and manufacture two different integrated circuits, or provide two sets of pads and accompanying cells, one per interface type, on the circuit. Either approach is more expensive than the preferred embodiments described herein, which supply multiple different interface types on the same pads, at no significant increase in circuit area or cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be best understood by reading the disclosure with reference to the drawing, wherein: 
     FIG. 1 illustrates a prior art single-ended output cell; 
     FIG. 2 shows an ESD equivalent circuit for the cell of FIG. 1; 
     FIG. 3 shows the ESD transistors of the FIG. 1 cell in cross-section; 
     FIG. 4 illustrates a multimode I/O cell according to a first embodiment of the invention; 
     FIG. 5 illustrates, in block diagram form, a multimode I/O cell according to a second embodiment of the invention; 
     FIG. 6 contains a circuit diagram for the driver/ESD block of FIG. 5; 
     FIG. 7 contains additional circuit details for the differential section of FIG. 6; 
     FIG. 8 illustrates a logic gate implementation for the driver logic circuit block of FIG. 5; 
     FIG. 9 contains a circuit diagram for the receiver circuit of FIG. 5; 
     FIG. 10 contains a circuit diagram for the current reference of FIG. 5; and 
     FIG. 11 shows a block diagram for part of an integrated circuit incorporating a block of I/O cells according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 4 contains a simplified block diagram for an input/output cell  50  according to one embodiment of the invention. Logic circuit  52  accepts two input signals S 0  and S 1 , and a mode signal MODE. Logic circuit  52  provides signals to the gates of four CMOS transistor pairs: pair P 1 , N 1 ; pair P 2 , N 2 ; pair P 3 , N 3 ; and pair P 4 , N 4 . The two transistors of each pair are connected at their drains to a drain node—the drain nodes of pairs P 1 , N 1  and P 3 , N 3  connect to a first conductive pad  54 , and the drain nodes of pairs P 2 , N 2  and P 4 , N 4  connect to a second conductive pad  56 . The source of each N-channel transistor (N 1 , N 2 , N 3 , N 4 ) couples to a reference or ground voltage Vss. The source of each P-channel transistor (P 1 , P 2 , P 3 , P 4 ) couples to a supply voltage Vdd. P 1  and P 2  couple to Vdd through a common current source  58 , placing pairs P 1 , N 1  and P 2 , N 2  in a differential configuration. 
     Although not necessary if the cell will be used only for signal output, cell  50  also includes two additional CMOS transistor pairs, P 21 , N 21  and P 22 , N 22 . Pair P 21 , N 21  inverts and drives the signal received on conductive pad  54 , producing an input signal C 1 . Pair P 22 , N 22  inverts and drives the signal received on conductive pad  56 , producing an input signal C 2 . 
     When MODE is set to a first output mode, logic circuit  52  turns off pairs P 3 , N 3  and P 4 , N 4 , e.g., by supplying Vdd to the gates of P 3  and P 4 , and Vss to the gates of N 3  and N 4 . S 0  is used in this mode to drive the gates of P 1 , P 2 , N 1 , and N 2  as a differential current-mode driver. For instance, when S 0  is at a logic low level, logic circuit  52  turns on P 1  and N 2  and turns off N 1  and P 2 , such that current I 0  flows out pad  54  and in pad  56 . And when S 0  transitions to a logic high level, logic circuit  52  reverses this on/off pattern, such that current I 0  flows out pad  56  and in pad  54 . 
     S 0  could optionally be an analog output signal instead of a logic signal, in which case logic circuit  52  can create appropriate analog drive signals for pairs P 1 , N 1  and P 2 , N 2 . 
     When MODE is set to a second output mode, logic circuit  52  turns off pairs P 1 , N 1  and P 2 , N 2 , e.g., by supplying Vss to the gates of all four transistors and turning off current source  58 . S 0  is used in this mode to drive the gates of P 3  and N 3  as a single-ended voltage driver. In the second mode, logic circuit  52  can drive the gates of P 4  and N 4  as a second single-ended voltage driver. MODE can of course have multiple sub-modes in which the mapping of signals S 0  and S 1  onto bonding pads  54 ,  56  can be one of the following: S 0 , S 1 ; S 1 , S 0 ; S 0 , none; S 1 , none; none, S 0 ; or none, S 1 . Note that if one of the pairs P 3 , N 3  and P 4 , N 4  is never used as a voltage driver, the logic circuit need not control the gates of that pair, and that pair can be configured as a conventional ESD circuit by connecting the gates of that pair permanently to their respective voltage rails. 
     For the embodiment shown in FIG. 4, MODE can also be set to an input mode that turns off all output drivers, placing all in an ESD mode. Input signals can then be received on one or both of pads  54  and  56 , and passed to the integrated circuit as C 1  and C 2 . 
     FIG. 5 illustrates, in block diagram form, a specific input/output cell embodiment  100 . Cell  100  interfaces on the integrated circuit side with core logic operating at 1.8 V. Cell  100  itself operates at 3.3 V. In one mode, cell  100  outputs signaling compatible with Reduced Swing Differential Signaling (RSDS, a trademark of National Semiconductor Corp., as described in RSDS™ Specification, Rev. 0.95, May 2001). When driven into a 100-ohm load placed across PAD 0  and PAD 1 , the differential voltage across the pads will be roughly 250 mV, with an offset voltage V off  of approximately V ref =1.3 V. 
     In a second mode, cell  100  outputs either one or two CMOS/TTL (3.3 V logic) signals, one on PAD 0  and the other on PAD 1 . 
     In a third mode, cell  100  receives either one or two CMOS/TTL signals, one on PAD 0  and the other on PAD 1 . 
     Cell  100  contains four functional blocks. Driver/ESD circuit  200  produces output signals in the various output modes, and provides ESD protection against spurious transients on PAD 0  and PAD 1 . Driver logic circuit  300  receives 1.8 V signals from the circuit core, and converts these signals to control signals for driver/ESD circuit  200 . Receiver circuit  400  performs the signal input functions for PAD 0  and PAD 1 , providing corresponding 1.8 V signals to the circuit core on C 0  and C 1 . Current reference  500  provides a biasing current reference IREF for the differential circuitry of driver/ESD circuit  200 . 
     An implementation example for each block of cell  100  will now be described with reference to FIGS. 6-10. 
     FIG. 6 contains a more detailed version of driver/ESD circuit  200  of FIG.  5 . The operation of that circuit will be described first for a differential output mode, then for a single-ended output mode, and finally for a single-ended input mode. 
     In differential output mode, signal DIFFEN is asserted (and complementary signal DIFFEN# is deasserted) in order to activate the differential circuitry. Signals DIFF+ and DIFF− form the differential inputs used to control the differential driver transistor pairs P 1 , N 1  and P 2 , N 2 . Signal IREF provides a reference current I 0  for generating an appropriate RSDS current level, and signal VREF provides a reference voltage for generating an appropriate RSDS bias voltage. The remaining control signals (SEAP 0 , SEAN 0 , SEAP 1 , SEAN 1 , SEBP 0 , SEBN 0 , SEBP 1 , and SEBN 1 ) each control one of the single-ended output transistors (respectively P 3 , N 3 , P 5 , N 5 , P 4 , N 4 , P 6 , and N 6 ). In differential mode, each SE signal controlling a PMOS transistor is driven high, and each SE signal controlling an NMOS transistor is driven low, placing the SE transistors in an ESD mode. 
     Gated current mirror  210  is off when DIFFEN# is asserted, but otherwise replicates IREF, supplying a reference current of magnitude I 0  to current mirrors  212  and  214  (which use a common mirror transistor). Current mirror  212  in turn supplies a reference current of magnitude  10  to current mirrors  216  and  218  (which also use a common mirror transistor). 
     Gated averaging circuit  220  is on when DIFFEN is asserted. When on, averaging circuit  220  supplies a sample voltage VAVG, representing the instantaneous average of the voltage on PAD 0  and the voltage on PAD 1 , to voltage error amplifier  230 . 
     Voltage error amplifier  230  compares VREF with VAVG. Error amplifier  230  splits a reference current of magnitude  2 I 0  (from current mirror  216 ), such that when VREF and VAVG are equal, a reference current of magnitude I 0  is supplied to current mirror  232 . But when VAVG rises above VREF, error amplifier  230  increases the reference current supplied to current mirror  232  (up to a maximum value of  2 I 0  if necessary). Conversely, when VAVG dips below VREF, error amplifier  230  decreases the reference current supplied to current mirror  232  (down to a minimum value of zero, if necessary). 
     Current mirror  218  supplies a current of magnitude  26 I 0  to the coupled sources of differential driver transistors P 1  and P 2  when DIFFEN is asserted. Likewise, current mirrors  214  and  232  combine to drain a current of magnitude  26 I 0  ( 8 I 0  from mirror  232  and  18 I 0  from mirror  214 ) from the coupled sources of differential driver transistors N 1  and N 2  when DIFFEN is asserted. 
     In differential output mode signaling, one of DIFF+ or DIFF− will be a logic high, and the other will be a logic low. Gate  240  passes DIFF+ to the gates of P 1  and N 1 ; DIFF− is supplied directly to the gates of P 2  and N 2 . Thus when DIFF+ is logic high, a current of magnitude  26 I 0  will flow through P 2 , out PAD 1  through the differential load, back in PAD 0 , and through N 1 . When DIFF+ is logic low, this current will reverse, flowing through P 1 , out PAD 0  and through the differential load in the opposite direction, back in PAD 1 , and through N 2 . 
     ESD continuity circuits  242 ,  244 , and  246  each contain transistors that are biased off, with sources tied to a voltage rail. The drains of the continuity circuit transistors connect to source/drain regions of differential circuit transistors that are not tied directly to a voltage rail and have their other source/drain region connected to a pad (e.g., P 1 , P 2 , N 1 , and N 2 ). 
     In single-ended output mode, DIFFEN is deasserted (and DIFFEN# is asserted). This turns off gated current mirror  210 , which zeros all of the differential bias currents in driver/ESD circuit  200 . Rail-gated current mirrors  214  and  218  have their mirror connections opened, and their gates referenced instead to the voltage rail that biases those circuits off. Averaging circuit  220  is also turned off. Gate  240  disconnects DIFF+ from P 1  and N 1 , instead connecting these transistors to Vdd (leaving P 1  off and N 1  on). DIFF− is driven low, such that P 2  is on and N 2  is off. Note that although N 1  and P 2  are technically on, each has its source coupled to a high impedance and thus the differential outputs are disabled. Optionally, each of P 1 , P 2 , N 1 , and N 2  could be driven by a separate input, such that all four transistors can be turned off in single-ended mode. 
     The SE gate signals are potentially active in single-ended output mode. When a single-ended signal is driven on PAD 0 , two drive strengths are available. One drive strength drives SEAP 0  and SEBP 0  in synchronism, and SEAN 0  and SEBN 0  in synchronism (but complementary to SEAP 0  and SEBP 0 ). A lesser drive strength drives only one P 0  and one N 0  transistor, leaving the others biased off. 
     A second single-ended signal can also be driven concurrently on PAD 1  using the remaining SE gate signals in similar fashion. 
     In single-ended input mode, the differential circuitry signals are set as in single-ended output mode. Further, the SE signals are set as in differential output mode. This setting places driver circuitry connected to a pad in a high-impedance state. 
     FIG. 7 illustrates further detail for the differential circuitry portions of driver/ESD circuit  200  in one embodiment, with the ESD continuity circuits and single-ended drivers removed for clarity. 
     Gated current mirror  210  comprises matched transistors P 7  and P 8 , with common sources tied to Vdd and common gates. P 7  has its gate and drain shorted to a switch transistor P 9  that allows IREF to flow through P 7  whenever DIFFEN# is deasserted. Thus in single-ended modes, current mirror  210  is off, and in differential mode, P 8  mirrors IREF. 
     Current mirror  212  comprises matched transistors N 7  and N 8 , with common sources tied to Vss and common gates. N 7  has its gate and drain shorted to the drain of P 8 , such that in differential mode, mirror  212  replicates IREF at the N 8  drain node. 
     Current mirror  214  shares transistor N 7  with current mirror  212 . When DIFFEN is asserted, switch transistor N 10  couples the gate of transistor N 9  to the gate of transistor N 7 . Transistor N 9  has 18 parallel channels, each dimensionally identical to the single channel of N 7 , such that N 9  mirrors 18 times IREF when on. Note that when DIFFEN is deasserted, not only is the gate of N 9  disconnected from the gate of N 7 , but the N 9  gate is biased to Vss instead through switch transistor N 11 , which uses DIFFEN# as a gate signal. 
     Current mirror  216  comprises transistors P 10  and P 14 , with common sources tied to Vdd and common gates. P 10  has its gate and drain shorted to the drain of N 8 , such that in differential mode, mirror  216  is referenced to IREF. Transistor P 14  has two parallel channels, each dimensionally identical to the single channel of P 10 , such that P 14  mirrors twice IREF when on. 
     Current mirror  218  shares transistor P 10  with current mirror  216 . When DIFFEN# is deasserted, switch transistor P 12  couples the gate of transistor P 11  to the gate of transistor P 10 . Transistor P 11  has 26 parallel channels, each dimensionally identical to the single channel of P 10 , such that P 11  mirrors 26 times IREF when on. Note that when DIFFEN is deasserted, not only is the gate of P 11  disconnected from the gate of P 10 , but the P 11  gate is biased to Vdd instead through switch transistor P 13 , which used DIFFEN as a gate signal. 
     Voltage error amplifier  230  receives the  2  IREF-magnitude current produced by mirror  216 , and apportions that current between two identical paths to Vss. Each path comprises a P-channel transistor with its source coupled to the drain of P 14 , and an N-channel transistor with its source coupled to Vss, the drain of the P-channel transistor coupled to the drain and gate of the N-channel transistor. 
     In one path, the P-channel transistor P 15  receives a gate signal VREF, and in the other path, the P-channel transistor P 16  receives a gate signal VAVG. It can be appreciated that when VAVG≈≈VREF, a current of magnitude IREF will flow through each path. When VAVG is greater than VREF, P 16  will carry less current than P 15 ; when VAVG is less than VREF, P 15  will carry less current than P 16 . 
     The current that passes through P 15  also passes through N 15 . N 15  and N 24  share common source and gate nodes. Transistor N 24  has eight parallel channels, each dimensionally identical to the single channel of N 15 , such that N 24  mirrors eight times the current passing through N 15 . 
     Gated averaging circuit  220  comprises the serial combination of transistor N 12 , two resistors of resistance R (e.g., R=2.8 kΩ), and transistor N 13 , bridged between PAD 0  and PAD 1 . Transistors N 12  and N 13  are identical switch transistors driven by a common gate signal DIFFEN. N 12  has one source/drain node connected to PAD 0 , and N 13  has one source/drain node connected to PAD 1 . When DIFFEN is asserted, the two series resistors are effectively connected across PAD 0  and PAD 1 . The voltage VAVG, measured between the two resistors, thus represents a voltage midway between the PAD 0  and the PAD 1  voltage, no matter which of PAD 0  or PAD 1  is at a higher voltage. 
     Finally, gate  240  contains switch transistors P 17  and N 17 , each driven by a gate signal DIFFEN. When DIFFEN is asserted, N 17  is on, and DIFF+ drives P 1  and N 1 . When DIFFEN is deasserted, P 17  is on, and pulls the gates of P 1  and N 1  high. 
     All P-channel transistors in FIGS. 6 and 7 have their n-wells referenced to Vdd. 
     FIG. 8 shows one implementation for a driver logic circuit  300 . 1.8 V logic signals S 0 , S 1 , OEN#, DIFFSEL, and DRVSEL are inputs to logic circuit  300 . The input inverters identified with a “C” are conditioning inverters that accept a 1.8 V logic input and provide a 3.3 V logic output. The remaining single-ended control logic gates in circuit  300  operate as 3.3 V logic gates (all of the differential control logic operates at 1.8 V). The identifiers within those gates, ending in “x”, indicate the relative size of each gate. 
     Signal DIFFSEL determines whether the differential driver circuitry will be enabled. DIFFSEL is supplied to the enable (E) input of differential gate signal generator  310 . 
     Differential gate signal generator  310  accepts S 0  as a 1.8 V input signal IN, and creates two 1.8 V output signals OUT+ and OUT−. One embodiment for generator  310  uses two serial inverters to create OUT+ from IN, and three faster serial inverters to create OUT− from IN with approximately the same timing but opposite phase. When E is deasserted, however, both OUT+ and OUT− produce logic low signals regardless of the signal present at S 0 . The signals generated at OUT+ and OUT− are buffered up to a higher drive strength (but remain 1.8 V logic signals) to form output signals DIFF+ and DIFF−, respectively. 
     Signal OEN# is asserted (low) whenever any output driver circuitry will be enabled. When asserted at the same time as DIFFSEL, however, DIFFSEL blocks the single-ended logic circuitry from responding to OEN#. Thus when OEN# is logic high or DIFFSEL is logic high, all single-ended outputs will be set to turn off their respective SE driver transistors regardless of the state of S 0  and S 1 . When both OEN# and DIFFSEL are logic low, at least some of the single-ended outputs will respond to S 0  and S 1 . 
     Which single-ended outputs respond to S 0  and S 1  depends in part on the state of DRVSEL. In single-ended mode, all “SEAxy” outputs respond to Sy. Further, when DRVSEL is set to logic high, all “SEBxy” outputs respond to Sy as well; otherwise, the “SEBxy” outputs continue to turn off their respective SE driver transistors. 
     Note that in this embodiment, signal S 0  provides an input for a drive signal in both single-ended and differential output modes, and S 1  provides an input for a drive signal in single-ended mode. It is straightforward to modify circuit  300  to provide different behavior, e.g., the ability to output one but not both S 0  and S 1  in a single-ended mode, the ability to use a separate input, even an analog input, for the differential channel, etc. 
     FIG. 9 illustrates one embodiment for receiver circuit  400  of FIG.  5 . PAD 0  connects through a resistance R 1  (e.g., 622 Ω) to the gates of transistors P 20  and N 20  (which share a common drain node B 0 ), and to the drain of transistor N 25  (which has a source connected to Vss). A transistor N 24 , connected between the source of N 20  and Vss, determines whether N 20  can pull node B 0  low. When DIFFEN# is low (indicating differential output mode), N 24  and N 25  are off, and circuit  400  presents a high impedance to the differential driver. When DIFFEN# is high (indicating either single-ended input or output mode), N 24  and N 25  are on, allowing: pair P 20 , N 20  to produce at B 0  an inverted version of the signal present at PAD 0 , when PAD 0  is driven; N 25  to pull PAD 0  low through R 1 , when PAD 0  is not driven. Note that N 25  is a weak device, e.g., a long-channel transistor, such that a drive transistor can easily dominate the signal at PAD 0  despite the pulldown effect through N 25 . 
     Node B 0  drives the gates of P 22  and N 22 , which are connected in a conventional inverter configuration between Vdd 1  and Vss 1  (e.g., 1.8 V logic rails), with an output at node C 0 . C 0  thus replicates the logical condition present at PAD 0 , in 1.8 V logic, when PAD 0  is not in differential mode. 
     Transistors P 21 , N 21 , P 23 , N 23 , N 26 , and N 27  perform a similar function (for PAD 1 ) to that just described for the transistors serving PAD 0 . 
     FIG. 10 shows one possible implementation for current reference  500  of FIG.  5 . Signal DIFFSEL turns on transistor N 30  when asserted, allowing reference  500  to generate reference current IREF. Mirror transistors P 31  and P 32  are identical. Mirror transistor N 35  contains two parallel channel regions, each identical to the single channel of mirror transistor N 34 , and thus generates twice the current as N 34 . Reference  500  is designed to produce a  50  μA current through N 34  (and thus a 100 μA current IREF) when Vdd−Vss=3.3 V. 
     FIG. 11 illustrates one application of an I/O cell embodiment in an integrated circuit, e.g., an image processor circuit that manipulates input video and/or graphics signals (not shown) to produce signals appropriate for a display device. Programmable timing controller (TCON)  620  accepts display data, e.g., as n-bit-wide data words. Microprocessor  610  configures TCON  620 , using bus signaling on bus  612 , in one of several possible output modes. For instance in one mode, TCON  620  could drive all n bits of a data word in parallel—through the n/2 I/O cells  100 —on Display Port pads PAD 0  through PAD(n−1) in one output clock cycle as CMOS/TTL single-ended outputs. In another mode, TCON  620  could drive n/2 of the n bits in parallel in two consecutive output clock cycles—this time using each I/O cell  100  to drive one bit differentially—across two pads—during each clock cycle. Or, in another mode TCON  620  could read an input word from the I/O cells and transmit the word to the microprocessor. In each mode, TCON  620  generates the appropriate DRVSEL, OEN, and DIFFSEL signals to each I/O cell  100  to configure the I/O cell in the appropriate mode. 
     For comparison, a general-purpose I/O (GPIO) interface  630  is also shown connected to bus  612 . GPIO unit  630  connects to Port A pads GP 0  to GP(m−1) through conventional I/O cells  640 . Although not shown exactly to scale, the comparison is intended to represent that the multimode I/O cells  100  take up no more room, on a per-pad basis, than the conventional cells  640 . 
     The multimode examples presented above are merely exemplary—for instance, the data word width and number of Display Port pads need not match, and the timing need not be as expressed. TCON  620  can use any of a variety of multiplexing schemes to drive data on the output pads. The illustrations are intended only to demonstrate the flexibility of such an integrated circuit in interfacing with different external display circuitry. 
     One of ordinary skill in the art will recognize that the concepts taught herein can be tailored to a particular application in many other advantageous ways. For instance, although RSDS signaling is shown, another signaling format, such as LVDS (Low Voltage Differential Signaling) could be employed—or configurable voltage and current references could be used to supply signals in multiple programmable differential formats. In general, the voltages, currents, resistance values, transistor ratios and configurations, etc. disclosed herein merely demonstrate a few implementations, and can be readily adapted to other implementations. Although a “pad” includes bonding pads such as typical in the industry, the exact mechanism used to interface the circuit with external circuitry is not critical to the invention, and thus a “pad” could include any such mechanism. Such minor modifications are encompassed within the invention, and are intended to fall within the scope of the claims. 
     The preceding embodiments are exemplary. Although the specification may refer to an “one”, “another”, or “some” embodiment(s) in several locations, this does not necessarily mean that each such reference is to the same embodiment(s), or that the feature only applies to a single embodiment.