2.7 I/O Cells

Figure 2.33 shows a three-state bidirectional output buffer (Tri-State ^® is a registered trademark of National Semiconductor). When the output enable (OE) signal is high, the circuit functions as a noninverting buffer driving the value of DATAin onto the I/O pad. When OE is low, the output transistors or drivers , M1 and M2, are disconnected. This allows multiple drivers to be connected on a bus. It is up to the designer to make sure that a bus never has two drivers—a problem known as contention .

In order to prevent the problem opposite to contention—a bus floating to an intermediate voltage when there are no bus drivers—we can use a bus keeper or bus-hold cell (TI calls this Bus-Friendly logic). A bus keeper normally acts like two weak (low drive-strength) cross-coupled inverters that act as a latch to retain the last logic state on the bus, but the latch is weak enough that it may be driven easily to the opposite state. Even though bus keepers act like latches, and will simulate like latches, they should not be used as latches, since their drive strength is weak.

Transistors M1 and M2 in Figure 2.33 have to drive large off-chip loads. If we wish to change the voltage on a C = 200 pF load by 5 V in 5 ns (a slew rate of 1 Vns ^–1 ) we will require a current in the output transistors of I _DS = C (d V /d t ) = (200 ¥ 10 ^–12 ) (5/5 ¥ 10 ^–9 ) = 0.2 A or 200 mA.

Such large currents flowing in the output transistors must also flow in the power supply bus and can cause problems. There is always some inductance in series with the power supply, between the point at which the supply enters the ASIC package and reaches the power bus on the chip. The inductance is due to the bond wire, lead frame, and package pin. If we have a power-supply inductance of 2 nH and a current changing from zero to 1 A (32 I/O cells on a bus switching at 30 mA each) in 5 ns, we will have a voltage spike on the power supply (called power-supply bounce ) of L (d I /d t ) = (2 ¥ 10 ^–9 )(1/(5 ¥ 10 ^–9 )) = 0.4 V.

We do several things to alleviate this problem: We can limit the number of simultaneously switching outputs (SSOs), we can limit the number of I/O drivers that can be attached to any one VDD and GND pad, and we can design the output buffer to limit the slew rate of the output (we call these slew-rate limited I/O pads). Quiet-I/O cells also use two separate power supplies and two sets of I/O drivers: an AC supply (clean or quiet supply) with small AC drivers for the I/O circuits that start and stop the output slewing at the beginning and end of a output transition, and a DC supply (noisy or dirty supply) for the transistors that handle large currents as they slew the output.

The three-state buffer allows us to employ the same pad for input and output— bidirectional I/O . When we want to use the pad as an input, we set OE low and take the data from DATAin. Of course, it is not necessary to have all these features on every pad: We can build output-only or input-only pads.

FIGURE 2.32 A three-state bidirectional output buffer. When the output enable, OE, is '1' the output section is enabled and drives the I/O pad. When OE is '0' the output buffer is placed in a high-impedance state.

We can also use many of these output cell features for input cells that have to drive large on-chip loads (a clock pad cell, for example). Some gate arrays simply turn an output buffer around to drive a grid of interconnect that supplies a clock signal internally. With a typical interconnect capacitance of 0.2pFcm ^–1 , a grid of 100 cm (consisting of 10 by 10 lines running all the way across a 1 cm chip) presents a load of 20 pF to the clock buffer.

Some libraries include I/O cells that have passive pull-ups or pull-downs (resistors) instead of the transistors, M1 and M2 (the resistors are normally still constructed from transistors with long gate lengths). We can also omit one of the driver transistors, M1 or M2, to form open-drain outputs that require an external pull-up or pull-down. We can design the output driver to produce TTL output levels rather than CMOS logic levels. We may also add input hysteresis (using a Schmitt trigger) to the input buffer, I1 in Figure 2.33, to accept input data signals that contain glitches (from bouncing switch contacts, for example) or that are slow rising. The input buffer can also include a level shifter to accept TTL input levels and shift the input signal to CMOS levels.

The gate oxide in CMOS transistors is extremely thin (100 Å or less). This leaves the gate oxide of the I/O cell input transistors susceptible to breakdown from static electricity ( electrostatic discharge , or ESD ). ESD arises when we or machines handle the package leads (like the shock I sometimes get when I touch a doorknob after walking across the carpet at work). Sometimes this problem is called electrical overstress (EOS) since most ESD-related failures are caused not by gate-oxide breakdown, but by the thermal stress (melting) that occurs when the n -channel transistor in an output driver overheats (melts) due to the large current that can flow in the drain diffusion connected to a pad during an ESD event.

To protect the I/O cells from ESD, the input pads are normally tied to device structures that clamp the input voltage to below the gate breakdown voltage (which can be as low as 10 V with a 100 Å gate oxide). Some I/O cells use transistors with a special ESD implant that increases breakdown voltage and provides protection. I/O driver transistors can also use elongated drain structures (ladder structures) and large drain-to-gate spacing to help limit current, but in a salicide process that lowers the drain resistance this is difficult. One solution is to mask the I/O cells during the salicide step. Another solution is to use pnpn and npnp diffusion structures called silicon-controlled rectifiers (SCRs) to clamp voltages and divert current to protect the I/O circuits from ESD.

There are several ways to model the capability of an I/O cell to withstand EOS. The human-body model ( HBM ) represents ESD by a 100 pF capacitor discharging through a 1.5 k W resistor (this is an International Electrotechnical Committee, IEC, specification). Typical voltages generated by the human body are in the range of 2–4 kV, and we often see an I/O pad cell rated by the voltage it can withstand using the HBM. The machine model ( MM ) represents an ESD event generated by automated machine handlers. Typical MM parameters use a 200 pF capacitor (typically charged to 200 V) discharged through a 25 W resistor, corresponding to a peak initial current of nearly 10 A. The charge-device model ( CDM , also called device charge–discharge) represents the problem when an IC package is charged, in a shipping tube for example, and then grounded. If the maximum charge on a package is 3 nC (a typical measured figure) and the package capacitance to ground is 1.5 pF, we can simulate this event by charging a 1.5 pF capacitor to 2 kV and discharging it through a 1 W resistor.

If the diffusion structures in the I/O cells are not designed with care, it is possible to construct an SCR structure unwittingly, and instead of protecting the transistors the SCR can enter a mode where it is latched on and conducting large enough currents to destroy the chip. This failure mode is called latch-up . Latch-up can occur if the pn -diodes on a chip become forward-biased and inject minority carriers (electrons in p -type material, holes in n -type material) into the substrate. The source–substrate and drain–substrate diodes can become forward-biased due to power-supply bounce or output undershoot (the cell outputs fall below V _SS ) or overshoot (outputs rise to greater than V _DD ) for example. These injected minority carriers can travel fairly large distances and interact with nearby transistors causing latch-up. I/O cells normally surround the I/O transistors with guard rings (a continuous ring of n -diffusion in an n -well connected to VDD, and a ring of p -diffusion in a p -well connected to VSS) to collect these minority carriers. This is a problem that can also occur in the logic core and this is one reason that we normally include substrate and well connections to the power supplies in every cell.

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