PROGRAMMABLE
ASIC I/O CELLS
All programmable ASICs contain some type of
input/output cell
(
I/O cell
). These I/O cells handle driving logic signals off-chip, receiving and conditioning external inputs, as well as handling such things as electrostatic protection. This chapter explains the different types of I/O cells that are used in programmable ASICs and their functions.
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DC output.
Driving a resistive load at DC or low frequency (less than 1 MHz). Example loads are light-emitting diodes (LEDs), relays, small motors, and such. Can we supply an output signal with enough voltage, current, power, or energy?
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AC output.
Driving a capacitive load with a high-speed (greater than 1 MHz) logic signal off-chip. Example loads are other logic chips, a data or address bus, ribbon cable. Can we supply a valid signal fast enough?
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DC input.
Example sources are a switch, sensor, or another logic chip. Can we correctly interpret the digital value of the input?
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AC input.
Example sources are high-speed logic signals (higher than 1 MHz) from another chip. Can we correctly interpret the input quickly enough?
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Clock input.
Examples are system clocks or signals on a synchronous bus. Can we transfer the timing information from the input to the appropriate places on the chip correctly and quickly enough?
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Power input.
We need to supply power to the I/O cells and the logic in the core, without introducing voltage drops or noise. We may also need a separate power supply to program the chip.
These issues are common to all FPGAs (and all ICs) so that the design of FPGA I/O cells is driven by the I/O requirements as well as the programming technology.