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<H1 CLASS="ChapterTitle">
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PROGRAMMABLE <BR>
ASIC I/O CELLS</H1>
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All programmable ASICs contain some type of <A NAME="marker=82693">
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input/output cell</SPAN>
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I/O cell</SPAN>
). 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.</P>
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The following are different types of I/O requirements.</P>
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DC output.</SPAN>
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?</LI>
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AC output.</SPAN>
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?</LI>
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DC input. </SPAN>
Example sources are a switch, sensor, or another logic chip. Can we correctly interpret the digital value of the input?</LI>
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AC input.</SPAN>
Example sources are high-speed logic signals (higher than 1 MHz) from another chip. Can we correctly interpret the input quickly enough?</LI>
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Clock input.</SPAN>
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?</LI>
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Power input.</SPAN>
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.</LI>
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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.</P>
<H2 CLASS="Heading1TOC">
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6.1 DC Output</A>
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6.2 AC Output</A>
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6.3 DC Input </A>
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6.4 AC Input</A>
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6.5 Clock Input</A>
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6.6 Power Input</A>
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6.7 Xilinx I/O Block</A>
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6.8 Other I/O Cells</A>
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6.9 Summary</A>
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6.10 Problems</A>
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6.11 Bibliography</A>
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6.12 References</A>
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</DIV>
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