About the parallel port jtag programmer, can anyone say what is the reason we require resistors at the inputs to the buffers. I thought buffers have high input resistance anyway?
- posted
16 years ago
About the parallel port jtag programmer,
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- posted
16 years ago
Many CMOS devices have protection diodes between the inputs and the power supplies. That means that beyond a diode drop outside the supply voltage, the inputs are no longer high impedance, since the diodes start to conduct. The resisters are probably there to limit the diode current in this situation. This is especially true if the receiving logic device has a lower supply voltage than the sending device may drive - in that case, the external resistor and the internal diode function as a level shifter.
You can do the same trick to put 5v signals into some FPGA families (spartan 3 for example), the resistor needing to be sized to keep the maximum current through the protection diode within its data sheet limit.