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I'm implementing a JTAG Daisy Chain for the first time on a PCB.
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The device TDO's tri-state after the device is configured. I always put a pull-up on each device's TDO, so that the TDI of the next device is not floating. There's no harm in doing this on the JTAG header too.
Put series resistors on the TDO->TDI chain from the header and between devices. That way if you have trouble with a device, you can remove it from the JTAG chain.
Actually, put series resistors from all the header signals, so that you provide a small amount of ESD protection.
TCK and TMS should be buffered and driven individually to the two devices. You will get transmission line reflections from a daisy chain of a single signal, and depending on layout, one of the devices may see the reflections as double clocks.
Look at this schematic and borrow ideas:
http://www.ovro.caltech.edu/~dwh/carma_board/ http://www.ovro.caltech.edu/~dwh/carma_board/gda06rb004_carma_v0.87_dec03.pdf Cheers,
Dave
Honored Contributor
