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Forum: FPGA, VHDL & Verilog XC9572 JTAG with 3.3V programmer?


Author: Thomas Jespersen (mindthomas)
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Hi everyone.
As I'm making a CPLD board for the XC9572 5V CPLD from Xilinx I would 
like to know if it's possible to program this with a 3.3V compliant JTAG 
programmer from Digilent?
I mean, does the JTAG signals have to be 5V, if the chip is powered from 
a 5V source? Or can the JTAG signals be 3.3V, if I fx power the JTAG 
adapter from a 3.3V source and then connect the signals to the JTAG pins 
on the CPLD?

Best Regards
Thomas Jespersen

Author: Na sowas (Guest)
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> Or can the JTAG signals be 3.3V, if I fx power the JTAG adapter
> from a 3.3V source and then connect the signals to the JTAG pins
> on the CPLD?
As the DS does not state a difference between 3V3 and 5V VccIO you will 
be able to use the 3V3 JTAG programmer:
VIL Low-level input voltage  0    0.80 V
VIH High-level input voltage 2.0  VCCINT + 0.5 V
But be aware that the the TDO will be a 5V output signal. You must adapt 
this to the needs of your programmer.

Author: Thomas Jespersen (mindthomas)
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Hmm, so would this schematic work?
http://elec.tkjweb.dk/blog/wp-content/uploads/JTAG...

Author: Na sowas (Guest)
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As I mentioned, the only signal of interest is the TDO signal. And here 
i would give a try to a simple voltoage divider to convert the 5V TDO 
from the CPLD to the 3V3 level of the programmer.
With the values 330 Ohm (from CPLD TDO to prog) and 680 Ohm (from the 
prog to GND) I would the thing expect to work.

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