[time-nuts] Frequency Divider
cupido at mail.ua.pt
Fri Apr 3 02:48:08 UTC 2009
There is a trick...
That JTAG interface made with a 74HC244 is
powered from the target board,
If the target board runs at 5v so it will work
at 5v. No doubts here...
But if the target runs at 3v3 the 74HC244 gets powered
at 3v3 (and it works fine, no need for the target
device to be tolerant to anything, it gets within
whatever VDD it uses...)
and the 74HC244 seems to be tolerant to whatever comes
out of the PC LPT port, but has a few series resistors though.
Amazingly it even works at 2v5 !
The byteblasterMV from Altera is just like that and it
works great, and programs from the small CPLDs up to
most of the FPGA's (except for some more recent ones that
require someting else... but is not a voltage issue).
I'm running one for ages now with zero issues in both
5v chips and 3v3 chips.
I even made a byteblasterII (to use on the more recent FPGA's)
with the same 74HC244 (adding a few bits to the
original byteblasterMV for the additional features required)
and it works just fine even with Cyclone devices.
Bruce Griffiths wrote:
> Luis Cupido wrote:
>>> The CPLDs are programmed via the JTAG port.
>>> Suitable JTAG programming cables are readily availble.
>> Or you can build one to use the LPT port of your PC using just a 74HC244.
>> Luis Cupido.
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> Using a 74HC244 may be somewhat problematic with CPLDs that don't have
> 5V tolerant inputs.
> Even when using a device with 5V tolerant inputs a 74HCT244 may be more
> suitable for translating LVCMOS logic level outputs from the CPLD.
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