Isaac Marino Bavaresco wrote:
> If your FPGA's I/Os are 5V tolerant then you may use the 74HCT245, which
> work with TTL input levels (it will understand the 3.3V CMOS levels from
> the FPGA) and outputs 5V CMOS logic levels.

I'm not that lucky -- the FPGA I/Os are configurable for 3.3V, LVDS, 2.5V, 
1.8V and a ton of other I/O standards, but not 5VCMOS or 5VTTL. I usually use 
the Xilinx XC9500XL CPLD chips where I can, because they do have 5V tolerant 
inputs (as long as they're fed via a resistor to limit the current).

Anyway, I've found a few ICs that appear to do what I need -- the TI 
74LVC4245, and the 74ALVC164245. The first is an 8-bit level translator, the 
second is a 16-bit.

> If you need bi-directional communications then you will need to control
> the direction input to allow data to flow to the appropriated direction.
> If not, then you may tie the direction input to a fixed logic level or
> use the 74HCT541 (it is usually more expensive than the 245 and don't
> allow bi-directional communications, so I never use it).

It's partially bi-directional -- the SPP has an 8-bit bidirectional part which 
is used for data transfer, and 3 control lines (OE, ALE and DLE -- with the 
SPP in Mode 00, OESPP, CK1SPP and CK2SPP respectively). OE controls the 
direction of the data buffer, ALE is Address Logic Enable (which clocks a 
4-bit address into the FPGA) and DLE is Data Logic Enable (effectively a clock 
signal -- DLE=1 with OE=1 is a read from FPGA to PIC, DLE=1 with OE=0 is a 
write from PIC to FPGA).

It looks like it'll work... now to solder up an adapter for the ALVC164245 and 
try it out...

-- 
Phil.
piclist@philpem.me.uk
http://www.philpem.me.uk/
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