OK, I have a board with an XC2S150 on it, and a load of I/O pins; the wiring on the board and the configuration of the default IP blocks on the FPGA appears to set the pins up as LVTTL.
On the other side of the room, I have some photo-diodes and some LM324N op-amps. If I rig up an op-amp as a comparator, I can get a signal which swings from 0V to ... umm ... inspect the datasheet ... Vcc - 1.5V.
Is there some easy way I can set up the IOBs so that the FPGA believes that 0V is a zero and 1.8V a one? Or should I rig up another battery to increase the supply voltage to the op-amp by 1.5V or so; in that case I might be setting the input voltage on an FPGA pin to slightly greater than the supply voltage to the FPGA, which seems likely to be painful.
I've had a look at Schmidt triggers and the like, but, whilst something like a 7414 has output swinging neatly from one supply line to the other, it seems, when running from 3.3V, to want a 2.5V input to switch the output on.
These diode drops weren't such a problem when power supplies were 5V, but I can't see what to do when the total I have to play with is 3.3V.
Tom