You need a stiff current source therefore you must use a 5 volt regutator like an LM7805. They have extremely low impedance and are not susceptable to load variations unless their power rating is exceded which is typically one amp.
Posted on 2003-05-17 18:18:21 by mrgone
Hi, x86asm,

I think by now you have a pretty good idea about how TTL and CMOS devices work.
There are only a few things I would like to mention:

1. Use a 5V regulator. That will ensure things will operate correctly. (Ernie's reply is pretty clear on that).

2. The propagation delay is an important parameter, but you have to ask yourself what the REAL requirements are for each circuit. If the gate only drives an LED, for example, it simply doesn't matter what the propagation delay is. You will never see the difference! (This may not be the best example of all, but this is the kind of question you should ask yourself: how would it affect the performance?).

3. Always consider the worst-case input and output voltage levels given in the datasheet. That way, things will always operate. Can you imagine how frustrating it would be to find your circuit doesn't operate one day, although the day before was just fine? And in the end to find the only difference is that the temperature is 10C higher?...
For the PIC in question there are inputs with TTL buffers, which require only 2V min for a HIGH, but there are also inputs with Schmitt triggers, which need 4V. So with the former you can use regular TTL's, with the latter you should use CMOS or TTL's with pullups. So before starting the project give it some thought and choose the I/O's you will use depending on the options you have available.

Good luck.
Posted on 2003-05-17 21:12:45 by VVV
Mind if I step on your debate? :tongue:
The characters in the midle of the IC designation number indicate the following:
H=highspeed
C=cmos compatible
T=ttl compatible


If you use 'HCT' drivers, they will interface between cmos and ttl with no dramas.
Now you can worry a little less about whether 3 volts is high or not :)

If you don't wish to pay the few extra cents, just ensure you drive your logic to its extremes in terms of maximum and mimimum voltage for logical 1 and 0... the grey area is really about TTL not producing enough voltage to provide a solid cmos high.
This can be remedied using HCT, or alternatively using an extra transistor to provide the high.
Posted on 2003-06-11 00:46:37 by Homer
Is advanced high speed CMOS and is the fastest!
Posted on 2003-06-11 11:24:47 by mrgone