[ ttl output to cmos input ]

[texane]

Hi,

I am to interface an old TTL based device with
the PIC18f. The device has roughly 20 pins coming
from TTL based gates I have to read at a given time.

I did not know, but reading the PIC18f specs, some
of the GPIOs are CMOS Schmitt trigger input buffer in
digital IO mode. So I guess I have to translate the level,
from TTL to CMOS, because the TTL level may be too
low for the CMOS Schmitt trigger to switch correctly, I
am right?

In that case, do you know about a chip doing so?

Cheers,

Fabien.

[jimmc]

A pull-up resistor from the TTL output to V_CC will ensure that V_OH is CMOS compatible.
Value is not critical, say 4k7 lower for high speed operation. Minimum value is set by I_OL max of TTL device.
For ultimate speed use 74ACT series device (eg 74ACT04 hex inverter) as a level shifter, the ACT series has TTL compatible input thresholds.

Jim

[texane]

Hi,

Thanks for replying. I do not really care about
speed in this application. I did see the trick of
adding a pullup resistor, but I do not understand
it... I understand what a pullup resistor is I mean,
but could you explain why it raises the voltage
in this case? I mean 2 voltages from differents
nodes do not add...

Thanks for helping,

Fabien.

[septer012]

Maybe you need something like this?!

http://www.sparkfun.com/commerce/product_info.php?products_id=8745

I used one for a 3.3v GPS to 5v PIC/Shmidt Trigger Input

[texane]

Thanks, I will have a look. At a first glance
it seems a bit undersized since I would need
at least 6 pins, thus an octal buffer converter.

Cheers,

fabien.

[jimmc]

The standard TTL output stage is the 'Totem pole' shown here:
http://www.siliconfareast.com/ttl.htm as you can see that, on its own, the output cannot rise above 2 diode drops below V_CC.
Adding the pull-up resistor enables the output to rise to V_CC when the lower transistor (T4) is off.
Under these conditions the diode is reverse biased and T3 has no effect.
If you're not interested in speed the a higher value resistor may be used for the pull-up, it only has to overcome leakage in the TTL output stage, 22k will still give 4v with almost 50uA of leakage.

I am assuming that you are running the PIC from the 5v, the TTL only operates at 5v.

A quick Google on 'TTL to CMOS' gives this as one of the top results:
http://www.fairchildsemi.com/an/AN/AN-314.pdf

Jim