1
\$\begingroup\$

I need an IC for two separate functions.

The first is level translation from 3.3 V to 5V, so that I can use a TFT touch screen with an Arduino Uno. In order for the touch function to work, level translation from 3.3 V to 5 V is needed. Level translation from 5 V to 3.3 V can be done with voltage dividing resistors, but I hope the chip can do this as well.

The second function is to act as a buffer that has output enable, in order to control SPI output from a 74HC165 chip. That chip does not have output enable, and seems to be on all the time, interfering with other SPI signals.

I think that the 74HC125 Will perform both of these functions. My question is, what is the difference between 74HC125 and 74HCT125? I know that the latter is TTL, and the former is CMOS. But other than that, I am not sure which one I should get.

Any help would be greatly appreciated.

\$\endgroup\$
  • \$\begingroup\$ There's a bevy of dual rail voltage shifter ICs that operate in both directions. You need to figure out how many you need and for what directions, though. For example, see: ti.com/logic/docs/… \$\endgroup\$ – jonk Aug 19 '16 at 22:19
  • 1
    \$\begingroup\$ A schematic would be helpful. \$\endgroup\$ – CL. Aug 20 '16 at 7:05
4
\$\begingroup\$

The difference between HC and HCT is the input levels: for HC the levels are symmetric (switchover is Vcc/2), for HCT the 0-part is smaller than the 1 part (switchover is more like Vcc/3, but check the datasheet).

When the chip that generates the input runs at the same level as the logic chip, preferrably use a HC (but an HCT will still work). When the chip that generates the input runs at a lower Vcc, HCT is often better.

You mention a touch screen. Note that if it has a raw (direct) interface, you can operate it at 3V or 5V as you see fit.

\$\endgroup\$
  • \$\begingroup\$ So, are you saying that for function 1 (level translation), HC would be better, where as for function 2 (controlling the 74HC165 output), HCT would be better? \$\endgroup\$ – Fed Aug 19 '16 at 22:08
  • \$\begingroup\$ 3.3 V signals are not valid for HC inputs, but they happen to match 5 V TTL signals (HCT). \$\endgroup\$ – CL. Aug 19 '16 at 22:20
  • \$\begingroup\$ So I had it backwards. HCT for level translation, and HC for OE control of SPI output from the 165? \$\endgroup\$ – Fed Aug 20 '16 at 3:36
5
\$\begingroup\$

The HCT series has the same threshold as TTL at 1.4V but tighter allowed signal levels than TTL thus greater noise margins.

adding to the 1.4V TTL threshold , of +/-0.2 so Vih=1.6min to Vcc and Vil = 1.2 max. to Vdd(0V)

Thus this makes a perfect 3V to 5V buffer.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.