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.

  • \$\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
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    \$\begingroup\$ A schematic would be helpful. \$\endgroup\$ – CL. Aug 20 '16 at 7:05

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.

  • \$\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

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.


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