What kind of interfaces (open drain, push-pull, pull-up, all of them?) are compatible with the input/output of the ST2378E?

BNord.1 · ‎2020-08-03

Hello ST team,

I need to create a TTL interface between two microcontrollers. One of them is supplied with 3V and the other one with 5V.

The 3V microcontroller output can be configured with a pull-up or in open-drain.

The 5V microcontroller has a 5V push-pull interface only.

Would this IC be able to provide a bidirectional interface between them without damage?

I'm particularily thinking of the case where the open drain from the 3V-µC is activated and the output of the 5V-µC is pulled to high state.

Thanks a lot for your help!

Peter BENSCH · ‎2020-08-03

Well, it doesn't matter, if the whatever driving side is push-pull or open-drain. But of course, you can only transmit data at a certain time if one side is the receiver and the other side is the sender or vice versa.

To answer your question: yes, the ST2378E provides a birectional interface between two buses each with a voltage level between 1.71...5.5V, while a direction pin is not necessary.

Good luck!

/Peter

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BNord.1 · ‎2020-08-04

Hello Peter,

Thank you for your reply.

I understand what you mean. However, One cannot be 100% sure of a wrong activation at both sides in the opposite level at the same time.

In that case, wouldn't it be a risk that the transistor in series of the ST2378E to be damaged and wouldn't you suggest series resistors to reduce the current that would flow through it?

For information the 3V microcontroller can also be configured with pull-down resistor of 11k to 16kOhms to limit this risk. However, I think the resistor value would be too high to pull the Vl side to low level due to the internal 9kOhm pull-up.

One last question : I am not really sure to understand how the One-shot block exactly works. It seems to me it will switch on PMOS transistor to pull a direct high level voltage when the output of the opposite side is at low level, then act like an inverter. Could you please explain it?

Peter BENSCH · ‎2020-08-04

Some serial resistors might be a good idea to protect the drivers as well as the translator.

The one-shot blocks are some kind of mone-flops driving the PFETs bypassing the internal pull-ups during edge transitions. This ensures fast switching times.

Good luck!

/Peter

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BNord.1 · ‎2020-08-04

Understood!

Thanks for your help :)