Quick level-down shifter question

[maxporter]

Hi all,

I have been looking in some papers regarding leveldown shifters (3.3 -> 1.2). So far I have seen two types:

- simple two inverters: One connected to high Vdd, second to low VDD. All thick gate oxide.
- "regular" levelshifter (also often used for level-up): pmos cross coupled latches with nmos pulldown & some inverters.

What would be the benifit of the second? I think it's slower because of settling time of the latches, so why would you consider that type?

Regards

 

[Dinamovist]

If all you need is 3.3 to 1.2 the only difference is propagation delay and possibly some hard to define thresholds. If you need to send some signals back from 1.2 to 3.3 then the cross coupled one is a far better option

 

[maxporter]

...those were my thoughts also. And the thresholds could be defined with a Schmitt trigger. So why use a solution with more devices which has more delay...
Regards.

 

[erikl]

... the thresholds could be defined with a Schmitt trigger.

You won't need a Schmitt trigger, I think, as the first inverter should deliver well defined logic levels and fast transitions between them.

If you use a Schmitt trigger instead of the 2^nd inverter, this could cost more delay than the 2-latches shifter would provide.