5V to 3.3V communication

[tumbleweed]

Select 5V from PIC needs to be 3.3V I think
CLK 5V from PIC needs to be 3.3V I think

Yup. You should use more than just a series 1K resistor, though.
You could use a voltage divider on the 5V PIC outputs...say 2K series/3K shunt to gnd. That should work ok at slow speeds.
You could also use a pullup resistor to 3.3V and drive the outputs as open-drain, but that's not compatible with SHIFTIN.

Data 3.3V from MAX should be OK

For the input pin be sure to pick a pin that has TTL level inputs (PORTA and PORTB) and not ST levels, otherwise the 3.3V output from the MAX won't meet the input Vih spec.

[HenrikOlsson]

Hi,
There are bidirectional level shifter ICs available and you can do it with small MOSFETs and such but for a one-off I'd just use a voltage divider on the 5V to 3.3V signal and nothing on the 3.3V to 5V signal. (Doing that right now in my desk (PIC is 3.3V, device is 5V).

The MAX31855 logic high is min Vcc-0.4 so if you run it at 3.3V it'll put out 2.9V which is well above the 2.0V threshold of the PIC if you run that at 5V and using a non Schmitt trigger input...

/Henrik.

[MOUNTAIN747]

Thanks guys,
I will take that and run with it. I think I'll bread board with the 2k/3k then look at a level shifter IC's' on the finial board work.
Thanks again!
Wayne

[MichelJasmin]

Check out at Adafruit's breakout board. They have a nice solution: https://github.com/adafruit/Adafruit...breakout-board

Why not powering your PIC with 3.3V? That saves a lot of trouble.

[Dave]

Rather than buying expensive chips why not make your own with this app note:

[MichelJasmin]

Rather than buying expensive chips why not make your own with this app note:

The Adafruit board is quite pricey but look how they did the level shifting. I did use this trick with an ESP8266, worked fine.

[Dave]

I also used it for interfacing to some LINX RFM-433 receivers and LINX TFM-433 transmitters in my Hopps moisture monitoring system.

[andywpg]

I've used THIS on a couple of projects and love 'em.