PIC16LF88 otwg on 3V3

[nextdomo]

Hi all,

I am testing the low voltage version of the PIC16 called PIC16LF88 with the OTGW. It works great on 4V however I want to use it on 3V3.
Every time I lower the voltage below 4V it stops working and it looks like this is a typical case for the Brown-out circuit.

I have read the documentations of the chip and it looks like that you can disable the BOR in configuration register 2007h.
Can someone help me how to do this with my pic-chip?

Many thanks!

[hvxl]

The self programming code can't access the configuration registers. So you will have to use an actual PIC programmer to load a different set of configuration bits. How to do that depends on the PIC programmer. What should work with any programmer is to change bit 6 on line 18 of gateway.asm to 0 and rebuild it.

[nextdomo]

Many thanks for your quick reply, I use the ICD2 and indeed the bit change works.
I see the version number at startup and it works. thanks.

however it will also need a hardware change i guess because the thermostat reports his status but the pic-chip is not responding on it. only when i raise the voltage back to 4,5V it starts communicating with the thermostat again.
Can i just change the resistor value or is a software change needed?

thanks again.

[hvxl]

Am I assuming correctly that you changed the 5V to 3.3V throughout the device, not just the supply of the PIC? At least the base of Q4 should be at the same level as VDD of IC1.

If you didn't adjust the R5/R6 voltage divider to account for the lower supply voltage, you will probably have to raise the reference voltage setting. The default value of 6, which corresponds to 1.25V @ 5V, will only be 0.825V @ 3.3V. As a result, both high and low logical levels on the opentherm line may stay above the reference level, meaning the PIC doesn't see any signal. To get a reference voltage level of around 1.25V, you will need to use a CVR value of 9 (that's 1.24V @ 3.3V). You achieve that by issuing a VR=6 command (the command adds 3 before setting the CVR value).

I don't think any other areas need adjusting.