SparkFun Forums 

Where electronics enthusiasts find answers.

Questions relating to designing PCBs
By TokTok
#171309
Hi all,

I’m looking to change my design in terms of powering my PIC18F2580 (IC3 on my schematic below) from my MCP1703 +3.3v voltage regulator (IC2 on my schematic below) - see datasheet - http://ww1.microchip.com/downloads/en/D ... 22049e.pdf.
See my schematic screenshot
Currently I’m supplying power to my PIC18F2580 from my +5V LM7805 (IC1 on my schematic above). Based on my understanding of the “Absolute Maximum Ratings” PIC18F2580 datasheet it is possible to run the PIC at +3.3V i.e. Voltage on VDD with respect to VSS ......................................................................................................... -0.3V to +7.5V - see page 416 of the datasheet - http://ww1.microchip.com/downloads/en/D ... 39637b.pdf

The reason for the change is due to the feedback received from my board house whereby they mention by having two different supply voltages on my board I could have “unknown long term consequences”.

The reason for the change is due to the feedback received from my board house whereby they mention by having two different supply voltages on my board I could have “unknown long term consequences”.

What could these be? Could having two different supply voltages cause a problem with my Tx Rx interfacing between my ETRX357HR Zigbee module(IC7 on my schematic above) and my PIC18F2580.

As for the MCP1703 if I were to change my design this would mean directly providing the MCP1703 with incoming +12v supply voltage from my PSU which would be OK based on the MCP1703 “Absolute Maximum Ratings”.

Any help is appreciated.

Thanks,
TokTok.
You do not have the required permissions to view the files attached to this post.
#171321
I don't have experience with the PIC and the ZigBee but your setup is very similar to one I did using the Arduino and the RFM69 radio.

The problem is that the inputs on the 3.3v radio may not be 5v safe. (I say "may not be" because I have not read the datasheet for your radio, but that is likely the concern from your board house.) Connecting the radio to 5v logic levels will likely damage the radio inputs. You need to either add level converters or, as you propose, run the microcontroller at the same 3.3v.

Note that running the Arduino at 3.3v requires that you drop the clock to about 8 MHz to stay in spec (tho many report that it still runs 16 MHz just fine). The PIC may or may not behave similarly. Check this in your PIC datasheet.

Here is a link to my radio breakout board project if you would like to compare schematics.
https://github.com/uChip/Moteino_Periph ... alf-Shield
Scroll down a bit to see the schematic.

Good luck,
- Chip