Help to voltage regulator with communication

moryoav · Joined: 23 Feb 2014 Posts: 35

I'm using pic18f2553 and i want to connect a cellular module that work with 4V via rx-tx. I'd like to know if putting a step up/step down voltage regulator
between them will interfere and ruin the data transmission.

temtronic · Posted: Sat Apr 12, 2014 4:19 pm

You should post the cellular module part number or a link to its datasheet so we can look and see the specs. It's important to know if the interface is RS-232,TTL, or 'something' else.
4V seems odd to me, I assume all cell devices are '3 volt' dvices....if so, the quick and easy solution is to use the 'L' version of the PIC unless that PIC can operate at 3 volts. If you read the PIC data sheet there will be 'specs' and 'charts' covering the allowable operating voltages vs. speed. It is possible that PIC will run fine at '4 volts'.

hth
jay

moryoav · Joined: 23 Feb 2014 Posts: 35

I cannot afford to change the PIC unfortunately.
The cellular module is ADH8066, and I'm using the Sparkfun eval board for it.
Datasheet: http://www.scribd.com/doc/54829003/ADH8066
Eval board: https://www.sparkfun.com/products/retired/10164
Its datasheet says operation voltage ranges between 3.5 and 4.5 with a recommended voltage of 4V.
Hope that helps a little bit more.

temtronic · Posted: Sat Apr 12, 2014 7:26 pm

You've got the 5V PIC---3V peripheral problem. NOT the first one here having it either.

Using the 'L' version PIC would be the best solution....

You can use a 'logic level translator'. Sparkfun have them or you can make your own using FETs or a 'chip' designed for it.

Do a little 'googling' to see what other have done, what's cost effective for you,board layout, etc.

I understand you can't change the PIC which is too bad as the 'L' version is easier to do, less chance of errors in board layout.

hth
jay

moryoav · Joined: 23 Feb 2014 Posts: 35

leaving everything said aside, does putting a step up\down between the cellular module and the PIC will cause any problems in the data sent and received from both sides?

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

The recommended voltage supply for your unit is 4V, but internally this is regulated down to 3V. See table 2-4 of your modem's datasheet. So yes, you are having the common 3V to 5V problem and things are not going to work without you doing some extra work on the digital interface.

Short answer is that without precautions the 5V PIC voltage on the I/O pins will damage your modem. The other way around, the 3V output of the modem is not high enough to trigger the PIC inputs.
This problem has been discussed so many times on this forum that I don't want to go here into it again. Do a forum search.