using level translators for 5v PIC with 3v I2C

asmboy · Joined: 20 Nov 2007 Posts: 2128 Location: albany ny

has anybody had experience - willing to be shared- with mating 5V PICS with 3v I2C using the Philips PCA9509 ?

dyeatman · Joined: 06 Sep 2003 Posts: 1933 Location: Norman, OK

I have used 2n7000 FETs for years with no problems.
See this document:
http://www.adafruit.com/datasheets/an97055.pdf
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temtronic · Posted: Wed Mar 26, 2014 7:20 pm

Since there usually is a 3V version of most PICs, it's easier and cheaper to use the L series.
As well, some F series actually run fine at 3 volts, though maybe not at full speed.

hth
jay

languer · Joined: 09 Jan 2004 Posts: 144 Location: USA

Along the same lines of what dyeatman showed;

https://www.sparkfun.com/products/12009

https://www.sparkfun.com/products/11955

I've implemented the FET solution on some of my designs a few times with no issues. Sometimes a simple series resistor suffices. But in some cases I have +5V sensors which need to interface to a +3.3V system.

Ttelmah · Joined: 11 Mar 2010 Posts: 19506

I've used the P82B96 for this. Both dealing with the 'odd' occasion of having to mix voltages on one board, and when sending I2C at higher voltages over long distances. The PCA9509, is very similar, but without the high current drive capability, and high voltage support.
Have to agree though that in general now for 5v PIC to 3.3v I2C, I'd start by seeing if it was possible to switch the whole board to 3.3v. Many of the 3.3v PIC's, are faster, use less power, and are often cheaper than there older brethren. PIC's like the 18F26J50, with dual I2C busses (second with DMA on SPI), dual UART'e etc., make using the older chips look rather pointless.

Best Wishes

asmboy · Joined: 20 Nov 2007 Posts: 2128 Location: albany ny

all points well taken. as with many projects i am offered , this is an update of an existing product that is legacy 5V - with a new requirement to add a specific feature- in this case a LIS3LV02DL from STM - and the simplest way i can see is to use a level translator with the new device on its own slightly remote pcb-for robustness and confidence in the alteration.

thanks to all for the commentary Very Happy

temtronic · Posted: Thu Mar 27, 2014 7:38 am

Yes, maintaining 'legacy' devices can be fun...I can see 2 more 'minor updates' before you have to redo the whole PCB...newer PIC, couple more bells, a whistle,just one more I/O pin...it never ends.

cheers
Jay

Ttelmah · Joined: 11 Mar 2010 Posts: 19506

That makes total sense.

The translator chips are easy. Effectively two FET's, and the pull up resistors for the 3.3v part all in one package. VccB goes to your 5v. VccA to the 3.3v side. A connections to the 3.3v chip. B connections to the PIC (with external pull up resistors to 5v). EN pin pulled high.
The one I used, was specifically designed for really long interfaces, and really was as simple to use as anything.
Given the size of the surface mount part, it's a tidier solution than using the FET's, and has the advantage of better bus performance on the low voltage bus (active pull-ups rather than simple resistive). However the FET solution is cheaper if price is critical....

Best Wishes