On 18 May 00, at 15:22, David Duffy wrote: > At 11:55 5/17/00 +0100, you wrote: > >In-Reply-To: <4.3.1.2.20000517171853.00a79180@192.168.0.100> > > > >David Duffy wrote: > > > Hi All, > > > This is my 1st time using 3v3 devices (ISD5008 voice chip) and was > > > wondering about the best way to interface the PIC to it. (5v supply) > > Brian Gregory wrote; > >Use an open drain output type idea. > > > >When you have an output from the PIC (5V) to the voice chip (3V) put a > >pull up resistor to 3V and instead of driving the output high for logic > >one just set it to high impedance with the TRIS register. > >The other way round an output from the voice chip into a PIC input > >should work as is because the PIC will take 3V as logic one anyway. > > Hmmm... Seems like a good idea but... > Does the PIC input being held at 3V (near 5V mid-point) pose a problem? > If it's not a schmitt trigger input, I thought CMOS inputs are unhappy > about this. > Regards... It's the best ideea, I don't know how I forgot ... PIC will see as high level anything up to ~1,8V...2V . And a CMOS have thresolds at 1/3 and 2/3 from VDD so 2/3 from 5V is 3.3V that's the reason why CMOS are unhappy when is supplied at 5V and interfaced with a signal coming from a 3.3V circuit... and not because of schmitt trigger inputs... But a good (standard) CMOS chip will work either at 3V (with frequency limitation ) so no problem... Vasile **************************************** Surducan Vasile, engineer email:vasile@l30.itim-cj.ro URL:http://www.geocities.com/vsurducan ****************************************