On Thu, 16 Jul 1998, Steven Kosmerchock wrote:

> Friends,
>
> I am working on a project where I am using (2) 16C64A's and (1) 16F84. I
> have 1 of the 16C64A's going to the LCD's 10 data lines, the 16F84
> shares these lines but are off (+5V Physically isolated) at different
> times. For some reason the 16F84 is interfering with the 16C64A when it
> (16C64A) is powered up. The 16C64A DOES NOT interfere with the 16F84
> when it (16F84) is powered up. Any suggestions would be GREATLY
> appreciated! TIA
>
>                 Best regards,
>                 Steven


  I think that you will need some sort of switch to isolate the downed
chip, otherwise the input protection diodes will open on the downed chip
and supply power to it (or just load the transmitting chip's output until
the logic level is out of spec). There are several methods to do this, the
simplest being a wired OR with D's and R's. The second simplest method, is
to choose a PIC that has OD outputs and use those with a wired OR
configuration (such as RA4 on some PICs), with a resistor to '+' at the
consumer (the LCD in your case). Other types of switches include
transistors (bipolar or MOS) and MOS switch arrays (even a CD4066 will do
it).

  As a rule of thumb, all active switches that are not open drain or open
collector, MUST be installed and controlled at the target (which is
guaranteed to be powered in evey configuration).


Peter