Slave PIC to be used like CD4021 chip

[Ioannis]

This is a little weird but, has anyone used a PIC as a slave device like the 4021 PISO (Parallel Input Serial Output)?

I am trying to use some 4021 as Digital inputs and also get some analog values (8) and all read by the master controller through a Shiftin command.

The idea is to use a 65 bit buffer, 64 bits for the 8x8bit analog values, and 1 bit for the input of the device, so other can connect on the bus, like another 4021 or another 8 analog input device.

My main problem is how to shift the 65 bit array (preferably not in assembly...).

Thanks,
Ioannis

[skimask]

I don't see why it wouldn't work...as long as the PIC was clocked fast enough for you...
I don't quite see what you're getting at as far as hardware though...
Are you going to use a large PIC to get 64/65 bits and then shift them out, or the other way around?
Could you draw up a rough block diagram?

[Charles Linquis]

I have done what you describe. The "shifting" was done using SEROUT2 and SERIN2. The "master" PIC sent out 4 bytes of data (in my case) and the "slave" took the bytes
and loaded them into a buffer. The data was followed by a checksum byte. If the checksum matched, the data was written to the port pins. I used a second wire between the two PICs to act as an ACK to indicate a successful trasmission (a pulse LOW was proof that the data got to the slave). The data rate was 19.2 kbaud. Crystals were used on both parts. I used open-collector drive on both lines and 2K pull-ups. The PICs were 6' or so apart.

[skimask]

So, this might be something like an 18F8723, grabbing 64 bits worth of digital data from the various ports and funneling it out the serial port? A 64bit, single chip PISO?

[Charles Linquis]

You apparently are aware of my "chip of choice"!

[skimask]

You apparently are aware of my "chip of choice"!

Started off with the 'broken' 8720, moved up along the way as they've came out.
http://web.ndak.net/jdgrotte/tqfp648...fpadapter.html
Use it a lot, especially when I need to write 'sloppy' code with a lot of debugging type info. I use the extra pins for LEDs, LCDs, switches, etc.
Later on, clean up the code, get it down to size and put it on a 40 pin version of the '8723 (i.e. 18F452/4620/4685, whatever). Have a few of a second version that'll allow the '8723 to plug into a 28-pin socket. Same principle applies.
Very few code changes required as long as I don't use an '8723 specific module or function.