Advice on PIC to PIC comms.

Hi all:
I need to create a data bus system to have the bus carrying up to 8 bytes of data which is updated and transmitted sequentially by a master MCU 16f886.

The data can be updated every 20 msec, this is flexible.

The slave PIc's (up to 8 of them) (also 16F886's) must receive their single data byte that is individually flagged . Each slave uses 1 of the 8 data bytes (8bit).

The 8 data bytes can be repetitively transmitted from the Master PIC, every 20 msec.
The master PIC is already running a code loop that cycles faster than 1msec.

The less data wires used the better.

thanks for any advice.

First protocol that comes to mind is I2c (I²C - Wikipedia, the free encyclopedia), you just described a textbook I2c scenario. Next thought is SPI. Both should handle the task well.

I would use the USART.

You can transmit to multiple receivers, and it would all be one one wire. Each slave would receive all the data and only use its own byte.

If you need the slaves to transmit, you need a pull-up on the return line and you need to disable the slave transmitters unless they are actually transmitting.

IMO a synchronous bus like SPI or I2C is better than the USART. There's also the pin happy PSP bus.
At 20ms the I2C bus would be best and the least amount of I/O (2 pins)

How does USART, SPI,I2C stack up vs pins required, speed & overheads?

SPI is the fastest and has little overhead.

Blue, would u have a SPI link with some asm code and a circuit diagram?