pic16f877a spi

[Tom Jetland]

Hi, just a quickie,

Am I correct in thinking that if two PICs are connected together via spi (one master, the other slave, using SS line), that both of the following examples would be compatible: -

[PCM programmer]

See ckielstra's table of SPI modes:
http://www.ccsinfo.com/forum/viewtopic.php?t=47444&start=3

Figure 2 shows the sampling edge and the idle state for SCLK in each mode:
http://www.totalphase.com/support/kb/10045/#modes

Finally, see this Microchip slide show tutorial on SPI:
http://ww1.microchip.com/downloads/en/devicedoc/spi.pdf

[Tom Jetland]

Thanks PCM programmer. I'll have a look at this lot.

It's weird how I can only get the spi to work using the first example code block I specified, and not the second. My suspicion lies with the pic16f877A though, as microchip has a whole errata document on spi for this chip, clarifiying certain things, i.e.

http://ww1.microchip.com/downloads/en/DeviceDoc/80131e.pdf

If you can spare the time, please have a look at the 2. Module: MSSP (SPI Slave Mode) on page 2 and see if you can work out what they're talking about?

Thanks,

Tom

[Ttelmah]

The only one that will cause you problems here, is the level. Basically in your slave code, you need to monitor the SCK pin (by using n input on the pin), and only enable the SPI _after_ the pin goes to the idle state. Obviously for a low idle state, there is a probability the line will be low when the chip wakes, but for the high idle state, you will need to wait, or the slave won't work.

Best Wishes

[Tom Jetland]

Thanks guys,

I do sometimes struggle to understand some of the microchip datasheets, so it's great to get a second opinion.

:-)

Tom