#use spi() and changing SPI clock speed?

newguy · Joined: 24 Jun 2004 Posts: 1908

I need to clock a peripheral slowly until it is configured, and then I can increase the speed. I've tried changing the SPI clock speed by having two different #use spi(); first the slow one, clock out some data, then the fast one, and clock out different data. My SPI clock speed isn't changing. In the past I'd use the setup_spi() function to change speeds but I was under the impression that it's no longer supposed to be used when/if using #use spi() and spi_xfer()?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

See the spi_speed() function in the CCS manual. It's for use with #use spi().

newguy · Joined: 24 Jun 2004 Posts: 1908

That didn't work. I'm using the brand-spanking-new 5.076. Interestingly the help file has a spelling error in the spi_init() entry. It repeatedly refers to a spi_inspi_init() function that doesn't exist. Must be a typo because spi_init() works.

Anyway, I answered my own question and in case anyone else finds themselves in the same boat, this works:

Ttelmah · Joined: 11 Mar 2010 Posts: 19535

Er. You do realise you are trying to specify 10MHz, and 17.5MHz....

It'll go as fast as possible, so I doubt if it'll change much.

The chip supports up to 15Mhz (depending on your clock rate), _half duplex only_. For standard full duplex operation, 10MHz max.

newguy · Joined: 24 Jun 2004 Posts: 1908

Yep, 10 & 17.5MHz, aye. I'm running the chip @ 140MHz (70 MIPS) so 70 / 4 = 17.5 MHz, which is the default when you don't specify a speed/baud in the #use spi() set up.

Didn't see the note regarding 100ns minimum clock period. I'll adjust accordingly. Thanks for that.

For the clock speed change test I wasn't concerned about what the clock actually was exactly; what concerned me more was the fact that I couldn't change it using the built-in functions that should have allowed it to change. I set up the MSSP to use the max rate first, measured it with a scope, then changed the rate in the #use spi() and repeated. Then I decided to switch to "let's try to change this on the fly" and hit a wall.

Ttelmah · Joined: 11 Mar 2010 Posts: 19535

It's in the timing characteristics Table 30-43.

I ran into this, when using a 'non standard' prototype chip from MicroChip. It supported a clock rate on SPI faster than the 'off the shelf' chips. I had to use the device editor to allow it to work (or manually code), since the compiler (correctly) blocked rates above the data sheet max....