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I am interfacing a 32 GB microSD card with PIC32MX795F512L. I am following the examples provided in Lucio de Jasio's book.

As the author has described that to set a 250 kHz, they are driving their peripheral clock frequency by 144. Which means 36 MHz when divided by 144 gives 250 kHz. But then why are they setting the SPI1BRG = 71;? (SPI1BRG = SPI baudrate register.)

I am using 44 MHz as a peripheral clock frequency. What value should I use?

I am really confused in setting the clock frequency for the SD card.

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They probably set it to 71 as the relationship between the register value and the divider value is probably divider = (BRG register value + 1)*2. In this case, (71+1)*2=144. You just need to calculate the divider value, divide by two, and subtract 1.

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  • \$\begingroup\$ what is the importance of 144.? I didn't get it properly. Please point me to any document or explain more. \$\endgroup\$ – user007 Dec 22 '15 at 10:01
  • \$\begingroup\$ You said it yourself in your question...? \$\endgroup\$ – alex.forencich Dec 22 '15 at 10:04
  • \$\begingroup\$ Thanks I got. I am using 44MHZ that means dividing it by 176 will give 250KHZ. So the value I have to put in register is 87 (87+1)*2=176 Is this correct.? \$\endgroup\$ – user007 Dec 22 '15 at 10:08
  • \$\begingroup\$ Looks right to me. It also passes the basic sanity check that since you're using a faster clock, the divider value should also be larger. \$\endgroup\$ – alex.forencich Dec 22 '15 at 10:11
  • \$\begingroup\$ Where did you find this equation divider = (BRG register value + 1)*2 \$\endgroup\$ – user007 Dec 22 '15 at 10:13

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