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Lpc2000

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Max speed of SPI port for SD/MMC?

2006-01-19 by Ed Schlunder

I'm a little confused at what speeds can SD/MMC cards be accessed
using SPI on the LPC2103. Normal SD/MMC cards support max 25MHz for SD
and 20MHz for MMC.

If my LPC2103 is clocked at 70MHz, APB divider set to 1, then it looks
like the maximum speed I can get out of SPI0 is by setting the S0SPCCR
SPI clock counter register to it's minimum value of 8. 70MHz/8 =
8.75MHz, which seems pretty slow.

SPI1 is more complicated. Set APB divider to 1, SSPCPSR clock prescale
register to minimum value of 2 and SSPCR0.SCR to 0. This gives 70MHz /
(2 * (0+1)) = 35MHz, which is too fast. The next best setting would
then be SSPCPSR to 4 (only even values allowed), giving 70MHz / (4 *
(0+1)) = 17.5MHz.

If I wanted to hit the magic speed of 25MHz for SD Cards, I would have
to run the LPC2103 clocked at 50MHz. Then SSPCPSR set to 2 would
provide 50MHz / (2 * (0+1)) = 25MHz.

From what I understand, SPI1 also provides 8 byte FIFOs for
send/receive. It sounds like I need to use the SPI1 port, and forget
about the SPI0 port.

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