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PowerPC 8272 SPI

I was wondering if anyone had experience with the powerpc 8272 involving the use of the SPI port. I was reading in the manual and I quote the following in chapter 35 on the SPI hardware:

"The maximum sustained data rate that the SPI supports is SYSTEMCLK/50. However, the SPI can transfer a single character at much higher rates?SYSTEMCLK/4 in master mode and SYSTEMCLK/2 in slave mode. Gaps should be inserted between multiple characters to keep from exceeding the maximum sustained data rate."

This would indicate that with a 100MHz system clock you could at most support 2MHz sustained transfers. The manual doesn't describe very well what this means. The beginning of this chapter mentions speeds of slave mode up to 50MHz with the SPI and this seems a bit misleading. Can anyone explain what it means by "sustained" and why this limit exists. I can't figure it out from the manual and I was hoping someone with experience with this processor might have a quick explanation

Reply to
Don
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The FIFO buffers in the SPI (receive and transmit) are only two characters in depth. The SPI uses RX and TX buffers in the 16 Kbyte dual-port RAM to fill or empty the FIFOs. The SPI contends with a number of other peripherals for access to the dual-port RAM.

Chapter 13 of the Reference Manual explains the Communications Processor Module (CPM) and shows the relationship between it and the ten communications controllers, one of which is the SPI.

~Dave~

Reply to
Dave

I understand the character buffering is only two deep but since it uses the dual port RAM to buffer these incoming characters, I don't understand the limit imposed (aka the "sustained" comment) as long as you can clean out the circular buffers in the dual port RAM quickly enough. We won't be sharing the SPI bus with any of the other CPM hardware (ethernet, I2C, etc). I guess it just leaves the possibility of actually coding and empirical study, rather than being lucky enough for someone here having experienced this problem. The manual is vague as to this limit, I would have thought they (freescale) would have mentioned the limitations of their own device, rather than putting up some (almost) useless burst speed information. It's kind of like when speaker manufacturers highlight peak power and hide the RMS in small print.

Thanks for your comment!

Reply to
Don

A long time ago in a galaxy far, far away .. there was this 68360 ...

Reply to
Dave

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