SPI slave on LPC1343

[lpcware]

Content originally posted in LPCWare by jookie on Mon Apr 22 02:00:02 MST 2013

Hello,

just 2 quick (dumb) questions :)
- 14.7.5 SSP Clock Prescale Register -- 'In Slave mode, the SSP clock rate provided by the master must not exceed 1/12 of the SSP peripheral clock...' -- does this mean that when my LPC is running on 72 MHz, then maximum clock (SCK) in SPI slave mode can be 6 MHz? (~ 6 Mbit, 0.75 MB/s)

- does the SPI CS line have to toggle (go H and L) for every received byte in SPI slave mode? (found somewhere here, on forum).

Jookie

[lpcware]

Content originally posted in LPCWare by Rorrik on Tue Sep 23 12:30:08 MST 2014
Is this 1/12th rule still true of the LPC812? My master clock is only 100 kHz, but it seems like this might be my problem. How can make sure my peripheral clock is set high enough?

[lpcware]

Content originally posted in LPCWare by jookie on Mon Apr 22 06:56:06 MST 2013

Rolf,

thank you for reply, I somehow missed that CPHA=1 part in user manual. That CPHA=1 would help me, but that 6 MBit/s thing will force me to move to a different MCU, because I need to implement SPI slave for 16 MHz SCK. Too bad that I've hit this problem at the end of writing code - I've left the SPI part as last one, because I thought there can't be any problem, and I didn't check the manual soon enough :) That /12 rule would push my 16 MHz SCK to 192 MHz PCLK which is not possible with LPC1343, and it seems that any other LPC MCU doesn't go so high either...

Regards,

Jookie 

[lpcware]

Content originally posted in LPCWare by DF9DQ on Mon Apr 22 06:29:42 MST 2013

Hi Jookie,

6 MBit/s (=72 MHz / 12) is indeed the fastest clock speed for the SSP in slave mode.

The SSEL line must toggle between frames only in SPI modes 0 and 2, i.e. the modes where CPHA=0. This is enforced by the SSP in slave mode, and it will ignore the second and any further frame if you do not toggle SSEL in between.

SPI modes 1 and 3 (where CPHA = 1) can have SSEL continuosly low for multiple frames.

Regards,
Rolf