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8devices Community :: View topic - Carambola + slow SPI
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Carambola + slow SPI
https://www.8devices.com/community/viewtopic.php?f=1&t=79		 Page 1 of 1
Author:  jedas [ 06 Apr 2012, 05:50 ]
Post subject:  Carambola + slow SPI
Hi,

I would like to read data from slave SPI device, which is based on slow microcontroller, with maximum 250 kHz clock. I've managed to configure SPI interface at ~5 MHz clock or so, but spidev driver refuses to configure at low speed. As far as I've dig into driver, it checks for limit F_CPU / 128, which would be 320 MHz / 128 = 2.5 MHz. Is it possible to configure some kind of hardware pre-scaler for that, or maybe I should use some kind of bit banging SPI driver for such application?

j
Author:  Saulius [ 07 Apr 2012, 08:53 ]
Post subject:  Re: Carambola + slow SPI
I have not tried it yet, but if you tell more details (working code / register dump if you are using mmio to test it manual way) it would be clearer.
Author:  jedas [ 10 Apr 2012, 05:44 ]
Post subject:  Re: Carambola + slow SPI
Actually I'm using spidev_test utility from kernel source. My question would be if it's possible to configure SPI device below 2.5 MHz clock.

I've found similar device datasheet:
http://www.tracermcc.ru/foto/bender/RT3 ... 8_0902.pdf

SPICFG register, SPICLK bits, recifies SPI clock divide control, which minimum value is: rate / 128. So given the fact that system clock is 320 MHz, it isn't possible to achieve less than 2.5 MHz ?
Author:  gedass [ 10 Apr 2012, 11:02 ]
Post subject:  Re: Carambola + slow SPI
try to use software SPI for slow modes
Author:  Gerd [ 04 May 2012, 09:01 ]
Post subject:  Re: Carambola + slow SPI
System clock is 1/3 of the CPU CLK

320Mhz / 3 /128 = 833 kHz is the lowest speed on the hardware SPI
Author:  jedas [ 04 May 2012, 10:32 ]
Post subject:  Re: Carambola + slow SPI
Author:  Gerd [ 04 May 2012, 13:14 ]
Post subject:  Re: Carambola + slow SPI
thx jedas for this useful hint :-)
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