spi clock in am335x

Ajith Kumsi

Hi, I am using /dev/spidev1.0 spi node and initializing the speed(SPICLK) as 100Khz in my application, but when I measure SPICLK using the Oscilloscope it is showing as 93.75KHz. It is not generating exactly 100KHz. I am using McSpi as a spi master. Please suggest me in this regard.

over 10 years ago

0 Biser Gatchev-XID over 10 years ago

TI__Guru**** 393215 points

Hi Ajith,

Am335X TRM Rev. J, section 24.4.1.8 is relevant. The McSPI functional clock is 48MHz. You need to divide it by 480 to get 100kHz. Instead it's divided by 512, which gives you exactly 93.75kHz. The default divisor values are power of 2. What you should do is set MCSPI_CHxCONF bit CLKG to 1. Then the divisor value will be a concatenation of
MCSPI_CHxCONF[CLKD] and MCSPI_CHCTRL[EXTCLK] values. See also section 24.4.1.10 bitfield EXTCLK. In summary you should set:

MCSPI_CHxCONF[CLKG] = 1

MCSPI_CHxCONF[CLKD] = 479

MCSPI_CHxCTRL[EXTCLK] = 0

0 Ajith Kumsi over 10 years ago in reply to Biser Gatchev-XID

Intellectual 380 points

Hi, Biser,

Thank you for the support. I am using /dev/spidev1.0 SPI driver. How can I configure/access below registers directly from my application. Please teach me this as I am new to TI processors

MCSPI_CHxCONF[CLKG] = 1

MCSPI_CHxCONF[CLKD] = 479

MCSPI_CHxCTRL[EXTCLK] = 0

0 Miroslav Kiradzhiyski XID over 10 years ago in reply to Ajith Kumsi

TI__Mastermind 25235 points

Ajith, read about mmap() usage in your user-space application.

Useful readings:

http://www.embedded.com/design/operating-systems/4401769/Device-drivers-in-user-space

http://lwn.net/Kernel/LDD3/

Best regards,
Miroslav

0 Ajith Kumsi over 10 years ago in reply to Miroslav Kiradzhiyski XID

Intellectual 380 points

Hi Biser,

Thank you for the support. I have one doubt regarding MCSPI_CHxCONF[CLKD] = 479 its hexa value is 1DF which is 12bit,but CKLD is a four bit and I have configured CLKD as 'F' and EXTCLK as 0. Please teach me how to put value of CKLD as 479.

Regards,
Ajith

0 Biser Gatchev-XID over 10 years ago in reply to Ajith Kumsi

TI__Guru**** 393215 points

Sorry, my explanation was not correct. Here is the correct one:

479 = 0x1DF

MCSPI_CHxCONF[CLKG] = 1

MCSPI_CHxCONF[CLKD] = 0xF

MCSPI_CHxCTRL[EXTCLK] = 0x1D

0 argeebee over 10 years ago in reply to Biser Gatchev-XID

Intellectual 835 points

Why not use the spidev driver to change the speed? The code looks something like:

fd = open("/dev/spidev1.0", O_RDWR);

ioctl(fd, SPI_IOC_WR_MAX_SPEED_HZ, speed);

See the linux doc on spidev

0 Ajith Kumsi over 10 years ago in reply to argeebee

Intellectual 380 points

Hi,

I am trying to configure these below registers manually during the boot time using command but it gets reset when we reset the board, can you suggest me on how to configure these registers. Where should I make these changes, should I make the changes in board-am335xevm.c Please help me as I am new to this.

MCSPI_CHxCONF[CLKG] = 1

MCSPI_CHxCONF[CLKD] = 0xF

MCSPI_CHxCTRL[EXTCLK] = 0x1D

0 argeebee over 10 years ago in reply to Ajith Kumsi

Intellectual 835 points

Option 1: In kernel space via board-am335xevm.c:

p_spi_base_addr = ioremap_nocache( < Mcspi address here >, SZ_4K);

iowrite32( < register value here >, (p_spi_base_addr + < config offset here > ));

Option 2: In userspace, compile a c program with the spidev call to change the speed and run it. For example, if your program is called my_spi_speed.c, to run it from the linux command: ./my_spi_speed.c

For the spidev API, see spidev documentation in your kernel source tree: Documentation/spi

There are also many examples on the web for spidev. Just search on spidev examples.

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spi clock in am335x