Why is my zero SPI clock frequency different than requested?

Question

Using the SPIdev, I have setup the SPI clock frequency, but measuring the real signal frequency using oscilloscope shows, it differs extremely to the expectations. How is this possible?

spi_fd = open(device, O_WRONLY);
ret = ioctl(spi_fd, SPI_IOC_WR_MAX_SPEED_HZ, &speed_hz);

EDIT:

uname -a
Linux zero2 4.9.22+ #987 Fri Apr 14 23:13:48 BST 2017 armv6l GNU/Linux

lsmod | grep spi
spidev                  7034  0
spi_bcm2835             7424  0

cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq
700000
cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_max_freq
1000000

sharpener · Answer 1 · 2017-04-17T15:31:18.740

There are at least following reasons for this:

Considering the ioctl name, looks like the speed is only guaranteed not to be greater than the value requested.
Due to some confusings in the Broadcom datasheets, the older SPI driver spi-bcm2708 calculated the clock multiplier to be power of two instead of multiply of two. So the range of possible values was strongly reduced. This should be ok in the current driver spi-bcm2835. Check the current module loaded via lsmod.

Not sure why, but the SPI clock depends on the CPU clock. At least on my raspbian system, using the spi-bcm2835 driver, the real clock frequency meets the expectations on full speed only (1GHz). At the default idle speed (700MHz) the SPI clock is lower (e.g. 12.5MHz real for 20MHz request). Check/change the current CPU speed settings:

cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor                #current policy
echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor #max performance

cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_min_freq    #min CPU clock frequency
cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq    #current CPU clock frequency
cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_max_freq    #max CPU clock frequency

EDIT:

After some more research, regarding the overclocking, following parameters in /boot/config.txt affect the SPI clock. Using the values provided the real timing meets the expectations.

arm_freq=700
arm_freq_min=700   #effectively disable scaling
core_freq=200      #not sure about exact meaning, but this allows precise SPI clock setup
core_freq_min=200

There is a tool able to determine either configured or real clock values of the overclocking parameters:

vcgencmd get_config arm_freq
vcgencmd get_config core_freq
vcgencmd measure_clock arm
vcgencmd measure_clock core

So the overclocked system in full performance mode (correct SPI setting) is 1000(arm)/400(core)MHz.

Nice summary for raspberry pi config options

Why is my zero SPI clock frequency different than requested?

1 Answers1