baudrate config changes only after starting x

Question

I've changed an i2c config file so current baudrate is 300000. I do a reboot and start an i2c baudrate checking program. sudo ./checkbr But what I get seems to be usual baudrate settings(default). Then I do a startx and load a gui. I open a terminal and make same steps. After sudo ./checkbr I'm getting desired baudrate = 300000. Why can't I get 300000 without x server loaded? I've also tried removing i2c_bcm2708 and re-adding it with baudrate = 300000 but still no results unless I do startx. If it's important, I do check a baudrate with a c++ piece of code which reads data from i2c in a loop and measures time needed to read it. In case of X server off I get 3x slower read speed than with a X started.

EDIT: Something may be wrong with my code so here it goes.

  for (;;) {
        I2CInterface.readRegister(MPU6050_ADDRESS, MPU6050_RA_ACCEL_XOUT_H, 14, data);
        oldTime = curTime;
        clock_gettime(CLOCK_REALTIME, &curTime);
        dt = ( (static_cast<int64_t>(curTime.tv_sec)*1000000000 + static_cast<int64_t>     (curTime.tv_nsec)) - (static_cast<int64_t>(oldTime.tv_sec)*1000000000 + 
        static_cast<int64_t>(oldTime.tv_nsec)) ) /1000000000.0;
        printf( "%7f \n", dt);
   }

I2CInterface.readRegister last 2 parameters are number of bytes and where to store it.

Answer 1

My Pi i2c was working nicely. i2c baudrate as defined in /etc/modprobe.d/i2c.conf

With the newest upgrade with new kernel (using Device Tree), the problems started. Initially nothing worked. After googling I added "dtparam=i2c=on" to /boot/config.txt. The i2c-bus started working, but with default speed.

with "dtparam=i2c=on, i2c_baudrate=400000" in /boot/config.txt allowed me to increase the baudrate to 400k.

I am running my Pi as headless, so I do not know about any startx etc.

see:

http://www.raspberrypi.org/forums/viewtopic.php?t=97314

https://github.com/raspberrypi/firmware/blob/master/boot/overlays/README

Answer 2

I guess the reason for the observed differences in I2C speed are due to the different core frequencies of the Raspi 3B+ under idle and load conditions. When you set the baudrate to 400000, you will get only 250000 under idle conditions, where the core_freq is 250 MHz. Under load conditions, which may be invoked by starting the x-server or watching a Live web cam or a youtube video, the core_freq is switched to 400 MHz and you see the 400000 baud rate. For constant baudrate, you will have to set core_freq_min to the same value as core_freq.

Regards Kai