PWM Usage

The AIO-1688JD4 development board only exposes one PWM (FAN)
externally, but the Core-1688JD4's gold fingers expose multiple PWMs.
By running PMW read/write commands in the console or writing PWM
read/write programs in kernel or user mode, you can perform input and
output operations on the PWM.

PWM operates at a fixed frequency clock of 100MHz, with a total of 20
channels, each of which can be controlled individually.

The BM1688 has 5 PWM IPs (pwmchip0/ pwmchip4/ pwmchip8/ pwmchip12/
pwmchip16), each controlling 4 channels, allowing for a total of 20
channels.

In the Linux sysfs, the device nodes for pwm0~pwm3 are as follows:

   /sys/class/pwm/pwmchip0/pwm0~3

In the Linux sysfs, the device nodes for pwm4~pwm7 are as follows:

   /sys/class/pwm/pwmchip4/pwm0~3

Taking the fan's device tree configuration as an example:

   &fan0 {
   		compatible = "pwm-fan";
   		#cooling-cells = <2>;
   		pwms = <&pwm2 3 1000000 0>; 
   		cooling-levels = <255 170 100 1>; // max 1
   };

Here, "pwms = <&pwm2 3 1000000 0>;" indicates the use of the "PWM11"
pin, as "pwm2" is the third group, and "3" indicates the fourth
channel, leading to the conclusion: "3 x 4 = 12(0~11)"

And so on.

Operation Example

Example of PWM Operation Commands:

Use the echo command on the control panel to configure the PWM number
to be operated on, in this case PWM1:

   echo 1 > /sys/class/pwm/pwmchip0/export

Set the duration of one PWM cycle, in nanoseconds:

   echo 1000000 >/sys/class/pwm/pwmchip0/pwm1/period

Set the "ON" time within one cycle, in nanoseconds, i.e., duty cycle
=duty_cycle/period=50%:

   echo 500000 >/sys/class/pwm/pwmchip0/pwm1/duty_cycle

Enable the PWM:

   echo 1 >/sys/class/pwm/pwmchip0/pwm1/enable

Example of File IO Operation Program:

Configure the PWM number to be operated on, taking PWM1 as an example:

   fd = open("/sys/class/pwm/pwmchip0/export", O_WRONLY);
   if(fd < 0)
   {
   dbmsg("open export error\n");
   return -1;
   }
   ret = write(fd, "1", strlen("0"));
   if(ret < 0)
   {
   dbmsg("Export pwm1 error\n");
   return -1;
   }

Set the duration of one PWM cycle, in ns:

   fd_period = open("/sys/class/pwm/pwmchip0/pwm1/period", O_RDWR);
   ret = write(fd_period, "1000000", strlen("1000000"));
   if(ret < 0)
   {
   dbmsg("Set period error\n");
   return -1;
   }

Set the "ON" time within one cycle, in ns: (In this example, the duty
cycle is 50%)

   fd_duty = open("/sys/class/pwm/pwmchip0/pwm1/duty_cycle", O_RDWR);
   ret = write(fd_duty, "500000", strlen("500000"));
   if(ret < 0)
   {
   dbmsg("Set period error\n");
   return -1;
   }

Enable PWM

   fd_enable = open("/sys/class/pwm/pwmchip0/pwm1/enable", O_RDWR);
   ret = write(fd_enable, "1", strlen("1"));
   if(ret < 0)
   {
   dbmsg("enable pwm0 error\n");
   return -1;
   }