Compiler/AM3352: Using Timer to create square wave output

Richard_McAleer

Part Number: AM3352

Tool/software: TI C/C++ Compiler

I'm working on a custom board based around the beagle bone black and would like to generate a square wave from a specific pin to serve as a clock for a peripheral

that pins is GPMC_ADVN_ALE/TIMER4/GPIO2_2 and I would be looking for a frequency around 330kHz

I am wondering what would be the best way to control this?

I can see in my files system the folder /sys/kernel/debug/clk/timer4_fck but its not immediately obvious how I should be driving this

From my compiled device-tree (viewed with fdtdump) the relevant entries for timer 4 appear to be:

                          timer4_fck@510 {
                                #clock-cells = <0x00000000>;
                                compatible = "ti,mux-clock";
                                clocks = <0x00000016 0x00000009 0x00000015 0x00000000 0x00000000>;
                                reg = <0x00000510>;
                            };

                target-module@44000 {
                    compatible = "ti,sysc-omap4-timer", "ti,sysc";
                    ti,hwmods = "timer4";
                    reg = <0x00044000 0x00000004 0x00044010 0x00000004 0x00044014 0x00000004>;
                    reg-names = "rev", "sysc", "syss";
                    ti,sysc-mask = <0x00000001>;
                    ti,sysc-sidle = <0x00000000 0x00000001 0x00000002 0x00000003>;
                    clocks = <0x0000002f 0x00000050 0x00000000>;
                    clock-names = "fck";
                    #address-cells = <0x00000001>;
                    #size-cells = <0x00000001>;
                    ranges = <0x00000000 0x00044000 0x00001000>;
                    timer@0 {
                        compatible = "ti,am335x-timer";
                        reg = <0x00000000 0x00000400>;
                        interrupts = <0x0000005c>;
                        ti,timer-pwm;
                    };
                };

I would be very grateful for any suggestions as to how this might be done

Best regards,

- Richard

over 5 years ago

0 Nick Saulnier over 5 years ago

TI__Guru** 109440 points

Hello Richard,

There are multiple ways to get a PWM signal:
* the PWM subsystem (see our Linux PWM kernel driver documentation)
* bit-banging a PWM signal from PRU GPO pins
* use a timer (take a look at pwm-omap-dmtimer. Documentation under Documentation/devicetree/bindings/pwm/pwm-omap-dmtimer.txt, driver at drivers/pwm/pwm-omap-dmtimer.c).

We do not suggest using Linux to generate a PWM signal on a regular GPIO signal.

It sounds like you would probably want to look at the timer driver first. Note that TI did not write that driver or maintain it, so I will only be able to offer limited support if you run into challenges using it.

Regards,

Nick

0 Richard_McAleer over 5 years ago in reply to Nick Saulnier

Intellectual 460 points

Thanks for the pointers

as ever with this stuff it's finding out the intended driver that's the hard part

after adding a pwm entry to the device tree:

  pwm4: dmtimer-pwm@4 {
          compatible = "ti,omap-dmtimer-pwm";
          ti,timers = <&timer4>;
          #pwm-cells = <3>;
          pinctrl-names = "default";
          pinctrl-0 = <&timer4_pins>;
  };

and enabling the driver in my config

CONFIG_PWM_OMAP_DMTIMER=y

there is a directory in my rootfs /sys/class/pwm/pwmchip0

root@my-board:/sys/class/pwm/pwmchip0# ls -la
drwxr-xr-x    3 root     root             0 Jan  1 00:00 .
drwxr-xr-x    3 root     root             0 Jan  1 00:00 ..
lrwxrwxrwx    1 root     root             0 Jan  1 00:00 device -> ../../../dmtimer-pwm@4
--w-------    1 root     root          4096 Jan  1 00:00 export
-r--r--r--    1 root     root          4096 Jan  1 00:00 npwm
drwxr-xr-x    2 root     root             0 Jan  1 00:00 power
lrwxrwxrwx    1 root     root             0 Jan  1 00:00 subsystem -> ../../../../../class/pwm
-rw-r--r--    1 root     root          4096 Jan  1 00:00 uevent
--w-------    1 root     root          4096 Jan  1 00:00 unexport

after echo-ing 0 to export there appeared

root@my-board:/sys/class/pwm/pwmchip0/pwm0# ls -la
drwxr-xr-x    3 root     root             0 Jan  1 00:01 .
drwxr-xr-x    4 root     root             0 Jan  1 00:00 ..
-r--r--r--    1 root     root          4096 Jan  1 00:01 capture
-rw-r--r--    1 root     root          4096 Jan  1 00:01 duty_cycle
-rw-r--r--    1 root     root          4096 Jan  1 00:02 enable
-rw-r--r--    1 root     root          4096 Jan  1 00:01 period
-rw-r--r--    1 root     root          4096 Jan  1 00:01 polarity
drwxr-xr-x    2 root     root             0 Jan  1 00:01 power
-rw-r--r--    1 root     root          4096 Jan  1 00:01 uevent

using this was as simple as:

# echo 3000 > period

# echo 1500 > duty_cycle

# echo 1 > enable

giving me a square wave on the pin at 333kHz

Thanks again

- Richard

0 Nick Saulnier over 5 years ago in reply to Richard_McAleer

TI__Guru** 109440 points

Hello Richard,

Thanks for replying back with your solution! I'll make a note on my side to point future PWM customers to your thread.

Regards,

Nick

Processors

Processors forum

Compiler/AM3352: Using Timer to create square wave output