rpi-PWM

linux pwm driver using raspberry pi pin 18 - expansion header pin 12

This is a raspberry pi linux device driver using the linux pwm subsystem (I've tested on linux kernel 3.11, 3.13 and 3.14)

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Installation steps

1 - modify arch/arm/mach-bcm2708/bcm2708.c this is on the linux 3.14 source

line 222:

static struct resource bcm2835_pwm_resources[] = {
	{
	 .start = BCM2708_PERI_BASE,
	 .end = BCM2708_PERI_BASE + SZ_4K - 1,
	 .flags = IORESOURCE_MEM,
	 }
};

static struct platform_device pwm_bcm2835_device = {
	.name = "pwm-bcm2835",
	.id = 1,
	.resource = bcm2835_pwm_resources,
	.num_resources = ARRAY_SIZE(bcm2835_pwm_resources),
};

line 864:

    bcm_register_device(&pwm_bcm2835_device);

2 - add the pwm source to the pwm directory

git am < pwm-bcm2835.patch

3 - select the kernel driver

make ARCH=arm menuconfig
	> Device Drivers > [*] Pulse-Width Modulation (PWM) Support
	> Device Drivers > [*] Pulse-Width Modulation (PWM) Support > <*> BCM2835 PWM support 

4 - recompile everything

Using the driver

Export the pwm channel

echo 0 > /sys/class/pwm/pwmchip0/export			
cd /sys/class/pwm/pwmchip0/pwm0/

Set the duty cycle and period

!!!(minimum period is 1us)

echo 100000 > duty_cycle	//duty cycle is 100us
echo 30000  > period		//period of 30 us
echo 1 > enable

Inverse polarity

echo 0 > enable			//disable the pwm channel
echo inversed > polarity
echo 1 > enable			//enable the pwm channel

Back to normal mode

echo 0 > enable
echo normal > polarity
echo 1 > enable

Some additional information

This driver doesn't change the pin configuration of the pin and doesn't change the pwm clock divider. You can use the pinmux and the clock subsystem of the linux kernel. If you want to do is quick and durty, you can use the /dev/mem implementation of http://www.airspayce.com/mikem/bcm2835/ with this 2 lines (1Mhz pwm clock)

bcm2835_gpio_fsel(PIN, BCM2835_GPIO_FSEL_ALT5);
bcm2835_pwm_set_clock(19);