A Hardware PWM implementation for Cubie and Allwinner A10

A Hardware PWM Implementation for Cubie and Allwinner A10

by David H. Wilkins - Aug-23-2013 - posted in Hardware, Linux, PWM

Cubieboard-1 - powered by the Allwinner A10 Arm SOC

When I received my first Allwinner A10 board it lacked support for hardware PWM. Without a driver for hardware PWM it’s very hard to use this very powerful SOC for controlling motors

This article describes the design of one such hardware pwm driver interface.

The Arduino has a simple to use PWM interface and other implementations should strive to have a similarly developer friendly interface. The “analogWrite” Arduino interface is both easy to understand and implement. For instance the following code implements a 50% duty cycle on digital pin 5:

1 2	pinMode(5,OUTPUT); // Declare pin 5 as output analogWrite(5,128); // Output a valud between 0 (off) and 255 (full on)

One problem with the Arduino interface is the learning curve required to venture beyond the primary interface. For instance, the above code doesn’t specify the PWM period. Many motors (servos..) function best within a specific PWM period.

This implementation doesn’t allow specification of the signal polarity either. If the hardware interface requires an inverted signal, then duty will have to be computed as 1/duty manually.

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void TimerOne::setPeriod(long microseconds) // AR modified for atomic access { long cycles = (F_CPU / 2000000) * microseconds; // the counter runs backwards after TOP, interrupt is at BOTTOM so divide microseconds by 2 if(cycles < RESOLUTION) clockSelectBits = _BV(CS10); // no prescale, full xtal else if((cycles >>= 3) < RESOLUTION) clockSelectBits = _BV(CS11); // prescale by /8 else if((cycles >>= 3) < RESOLUTION) clockSelectBits = _BV(CS11) | _BV(CS10); // prescale by /64 else if((cycles >>= 2) < RESOLUTION) clockSelectBits = _BV(CS12); // prescale by /256 else if((cycles >>= 2) < RESOLUTION) clockSelectBits = _BV(CS12) | _BV(CS10); // prescale by /1024 else cycles = RESOLUTION - 1, clockSelectBits = _BV(CS12) | _BV(CS10); // request was out of bounds, set as maximum oldSREG = SREG; cli(); // Disable interrupts for 16 bit register access ICR1 = pwmPeriod = cycles; // ICR1 is TOP in p & f correct pwm mode SREG = oldSREG; TCCR1B &= ~(_BV(CS10) | _BV(CS11) | _BV(CS12)); TCCR1B |= clockSelectBits; // reset clock select register, and starts the clock }

*example code from Timer One Library for Arduino

The Linux OS and expanded ram of the cubieboard make possible a much more rich interface than the arduino affords. File based programming interfaces are available to extend the PWM control to many more languages. One possible implementation might include the following interface (examples in shell script):

/sys/class/pwm-sunxi/pwmX

• This is the sysfs directory that contains the PWM interface. X is the PWM channel. The Allwinner A10 has 2

period (r/w)

• • period that makes up a cycle. Can be expressed as hz, khz, ms, or us. Whole numbers only. Examples: echo 10hz > /sys/class/pwm-sunxi/pwm0/periodecho 1khz > /sys/class/pwm-sunxi/pwm0/period
echo 100ms > /sys/class/pwm-sunxi/pwm0/period
echo 100us > /sys/class/pwm-sunxi/pwm0/period
echo 150khz > /sys/class/pwm-sunxi/pwm0/period
    

duty (r/w)

• • portion of the period above that is “active” or on. Same units as duty.
    echo 100ms > /sys/class/pwm-sunxi/pwm0/period # Period is 100 milliseconds
    echo 25ms > /sys/class/pwm-sunxi/pwm0/duty # 25% duty 
    echo 50ms > /sys/class/pwm-sunxi/pwm0/duty # 50% duty 
    echo 75ms > /sys/class/pwm-sunxi/pwm0/duty # 75% duty 
    

duty_percent (r/w)

• • duty expressed as a percentage. Whole numbers only
    echo 100ms > /sys/class/pwm-sunxi/pwm0/period # Period is 100 milliseconds
    echo 25 > /sys/class/pwm-sunxi/pwm0/duty_percent # 25% duty
    echo 50 > /sys/class/pwm-sunxi/pwm0/duty_percent # 50% duty
    echo 75 > /sys/class/pwm-sunxi/pwm0/duty_percent # 75% duty
    

polarity (r/w)

• • Polarity of the pin during the duty portion. Note that the opposite state will be for the non-duty portion.
    1 = high, 0 = low

pulse (r/w)

• Output one pulse at the specified period and duty
  echo 1 > /sys/class/pwm-sunxi/pwm0/puls # Output 1 pulse when run is 1
  

pin (r/o)

• Name of the A10 pin this pwm outputs on. This is hardwired and informational only. Example:
  cat /sys/class/pwm-sunxi/pwm0/pin # output: PB2
  

run (r/w)

• Enable the PWM with the previously set parameters. Example:
  echo 1 > /sys/class/pwm-sunxi/pwm0/run # Start the pwm interface
  echo 0 > /sys/class/pwm-sunxi/pwm0/run # Stop the pwm interface
  

So, the above interface as a Linux kernel module can implement a 50% duty cycle hardware pwm with the following code:

1 2	echo 50 > /sys/class/pwm-sunxi/pwm0/duty_percent echo 1 > /sys/class/pwm-sunxi/pwm0/run

Or a 25% duty cycle at 250hz with the following code:

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echo 250hz > /sys/class/pwm-sunxi/pwm0/period echo 50 > /sys/class/pwm-sunxi/pwm0/duty_percent echo 1 > /sys/class/pwm-sunxi/pwm0/run

Or a 25% duty cycle at 500hz with negative polarity with the following code:

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echo 500hz > /sys/class/pwm-sunxi/pwm0/period echo 50 > /sys/class/pwm-sunxi/pwm0/duty\_percent echo 0 > /sys/class/pwm-sunxi/pwm0/polarity echo 1 > /sys/class/pwm-sunxi/pwm0/run

So, the design goal of a simple interface is achieved, with incremental functionality requiring only incremental amounts of additional code.

Next article – developing the pwm-sunxi pwm module