hpwm_avr_ocrnx

HPWM AVR OCRnx

Syntax:

     HPWM channel, frequency, duty cycle

Command Availability:

The HPWM command is available on Atmel AVR microcontrollers with an OCnx pin, and is compatible with the PIC HPWM command method. Due to the the unique way of AVR PWM implementation, and code efficiency, there are some notable differences in the HPWM initialization and its use.

This command supports the Fast PWM Mode and period registers for their respective devices. Typically Timer0 and Timer2 do not have a period register and the "A" channel is sacrificed to provide that function. Therefore, channel 1 and channel 6 will not be available, but are documented for possible future use. Some device Timers do not have an adjustable period register, so this command is not feasible (consult the datasheet).

Explanation:

The HPWM command sets up the hardware PWM module of the Atmel AVR microcontrollers to generate a PWM waveform of the given frequency and duty cycle. Once this command is called, the PWM will be emitted until the duty cycle parameter is written to zero.

If the need is just one particular frequency and duty cycle, one should use PWMOn and the constants PWM_Freq and PWM_Duty instead. PWMOn for the AVR is uniquely assigned to the OC0B pin, or channel 2. PWMOff will only shutdown the AVR HPWM channel 2.

channel described as 1, 2, 3,…​16 correspond to the pins OCR0A, OCR0B…​.OCR5C as detailed in the channel constant table. Channel 1 and channel 6 are not available.

frequency sets the frequency of the PWM output measured in Khz. The maximum value allowed is 255 KHz. In situations that do not require a specific PWM frequency, the PWM frequency should equal approximately 4 times the clock speed (GCB chipMHz) of the microcontroller (ie 63 KHz on a 16 MHz chip, 32 KHz on 8 MHz, 4 Khz on 1 MHz). This gives the best duty cycle resolution possible. Alternate frequencies with good duty cycle resolution are 1Khz, and 4Khz with chipMhz values of 16 and 8 respectively.

duty cycle specifies the desired duty cycle of the PWM signal, and ranges from 0 to 255 where 255 is 100% duty cycle. The AVR fast PWM mode has a small spike at the extreme setting of 0x00, on most devices, with each period register rollover. By using the HPWM command, and writing 0x00 to the duty cycle parameter, the PWM signal will shutdown completely and avoid the spike. The PWM signal can then be restarted again with a new HPWM command.

Note: Due to the AVR having a timer prescaler of just 1, 8, and 64; the AVR frequency and duty cycle resolution will be different from the PIC frequency and duty cycle resolution. The AVR HPWM parameters will likely need adjusting ,when substituted into an existing PIC program, and where accuracy is required.

HPWM Constants:

The AVR HPWM timer constants for channel number control are shown in the table below. Each timer constant needs to be defined for any one of the channels it controls.

Timer Constants	Controls	Options
AVRTC0	Specifies AVR TC0 associated with channel 1, and 2	Must be defined
AVRTC1	Specifies AVR TC1 associated with channel 3, 4 and 5 Channel 5 present on larger pinout devices	Must be defined
AVRTC2	Specifies AVR TC2 associated with channel 6, and 7	Must be defined
AVRTC3	Specifies AVR TC3 associated with channel 8, 9, and 10	Must be defined
AVRTC4	Specifies AVR TC4 associated with channel 11,12, and 13	Must be defined
AVRTC5	Specifies AVR TC5 associated with channel 14, 15, and 16	Must be defined

The Great Cow BASIC HPWM channel constants for output pin control are shown in the table below. Each HPWM channel used needs to be defined. The Port pin associated with each OCnx must be set to output.

Channel Constants	Controls	Options
AVRCHAN1	Specifies AVR HPWM channel 1 to the associated output pin OC0A OCR0A is used as period register and thus not available	N/A
AVRCHAN2	Specifies AVR HPWM channel 2 to the associated output pin OC0B	Must be defined
AVRCHAN3	Specifies AVR HPWM channel 3 to the associated output pin OC1A MUX’d with OC0A pin on some ATTiny’s	Must be defined
AVRCHAN4	Specifies AVR HPWM channel 4 to the associated output pin OC1B	Must be defined
AVRCHAN5	Specifies AVR HPWM channel 5 to the associated output pin OC1C On some larger pinout devices and MUX’d with OC0A pin	Must be defined
AVRCHAN6	Specifies AVR HPWM channel 6 to the associated output pin OC2A OCR2A is used as a period register and thus not available	N/A
AVRCHAN7	Specifies AVR HPWM channel 7 to the associated output pin OC2B	Must be defined
AVRCHAN8	Specifies AVR HPWM channel 8 to the associated output pin OC3A	Must be defined
AVRCHAN9	Specifies AVR HPWM channel 9 to the associated output pin OC3B	Must be defined
AVRCHAN10	Specifies AVR HPWM channel 9 to the associated output pin OC3C	Must be defined
AVRCHAN11	Specifies AVR HPWM channel 11 to the associated output pin OC4A	Must be defined
AVRCHAN12	Specifies AVR HPWM channel 12 to the associated output pin OC4B	Must be defined
AVRCHAN13	Specifies AVR HPWM channel 13 to the associated output pin OC4C	Must be defined
AVRCHAN14	Specifies AVR HPWM channel 14 to the associated output pin OC5A	Must be defined
AVRCHAN15	Specifies AVR HPWM channel 15 to the associated output pin OC5B	Must be defined
AVRCHAN16	Specifies AVR HPWM channel 16 to the associated output pin OC5C	Must be defined

Example:

        'Using HPWM command to alternate ramping leds with the UNO board
    #chip mega328,16

    '************pwm************************
    'Must define AVRTCx, AVRCHANx, and set OCnX pin dir to out

    #define AVRTC0    'Timer0
    #define AVRCHAN2
    dir PortD.5 Out   'OC0B, UNO pin 5

    #define AVRTC1    'Timer1
    #define AVRCHAN3
    #define AVRCHAN4
    dir PortB.1 out   'OC1A, UNO pin 9
    dir PortB.2 Out   'OC1B, UNO pin 10

    #define AVRTC2    'Timer2
    #define AVRCHAN7
    dir PortD.3 Out   'OC2B, UNO pin 3

    do

    '63khz works good with 16MHz
    '32khz with 8MHz intosc
    '4KHz with 8MHz intosc and ckDiv8 fuse
    freq = 63
      For PWMled1 = 0 to 255
        HPWM 2,freq,PWMled1
        PWMled2 = NOT PWMled1
        HPWM 3,freq,PWMled2
        HPWM 4,freq,PWMled2
        HPWM 7,freq,PWMled1
        wait 5 ms
      Next

      For PWMled1 = 255 to 0
        HPWM 2,freq,PWMled1
        PWMled2 = NOT PWMled1
        HPWM 3,freq,PWMled2
        HPWM 4,freq,PWMled2
        HPWM 7,freq,PWMled1
        wait 5 ms
      Next

    loop