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| <h1 id="hal_pwm">hal_pwm</h1> |
| <p>The hardware independent interface to Pulse Width Modulators</p> |
| <h3 id="description">Description</h3> |
| <p>Pulse Width Modulators (PWMs) are hardware devices that output digital waveforms |
| with programmable duty cycle and frequency. They use a digital waveform |
| but can contiuously adjust the amount of power delivered to their load via |
| duty cycle. They are often used to create analog waveforms via low pass filter, |
| drive LEDs at various intensities, and control DC or servo motors.</p> |
| <p>For a description of PSM, see <a href="https://en.wikipedia.org/wiki/Pulse-width_modulation">wikipedia</a></p> |
| <h3 id="definition">Definition</h3> |
| <p><a href="https://github.com/apache/incubator-mynewt-larva/blob/master/hw/hal/include/hal/hal_pwm.h">hal_pwm.h</a></p> |
| <h3 id="hal_pwm-theory-of-operation">HAL_PWM Theory Of Operation</h3> |
| <p>The HAL_PWM interface was designed with some specific use cases in mind. It |
| was also designed to exploit a subset of the functionality provided by |
| common PWM controllers.</p> |
| <p>Fundamentally, a PWM has at it root an N-bit COUNTER, a CLOCK source, one or |
| more CAPTURE registers, and sometimes a TOP register.</p> |
| <p>The clock source feeds the COUNTER register which is continuously counting |
| (up or down), When the COUNTER is less than the value of the CAPTURE |
| register, the PWM outputs a logic HIGH. When the counter is greater than |
| the CAPTURE register the PWM outputs logic LOW. When the counter reaches the TOP |
| register, it wraps back to zero. If the PWM has no TOP register, it simply |
| wraps back to zero after reaching 2^N-1.</p> |
| <p>The HAL_PWM abstracts this implementation and provides methods to set the |
| PWM frequency and duty cycle. The frequency is the rate at which the counter |
| wraps back to zero. The duty cycle is the fraction of time the PWM is in |
| logic HIGH state.</p> |
| <p>The HAL_PWM API provides a few methods to get information on the PWM.</p> |
| <table> |
| <thead> |
| <tr> |
| <th><strong>Method Name</strong></th> |
| <th>** Description **</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td>hal_pwm_get_source_clock_freq</td> |
| <td>gets the frequency of the CLOCK that is driving the COUNTER</td> |
| </tr> |
| <tr> |
| <td>hal_pwm_get_resolution_bits</td> |
| <td>gets the number of bits in the COUNTER</td> |
| </tr> |
| </tbody> |
| </table> |
| <p>To use a PWM via HAL_PWM, follow these simple steps.</p> |
| <p>1) initialize a PWM using <code>hal_pwm_init</code> -- this binds the driver to the device |
| for the specific PWM you are using |
| 2) Optionally set the frequency of the PWM using <code>hal_pwm_set_frequency</code> |
| -- PWMs that support a TOP register have fine control over their frequency |
| settings. If this command is not supported by the PWM it will return an error |
| and its likely that there is no TOP register; the frequency resolution is not |
| finely adjustable. See the BSP for coarse adjustment of the PWM frequency.<br /> |
| 3) Enable the PWM to output a specific duty cycle using the |
| <code>hal_pwm_enable_duty_cycle</code> API</p> |
| <p>To change the duty cycle dynamically, just issue another |
| <code>hal_pwm_enable_duty_cycle</code> command.</p> |
| <p>To disable the PWM waveform, use <code>hal_pwm_disable</code>.</p> |
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