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InputPulseWidth.md

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Overview

This page describes the configuration of RC servo pulse width input interface.

Wiring

The pulse signal is fed to PPM pin that must be 5v-tolerant.

	        +-----------------> +5v
	        |   +-------------< (pulse)
	        |   |   +---------> GND
	        |   |   |
	+-------+---+---+-----------------------------+
	|      +5v PPM GND                            |

WARNING: Refer to your hardware manual or look into src/hal/hw/... directory to find out actual pin mapping and voltage levels of your port.

Configuration

First you need to enable the appropriate mode of the PPM interface in HAL.

(pmc) reg hal.PPM_mode 1

Now you can see how the controller receives the control signal. If variable ap.ppm_FREQ is nonzero then pulse is caught. Use these registers to view pulse parameters.

(pmc) reg ap.ppm_PULSE
(pmc) reg ap.ppm_FREQ

Select the pulse width range in which you want to operate. Write the range to a PPM configuration or leave defaults. We use three point conversion from pulse width to the control value.

(pmc) reg ap.ppm_range0 <ms>
(pmc) reg ap.ppm_range1 <ms>
(pmc) reg ap.ppm_range2 <ms>

Choose what parameter you want to control. You can choose any of the registers available for writing. By default the pm.s_setpoint_speed_knob register is selected that mapped on current or speed percentage depending on control loop enabled.

(pmc) reg ap.ppm_reg_ID <reg>

Note that setting the control variable does not enable appropriate control loop automatically. You can select control loop if you need.

(pmc) reg pm.config_LU_DRIVE 2

Select the control variable range. So the input pulse width range will be converted to this control range.

(pmc) reg ap.ppm_control0 <x>
(pmc) reg ap.ppm_control1 <x>
(pmc) reg ap.ppm_control2 <x>

Now enable the machine startup control. The condition to start is the transition of pulse signal to the operating range from the low side.

(pmc) reg ap.ppm_STARTUP 1

When you change the configuration we recommend to disconnect pulse signal for safety.

Precision

You can change the timebase frequency of the timer that is used to measure the pulse width. By increasing the resolution you increase a minimum pulse rate that can be captured.

(pmc) reg hal.PPM_frequency <Hz>

The minimum pulse frequency is determined from the expression. Default timebase allows you to capture pulses from 31 Hz.

       frequency
min = -----------
         65536

Maximal timebase allowed is 84000000 Hz that gives a resolution about 12 ns with 1282 Hz minimal pulse frequency.

Disarm timeout

To ensure a safe startup it is required to hold low pulse signal for ap.timeout_DISARM milliseconds until disarmed state was reset.

(pmc) reg ap.timeout_DISARM <ms>