servo.c
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/*
* Copyright (C) 2014 Freie Universität Berlin
* Copyright (C) 2015 Eistec AB
*
* This file is subject to the terms and conditions of the GNU Lesser General
* Public License v2.1. See the file LICENSE in the top level directory for more
* details.
*/
/**
* @ingroup drivers_servo
* @{
*
* @file
* @brief Servo motor driver implementation
*
* @author Hauke Petersen <hauke.petersen@fu-berlin.de>
* @author Joakim Nohlgård <joakim.nohlgard@eistec.se>
*
* @}
*/
#include "servo.h"
#include "periph/pwm.h"
#include "timex.h" /* for US_PER_SEC */
#define ENABLE_DEBUG (0)
#include "debug.h"
#ifndef SERVO_FREQUENCY
#define SERVO_FREQUENCY (100U)
#endif
#ifndef SERVO_RESOLUTION
#define SERVO_RESOLUTION (US_PER_SEC / SERVO_FREQUENCY)
#endif
int servo_init(servo_t *dev, pwm_t pwm, int pwm_channel, unsigned int min, unsigned int max)
{
int actual_frequency;
actual_frequency = pwm_init(pwm, PWM_LEFT, SERVO_FREQUENCY, SERVO_RESOLUTION);
DEBUG("servo: requested %d hz, got %d hz\n", SERVO_FREQUENCY, actual_frequency);
if (actual_frequency < 0) {
/* PWM error */
return -1;
}
dev->device = pwm;
dev->channel = pwm_channel;
dev->min = min;
dev->max = max;
/* Compute scaling fractional */
/*
* The PWM pulse width can be written as:
*
* t = k / (f * r)
*
* where t is the pulse high time, k is the value set in the PWM peripheral,
* f is the frequency, and r is the resolution of the PWM module.
*
* define t0 as the desired pulse width:
*
* t0 = k0 / (f0 * r)
*
* where f0 is the requested frequency, k0 is the requested number of ticks.
* Introducing f1 as the closest achievable frequency and k1 as the set tick
* value yields:
*
* t1 = k1 / (f1 * r)
*
* setting t1 = t0 and substituting k1 = k0 * s yields:
*
* k0 / (f0 * r) = k0 * s / (f1 * r)
*
* solve for s:
*
* s = f1 / f0
*
* where s is the optimal scale factor to translate from requested position
* to actual hardware ticks.
*/
dev->scale_nom = actual_frequency;
dev->scale_den = SERVO_FREQUENCY;
return 0;
}
void servo_set(const servo_t *dev, unsigned int pos)
{
unsigned int raw_value;
if (pos > dev->max) {
pos = dev->max;
}
else if (pos < dev->min) {
pos = dev->min;
}
/* rescale value to match PWM peripheral configuration */
raw_value = (pos * dev->scale_nom) / dev->scale_den;
DEBUG("servo_set: pos %d -> raw %d\n", pos, raw_value);
pwm_set(dev->device, dev->channel, raw_value);
}