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*
* Copyright (C) 2012 PX4 Development Team. All rights reserved.
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* 1. Redistributions of source code must retain the above copyright
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/**
* @file ppm_decode.c
*
* PPM input decoder.
*/
#include <nuttx/config.h>
#include <stdint.h>
#include <string.h>
#include <drivers/drv_hrt.h>
#include "ppm_decode.h"
/*
* PPM decoder tuning parameters.
*
* The PPM decoder works as follows.
*
* Initially, the decoder waits in the UNSYNCH state for two edges
* separated by PPM_MIN_START. Once the second edge is detected,
* the decoder moves to the ARM state.
*
* The ARM state expects an edge within PPM_MAX_PULSE_WIDTH, being the
* timing mark for the first channel. If this is detected, it moves to
* the INACTIVE state.
*
* The INACTIVE phase waits for and discards the next edge, as it is not
* significant. Once the edge is detected, it moves to the ACTIVE stae.
*
* The ACTIVE state expects an edge within PPM_MAX_PULSE_WIDTH, and when
* received calculates the time from the previous mark and records
* this time as the value for the next channel.
*
* If at any time waiting for an edge, the delay from the previous edge
* exceeds PPM_MIN_START the frame is deemed to have ended and the recorded
* values are advertised to clients.
*/
#define PPM_MAX_PULSE_WIDTH 500 /* maximum width of a pulse */
#define PPM_MIN_CHANNEL_VALUE 800 /* shortest valid channel signal */
#define PPM_MAX_CHANNEL_VALUE 2200 /* longest valid channel signal */
#define PPM_MIN_START 2500 /* shortest valid start gap */
/* Input timeout - after this interval we assume signal is lost */
#define PPM_INPUT_TIMEOUT 100 * 1000 /* 100ms */
/* Number of same-sized frames required to 'lock' */
#define PPM_CHANNEL_LOCK 3 /* should be less than the input timeout */
/* decoded PPM buffer */
#define PPM_MIN_CHANNELS 4
#define PPM_MAX_CHANNELS 12
/*
* Public decoder state
*/
uint16_t ppm_buffer[PPM_MAX_CHANNELS];
unsigned ppm_decoded_channels;
hrt_abstime ppm_last_valid_decode;
static uint16_t ppm_temp_buffer[PPM_MAX_CHANNELS];
/* PPM decoder state machine */
static struct {
uint16_t last_edge; /* last capture time */
uint16_t last_mark; /* last significant edge */
unsigned next_channel;
unsigned count_max;
enum {
UNSYNCH = 0,
ARM,
ACTIVE,
INACTIVE
} phase;
} ppm;
void
ppm_input_init(unsigned count_max)
{
ppm_decoded_channels = 0;
ppm_last_valid_decode = 0;
memset(&ppm, 0, sizeof(ppm));
ppm.count_max = count_max;
}
void
ppm_input_decode(bool reset, unsigned count)
{
uint16_t width;
uint16_t interval;
unsigned i;
/* if we missed an edge, we have to give up */
if (reset)
goto error;
/* how long since the last edge? */
width = count - ppm.last_edge;
if (count < ppm.last_edge)
width += ppm.count_max; /* handle wrapped count */
ppm.last_edge = count;
/*
* If this looks like a start pulse, then push the last set of values
* and reset the state machine.
*
* Note that this is not a "high performance" design; it implies a whole
* frame of latency between the pulses being received and their being
* considered valid.
*/
if (width >= PPM_MIN_START) {
/*
* If the number of channels changes unexpectedly, we don't want
* to just immediately jump on the new count as it may be a result
* of noise or dropped edges. Instead, take a few frames to settle.
*/
if (ppm.next_channel != ppm_decoded_channels) {
static unsigned new_channel_count;
static unsigned new_channel_holdoff;
if (new_channel_count != ppm.next_channel) {
/* start the lock counter for the new channel count */
new_channel_count = ppm.next_channel;
new_channel_holdoff = PPM_CHANNEL_LOCK;
} else if (new_channel_holdoff > 0) {
/* this frame matched the last one, decrement the lock counter */
new_channel_holdoff--;
} else {
/* we have seen PPM_CHANNEL_LOCK frames with the new count, accept it */
ppm_decoded_channels = new_channel_count;
new_channel_count = 0;
}
} else {
/* frame channel count matches expected, let's use it */
if (ppm.next_channel > PPM_MIN_CHANNELS) {
for (i = 0; i < ppm.next_channel; i++)
ppm_buffer[i] = ppm_temp_buffer[i];
ppm_last_valid_decode = hrt_absolute_time();
}
}
/* reset for the next frame */
ppm.next_channel = 0;
/* next edge is the reference for the first channel */
ppm.phase = ARM;
return;
}
switch (ppm.phase) {
case UNSYNCH:
/* we are waiting for a start pulse - nothing useful to do here */
return;
case ARM:
/* we expect a pulse giving us the first mark */
if (width > PPM_MAX_PULSE_WIDTH)
goto error; /* pulse was too long */
/* record the mark timing, expect an inactive edge */
ppm.last_mark = count;
ppm.phase = INACTIVE;
return;
case INACTIVE:
/* this edge is not interesting, but now we are ready for the next mark */
ppm.phase = ACTIVE;
/* note that we don't bother looking at the timing of this edge */
return;
case ACTIVE:
/* we expect a well-formed pulse */
if (width > PPM_MAX_PULSE_WIDTH)
goto error; /* pulse was too long */
/* determine the interval from the last mark */
interval = count - ppm.last_mark;
ppm.last_mark = count;
/* if the mark-mark timing is out of bounds, abandon the frame */
if ((interval < PPM_MIN_CHANNEL_VALUE) || (interval > PPM_MAX_CHANNEL_VALUE))
goto error;
/* if we have room to store the value, do so */
if (ppm.next_channel < PPM_MAX_CHANNELS)
ppm_temp_buffer[ppm.next_channel++] = interval;
ppm.phase = INACTIVE;
return;
}
/* the state machine is corrupted; reset it */
error:
/* we don't like the state of the decoder, reset it and try again */
ppm.phase = UNSYNCH;
ppm_decoded_channels = 0;
}
