Adaptive time synchronization for TSCH
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Atis Elsts
Simon Duquennoy
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f9f91363d2
commit
d79ce957a1
9 changed files with 327 additions and 26 deletions
186
core/net/mac/tsch/tsch-adaptive-timesync.c
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186
core/net/mac/tsch/tsch-adaptive-timesync.c
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/*
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* Copyright (c) 2015, SICS Swedish ICT.
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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* 3. Neither the name of the Institute nor the names of its contributors
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* may be used to endorse or promote products derived from this software
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* without specific prior written permission.
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*
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* THIS SOFTWARE IS PROVIDED BY THE INSTITUTE AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE INSTITUTE OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*
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* This file is part of the Contiki operating system.
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*
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*/
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/**
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* \file
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* TSCH adaptive time synchronization
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* \author
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* Atis Elsts <atis.elsts@sics.se>
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*
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*/
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#include "tsch-adaptive-timesync.h"
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#include "tsch-log.h"
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#include <stdio.h>
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#if TSCH_ADAPTIVE_TIMESYNC
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/* Estimated drift of the time-source neighbor. Can be negative.
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* Units used: ppm multiplied by 256. */
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static int32_t drift_ppm;
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/* Ticks compensated locally since the last timesync time */
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static int32_t compensated_ticks;
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/* Number of already recorded timesync history entries */
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static uint8_t timesync_entry_count;
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/* Since last learning of the drift; may be more than time since last timesync */
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static uint32_t asn_since_last_learning;
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/* Units in which drift is stored: ppm * 256 */
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#define TSCH_DRIFT_UNIT (1000L * 1000 * 256)
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/*---------------------------------------------------------------------------*/
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/* Add a value to a moving average estimator */
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static int32_t
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timesync_entry_add(int32_t val, uint32_t time_delta)
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{
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#define NUM_TIMESYNC_ENTRIES 8
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static int32_t buffer[NUM_TIMESYNC_ENTRIES];
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static uint8_t pos;
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int i;
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if(timesync_entry_count == 0) {
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pos = 0;
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}
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buffer[pos] = val;
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if(timesync_entry_count < NUM_TIMESYNC_ENTRIES) {
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timesync_entry_count++;
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}
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pos = (pos + 1) % NUM_TIMESYNC_ENTRIES;
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val = 0;
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for(i = 0; i < timesync_entry_count; ++i) {
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val += buffer[i];
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}
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return val / timesync_entry_count;
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}
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/*---------------------------------------------------------------------------*/
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/* Learn the neighbor drift rate at ppm */
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static void
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timesync_learn_drift_ticks(uint32_t time_delta_asn, int32_t drift_ticks)
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{
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/* should fit in 32-bit unsigned integer */
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uint32_t time_delta_ticks = time_delta_asn * tsch_timing[tsch_ts_timeslot_length];
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int32_t real_drift_ticks = drift_ticks + compensated_ticks;
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int32_t last_drift_ppm = (int32_t)((int64_t)real_drift_ticks * TSCH_DRIFT_UNIT / time_delta_ticks);
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drift_ppm = timesync_entry_add(last_drift_ppm, time_delta_ticks);
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}
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/*---------------------------------------------------------------------------*/
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/* Either reset or update the neighbor's drift */
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void
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tsch_timesync_update(struct tsch_neighbor *n, uint16_t time_delta_asn, int32_t drift_correction)
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{
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/* Account the drift if either this is a new timesource,
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* or the timedelta is not too small, as smaller timedelta
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* means proportionally larger measurement error. */
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if(last_timesource_neighbor != n) {
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last_timesource_neighbor = n;
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drift_ppm = 0;
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timesync_entry_count = 0;
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compensated_ticks = 0;
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asn_since_last_learning = 0;
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} else {
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asn_since_last_learning += time_delta_asn;
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if(asn_since_last_learning >= 4 * TSCH_SLOTS_PER_SECOND) {
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timesync_learn_drift_ticks(asn_since_last_learning, drift_correction);
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compensated_ticks = 0;
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asn_since_last_learning = 0;
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} else {
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/* Too small timedelta, do not recalculate the drift to avoid introducing error. instead account for the corrected ticks */
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compensated_ticks += drift_correction;
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}
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}
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}
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/*---------------------------------------------------------------------------*/
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/* Error-accumulation free compensation algorithm */
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static int32_t
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compensate_internal(uint32_t time_delta_usec, int32_t drift_ppm, int32_t *remainder, int16_t *tick_conversion_error)
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{
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int64_t d = (int64_t)time_delta_usec * drift_ppm + *remainder;
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int32_t amount = d / TSCH_DRIFT_UNIT;
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int32_t amount_ticks;
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*remainder = (int32_t)(d - amount * TSCH_DRIFT_UNIT);
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amount += *tick_conversion_error;
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amount_ticks = US_TO_RTIMERTICKS(amount);
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*tick_conversion_error = amount - RTIMERTICKS_TO_US(amount_ticks);
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if(ABS(amount_ticks) > RTIMER_ARCH_SECOND / 128) {
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TSCH_LOG_ADD(tsch_log_message,
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snprintf(log->message, sizeof(log->message),
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"!too big compensation %ld delta %ld", amount_ticks, time_delta_usec));
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amount_ticks = (amount_ticks > 0 ? RTIMER_ARCH_SECOND : -RTIMER_ARCH_SECOND) / 128;
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}
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return amount_ticks;
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}
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/*---------------------------------------------------------------------------*/
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/* Do the compensation step before scheduling a new timeslot */
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int32_t
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tsch_timesync_adaptive_compensate(rtimer_clock_t time_delta_ticks)
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{
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int32_t result = 0;
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uint32_t time_delta_usec = RTIMERTICKS_TO_US_64(time_delta_ticks);
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/* compensate, but not if the neighbor is not known */
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if(drift_ppm && last_timesource_neighbor != NULL) {
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static int32_t remainder;
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static int16_t tick_conversion_error;
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result = compensate_internal(time_delta_usec, drift_ppm,
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&remainder, &tick_conversion_error);
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compensated_ticks += result;
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}
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if(TSCH_BASE_DRIFT_PPM) {
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static int32_t base_drift_remainder;
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static int16_t base_drift_tick_conversion_error;
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result += compensate_internal(time_delta_usec, 256L * TSCH_BASE_DRIFT_PPM,
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&base_drift_remainder, &base_drift_tick_conversion_error);
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}
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return result;
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}
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/*---------------------------------------------------------------------------*/
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#else /* TSCH_ADAPTIVE_TIMESYNC */
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/*---------------------------------------------------------------------------*/
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void
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tsch_timesync_update(struct tsch_neighbor *n, uint16_t time_delta_asn, int32_t drift_correction)
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{
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}
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/*---------------------------------------------------------------------------*/
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int32_t
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tsch_timesync_adaptive_compensate(rtimer_clock_t delta_ticks)
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{
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return 0;
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}
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/*---------------------------------------------------------------------------*/
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#endif /* TSCH_ADAPTIVE_TIMESYNC */
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