Contiki-NG
tsch-adaptive-timesync.c
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1/*
2 * Copyright (c) 2015, SICS Swedish ICT.
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
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10 * 2. Redistributions in binary form must reproduce the above copyright
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14 * may be used to endorse or promote products derived from this software
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29 * This file is part of the Contiki operating system.
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31 */
32
33/**
34 * \file
35 * TSCH adaptive time synchronization
36 * \author
37 * Atis Elsts <atis.elsts@sics.se>
38 *
39 */
40
41/**
42 * \addtogroup tsch
43 * @{
44*/
45
46#include "net/mac/tsch/tsch.h"
47#include <stdio.h>
48#include <inttypes.h>
49
50#if TSCH_ADAPTIVE_TIMESYNC
51
52/* Estimated drift of the time-source neighbor. Can be negative.
53 * Units used: ppm multiplied by 256. */
54static int32_t drift_ppm;
55/* Ticks compensated locally since the last timesync time */
56static int32_t compensated_ticks;
57/* Number of already recorded timesync history entries */
58static uint8_t timesync_entry_count;
59/* Since last learning of the drift; may be more than time since last timesync */
60static uint32_t asn_since_last_learning;
61/* The last neighbor used for timesync */
62struct tsch_neighbor *last_timesource_neighbor;
63
64/* Units in which drift is stored: ppm * 256 */
65#define TSCH_DRIFT_UNIT (1000L * 1000 * 256)
66
67/*---------------------------------------------------------------------------*/
68long int
70{
71 return (long int)drift_ppm / 256;
72}
73/*---------------------------------------------------------------------------*/
74/* Add a value to a moving average estimator */
75static int32_t
76timesync_entry_add(int32_t val)
77{
78#define NUM_TIMESYNC_ENTRIES 8
79 static int32_t buffer[NUM_TIMESYNC_ENTRIES];
80 static uint8_t pos;
81 int i;
82 if(timesync_entry_count == 0) {
83 pos = 0;
84 }
85 buffer[pos] = val;
86 if(timesync_entry_count < NUM_TIMESYNC_ENTRIES) {
87 timesync_entry_count++;
88 } else {
89 /* We now have accurate drift compensation.
90 * Increase keep-alive timeout. */
91 tsch_set_ka_timeout(TSCH_MAX_KEEPALIVE_TIMEOUT);
92 }
93 pos = (pos + 1) % NUM_TIMESYNC_ENTRIES;
94
95 val = 0;
96 for(i = 0; i < timesync_entry_count; ++i) {
97 val += buffer[i];
98 }
99 return val / timesync_entry_count;
100}
101/*---------------------------------------------------------------------------*/
102/* Learn the neighbor drift rate at ppm */
103static void
104timesync_learn_drift_ticks(uint32_t time_delta_asn, int32_t drift_ticks)
105{
106 /* should fit in a 32-bit integer */
107 int32_t time_delta_ticks = time_delta_asn * tsch_timing[tsch_ts_timeslot_length];
108 int32_t real_drift_ticks = drift_ticks + compensated_ticks;
109 int32_t last_drift_ppm = (int32_t)(((int64_t)real_drift_ticks * TSCH_DRIFT_UNIT) / time_delta_ticks);
110
111 drift_ppm = timesync_entry_add(last_drift_ppm);
112
113 TSCH_LOG_ADD(tsch_log_message,
114 snprintf(log->message, sizeof(log->message),
115 "drift %ld ppm (min/max delta seen: %"PRId32"/%"PRId32")",
117 min_drift_seen, max_drift_seen));
118}
119/*---------------------------------------------------------------------------*/
120/* Either reset or update the neighbor's drift */
121void
122tsch_timesync_update(struct tsch_neighbor *n, uint16_t time_delta_asn, int32_t drift_correction)
123{
124 /* Account the drift if either this is a new timesource,
125 * or the timedelta is not too small, as smaller timedelta
126 * means proportionally larger measurement error. */
127 if(last_timesource_neighbor != n) {
129 last_timesource_neighbor = n;
130 } else {
131 asn_since_last_learning += time_delta_asn;
132 if(asn_since_last_learning >= 4 * TSCH_SLOTS_PER_SECOND) {
133 timesync_learn_drift_ticks(asn_since_last_learning, drift_correction);
134 compensated_ticks = 0;
135 asn_since_last_learning = 0;
136 } else {
137 /* Too small timedelta, do not recalculate the drift to avoid introducing error. instead account for the corrected ticks */
138 compensated_ticks += drift_correction;
139 }
140 }
141 min_drift_seen = MIN(drift_correction, min_drift_seen);
142 max_drift_seen = MAX(drift_correction, max_drift_seen);
143}
144/*---------------------------------------------------------------------------*/
145/* Error-accumulation free compensation algorithm */
146static int32_t
147compensate_internal(uint32_t time_delta_usec, int32_t drift_ppm, int32_t *remainder, int16_t *tick_conversion_error)
148{
149 int64_t d = (int64_t)time_delta_usec * drift_ppm + *remainder;
150 int32_t amount = d / TSCH_DRIFT_UNIT;
151 int32_t amount_ticks;
152
153 *remainder = (int32_t)(d - amount * TSCH_DRIFT_UNIT);
154
155 amount += *tick_conversion_error;
156 amount_ticks = US_TO_RTIMERTICKS(amount);
157 *tick_conversion_error = amount - RTIMERTICKS_TO_US(amount_ticks);
158
159 if(ABS(amount_ticks) > RTIMER_ARCH_SECOND / 128) {
160 TSCH_LOG_ADD(tsch_log_message,
161 snprintf(log->message, sizeof(log->message),
162 "!too big compensation %ld delta %ld", (long int)amount_ticks, (long int)time_delta_usec));
163 amount_ticks = (amount_ticks > 0 ? RTIMER_ARCH_SECOND : -RTIMER_ARCH_SECOND) / 128;
164 }
165
166 return amount_ticks;
167}
168/*---------------------------------------------------------------------------*/
169/* Do the compensation step before scheduling a new timeslot */
170int32_t
171tsch_timesync_adaptive_compensate(rtimer_clock_t time_delta_ticks)
172{
173 int32_t result = 0;
174 uint32_t time_delta_usec = RTIMERTICKS_TO_US_64(time_delta_ticks);
175
176 /* compensate, but not if the neighbor is not known */
177 if(drift_ppm && last_timesource_neighbor != NULL) {
178 static int32_t remainder;
179 static int16_t tick_conversion_error;
180 result = compensate_internal(time_delta_usec, drift_ppm,
181 &remainder, &tick_conversion_error);
182 compensated_ticks += result;
183 }
184
185 if(TSCH_BASE_DRIFT_PPM) {
186 static int32_t base_drift_remainder;
187 static int16_t base_drift_tick_conversion_error;
188 result += compensate_internal(time_delta_usec, 256L * TSCH_BASE_DRIFT_PPM,
189 &base_drift_remainder, &base_drift_tick_conversion_error);
190 }
191
192 return result;
193}
194/*---------------------------------------------------------------------------*/
195void
197{
198 last_timesource_neighbor = NULL;
199 drift_ppm = 0;
200 timesync_entry_count = 0;
201 compensated_ticks = 0;
202 asn_since_last_learning = 0;
203}
204/*---------------------------------------------------------------------------*/
205#else /* TSCH_ADAPTIVE_TIMESYNC */
206/*---------------------------------------------------------------------------*/
207void
208tsch_timesync_update(struct tsch_neighbor *n, uint16_t time_delta_asn, int32_t drift_correction)
209{
210}
211/*---------------------------------------------------------------------------*/
212int32_t
213tsch_timesync_adaptive_compensate(rtimer_clock_t delta_ticks)
214{
215 return 0;
216}
217void
219{
220}
221/*---------------------------------------------------------------------------*/
222long int
224{
225 return 0;
226}
227/*---------------------------------------------------------------------------*/
228#endif /* TSCH_ADAPTIVE_TIMESYNC */
229/** @} */
#define TSCH_LOG_ADD(log_type, init_code)
Use this macro to add a log to the queue (will be printed out later, after leaving interrupt context)
Definition: tsch-log.h:141
void tsch_adaptive_timesync_reset(void)
Reset the status of the module.
void tsch_set_ka_timeout(uint32_t timeout)
Set the desynchronization timeout after which a node sends a unicasst keep-alive (KA) to its time sou...
Definition: tsch.c:190
int32_t tsch_timesync_adaptive_compensate(rtimer_clock_t delta_ticks)
Computes time compensation for a given point in the future.
long int tsch_adaptive_timesync_get_drift_ppm(void)
Gives the estimated clock drift w.r.t.
void tsch_timesync_update(struct tsch_neighbor *n, uint16_t time_delta_asn, int32_t drift_correction)
Updates timesync information for a given neighbor.
TSCH neighbor information.
Definition: tsch-types.h:109
Main API declarations for TSCH.