samples: bluetooth: central_iso: fix rounding error

This change introduces a timing approach that does not accrue rounding
errors over time, thereby further improving deterministic behavior of
the sample relative to the BLE host's sleep clock.

Signed-off-by: Florian Grandel <[email protected]>

samples/bluetooth/central_iso/src/main.c

@@ -66,7 +66,7 @@ static void iso_send_sdu(struct k_work *work)
 
 	net_buf_add_mem(buf, buf_data, len_to_send);
 
-	ret = bt_iso_chan_send(&iso_chan, buf, seq_num++);
+	ret = bt_iso_chan_send(&iso_chan, buf, seq_num);
 
 	if (ret < 0) {
 		printk("Failed to send ISO data (%d)\n", ret);
@@ -83,9 +83,29 @@ static K_WORK_DEFINE(iso_send_sdu_work, iso_send_sdu);
 
 static void iso_timer_timeout(struct k_timer *timer)
 {
-	ARG_UNUSED(timer);
+	int64_t cur_tick = sys_clock_tick_get(), next_tick;
+	static int64_t first_tick = 0;
 
+	if (unlikely(first_tick == 0)) {
+		first_tick = cur_tick;
+	}
+
+	seq_num++;
+
+	/* iso_send_sdu_work will access seq_num, so be sure to update it
+	 * before submitting the work.
+	 */
 	k_work_submit(&iso_send_sdu_work);
+
+	/* Calculate the next tick such that that the next measurement will be
+	 * delivered with minimal latency and w/o accruing rounding errors.
+	 */
+	next_tick = first_tick +
+		    (((int64_t)seq_num * INTERVAL_US) *
+		     CONFIG_SYS_CLOCK_TICKS_PER_SEC) /
+			    USEC_PER_SEC -
+		    cur_tick;
+	k_timer_start(timer, K_TICKS(next_tick), K_FOREVER);
 }
 
 static K_TIMER_DEFINE(iso_timer, iso_timer_timeout, NULL);
@@ -148,7 +168,7 @@ static void iso_connected(struct bt_iso_chan *chan)
 	seq_num = 0U;
 
 	/* Start send timer */
-	k_timer_start(&iso_timer, K_NO_WAIT, K_USEC(INTERVAL_US));
+	k_timer_start(&iso_timer, K_NO_WAIT, K_FOREVER);
 }
 
 static void iso_disconnected(struct bt_iso_chan *chan, uint8_t reason)