How can ESL establish periodic sync with the special AP?

RE: How can ESL establish periodic sync with the special AP?

Hung Bui — Mon, 02 Mar 2026 21:00:43 GMT

Hi Scott,

Please create a new ticket for your question. As far as I know the scanning for APs is done in the application, not in the stack or driver, so you just need to modify the application/sample to do so. I would suggest to take a look at the periodic_sync_conn sample.

RE: How can ESL establish periodic sync with the special AP?

Scott Grubb — Mon, 02 Mar 2026 17:42:53 GMT

Hi,

I'm trying to implement a similar function in the Nordic ESL Getting Started demo. In my application, there may be multiple APs and I would like the tags to associate with the AP with the strongest signal. However, I can't find any simple way to achieve this in the demo code without modifying the SDK. Are there any existing functions/methods in the BLE driver that would allow a tag to listen for APs before it starts advertising?

Thanks

Scott

RE: How can ESL establish periodic sync with the special AP?

Hung Bui — Wed, 06 Nov 2024 14:23:57 GMT

Hi Tan,
As you can see in your log:

Creating Periodic Advertising Sync is printed right after the first AP is captured. So you need to check what's wrong with the check if (!per_adv_found && second_cnt>5) . Is 5 seconds too short. Or if the second_cnt is counting correctly or not or the if condition was correct or not. Please try to print the log out.

I don't think this is something very hard to debug. Please show the log with all the output, for example the second_cnt print out in the timer handler and in the if (!per_adv_found && second_cnt>5)

I think what you implement is OK , about the timer. You can increase the timer to 10 sec just to test.

RE: How can ESL establish periodic sync with the special AP?

always18 — Wed, 06 Nov 2024 01:22:40 GMT

ok,

I add a timer like this:

k_timer_start(&my_timer, K_SECONDS(1), K_SECONDS(1));

void timer_func(struct k_timer *timer_id)

{

secondTick++;

}

Init startScan when scan starts: startScan = secondTick;

static void scan_AP(const struct bt_le_scan_recv_info *info)

{

if (info->rssi > AP_info.rssi){

AP_info.sid = info->sid;

AP_info.rssi = info->rssi;

bt_addr_le_copy(&(AP_info.addr), info->addr);

printk("Get better rssi ap:sid=%d,rssi=%i,addr=0x%X\n", AP_info.sid, AP_info.rssi, AP_info.addr.a.val[0]);

}

static void scan_recv(const struct bt_le_scan_recv_info *info, struct net_buf_simple *buf)

{

char name[NAME_LEN] = {0};

char le_addr[BT_ADDR_LE_STR_LEN];

(void)memset(name, 0, sizeof(name));

bt_data_parse(buf, data_cb, name);

bt_addr_le_to_str(info->addr, le_addr, sizeof(le_addr));

if (0 == strstr(name, "PAwR conn sample") || 0 == info->interval){

return;

}

printk("scan_recv:%s,sid:%d,rssi:%i,type:%d,addr:%s\n", name, info->sid, info->rssi, info->addr->type, le_addr);

scan_AP(info);

if ( !per_adv_found && (secondTick-startScan>5) ){

per_sid = AP_info.sid;

bt_addr_le_copy(&per_addr, &(AP_info.addr));

per_adv_found = true;

k_sem_give(&sem_per_adv);

}

Next is the log：

scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-49,type:1,addr:71:8E:E0:39:A6:F6 (random)
Get better rssi ap:sid=1,rssi=-49,addr=0xF6
Creating Periodic Advertising Sync
Waiting for periodic sync
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-48,type:1,addr:71:8E:E0:39:A6:F6 (random)
Get better rssi ap:sid=1,rssi=-48,addr=0xF6
scan_recv:sid=3_PAwR conn sample,sid:3,rssi:-53,type:1,addr:60:A5:88:64:00:6F (random)
scan_recv:sid=3_PAwR conn sample,sid:3,rssi:-52,type:1,addr:60:A5:88:64:00:6F (random)
scan_recv:sid=2_PAwR conn sample,sid:2,rssi:-43,type:1,addr:79:D7:19:CA:C1:9D (random)
Get better rssi ap:sid=2,rssi=-43,addr=0x9D
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-48,type:1,addr:71:8E:E0:39:A6:F6 (random)
scan_recv:sid=3_PAwR conn sample,sid:3,rssi:-54,type:1,addr:60:A5:88:64:00:6F (random)
scan_recv:sid=2_PAwR conn sample,sid:2,rssi:-43,type:1,addr:79:D7:19:CA:C1:9D (random)
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-49,type:1,addr:71:8E:E0:39:A6:F6 (random)
scan_recv:sid=3_PAwR conn sample,sid:3,rssi:-51,type:1,addr:60:A5:88:64:00:6F (random)
scan_recv:sid=2_PAwR conn sample,sid:2,rssi:-50,type:1,addr:79:D7:19:CA:C1:9D (random)
scan_recv:sid=2_PAwR conn sample,sid:2,rssi:-45,type:1,addr:79:D7:19:CA:C1:9D (random)
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-48,type:1,addr:71:8E:E0:39:A6:F6 (random)
scan_recv:sid=2_PAwR conn sample,sid:2,rssi:-44,type:1,addr:79:D7:19:CA:C1:9D (random)
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-48,type:1,addr:71:8E:E0:39:A6:F6 (random)
scan_recv:sid=3_PAwR conn sample,sid:3,rssi:-54,type:1,addr:60:A5:88:64:00:6F (random)
scan_recv:sid=2_PAwR conn sample,sid:2,rssi:-47,type:1,addr:79:D7:19:CA:C1:9D (random)
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-49,type:1,addr:71:8E:E0:39:A6:F6 (random)
scan_recv:sid=2_PAwR conn sample,sid:2,rssi:-45,type:1,addr:79:D7:19:CA:C1:9D (random)
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-52,type:1,addr:71:8E:E0:39:A6:F6 (random)
scan_recv:sid=2_PAwR conn sample,sid:2,rssi:-48,type:1,addr:79:D7:19:CA:C1:9D (random)
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-48,type:1,addr:71:8E:E0:39:A6:F6 (random)
scan_recv:sid=2_PAwR conn sample,sid:2,rssi:-44,type:1,addr:79:D7:19:CA:C1:9D (random)
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-49,type:1,addr:71:8E:E0:39:A6:F6 (random)
scan_recv:sid=2_PAwR conn sample,sid:2,rssi:-47,type:1,addr:79:D7:19:CA:C1:9D (random)
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-47,type:1,addr:71:8E:E0:39:A6:F6 (random)
scan_recv:sid=2_PAwR conn sample,sid:2,rssi:-49,type:1,addr:79:D7:19:CA:C1:9D (random)
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-49,type:1,addr:71:8E:E0:39:A6:F6 (random)
scan_recv:sid=3_PAwR conn sample,sid:3,rssi:-54,type:1,addr:60:A5:88:64:00:6F (random)
scan_recv:sid=2_PAwR conn sample,sid:2,rssi:-45,type:1,addr:79:D7:19:CA:C1:9D (random)
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-58,type:1,addr:71:8E:E0:39:A6:F6 (random)
scan_recv:sid=2_PAwR conn sample,sid:2,rssi:-45,type:1,addr:79:D7:19:CA:C1:9D (random)
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-52,type:1,addr:71:8E:E0:39:A6:F6 (random)
scan_recv:sid=2_PAwR conn sample,sid:2,rssi:-46,type:1,addr:79:D7:19:CA:C1:9D (random)
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-49,type:1,addr:71:8E:E0:39:A6:F6 (random)
scan_recv:sid=2_PAwR conn sample,sid:2,rssi:-45,type:1,addr:79:D7:19:CA:C1:9D (random)
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-48,type:1,addr:71:8E:E0:39:A6:F6 (random)
scan_recv:sid=2_PAwR conn sample,sid:2,rssi:-46,type:1,addr:79:D7:19:CA:C1:9D (random)
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-48,type:1,addr:71:8E:E0:39:A6:F6 (random)
scan_recv:sid=1_PAwR conn sample,sid:1,rssi:-48,type:1,addr:71:8E:E0:39:A6:F6 (random)
Synced to sid=1,addr_info_71:8E:E0:39:A6:F6 (random),service_data_0 has 128 subevents,phy=2,reassembly.len=0:0 0 0.
Rx_2, sub_0, RSSI=-47, len=0, data: 0xde_92_35
Periodic sync established.
Stopped scanning
Rx_2, sub_1, RSSI=-48, len=0, data: 0xde_92_35

RE: How can ESL establish periodic sync with the special AP?

Hung Bui — Tue, 05 Nov 2024 15:32:54 GMT

Please show the code where you edited it. How you expect it to work ? And what doesn't work ?

RE: How can ESL establish periodic sync with the special AP?

always18 — Tue, 05 Nov 2024 14:43:39 GMT

Hi Hung,

I add a timeout,but it don`t work like we want to.

RE: How can ESL establish periodic sync with the special AP?

Hung Bui — Tue, 05 Nov 2024 14:35:34 GMT

Hi Tan,

Did you make any modification ?
Do you have any question regarding my explanation ?

I don't see the point of trying your code on my board. It will show the same thing what you observed, it will connect to the first AP it scans.

RE: How can ESL establish periodic sync with the special AP?

always18 — Tue, 05 Nov 2024 11:05:04 GMT

devzone.nordicsemi.com/.../PAwR-test-code.zip

Hi,would you please build the code on your board?

thanks

RE: How can ESL establish periodic sync with the special AP?

Hung Bui — Mon, 04 Nov 2024 08:57:05 GMT

Hi Tan,

What I can see in the log is that it will sync to the first one it find (you can see Creating Periodic Advertising Sync is printed). If you do what you do right now, it will give the semaphore after any AP has RSSI > the default RSSI = -80.
That's why I suggested that you need to use a timer. You need to run for a certain time before you decide which AP it should connect to. You should not give the semaphore like what you are doing now.

The function bt_le_per_adv_sync_create() will not update if you call it again. If you want to change the AP it should try to sync to you need to call bt_le_per_adv_sync_delete() to stop that before you call it again. But anyway, the correct way is to implement a timer so that you only connect to the best AP after you have scanned all of them or scanned for a certain period of time.

RE: How can ESL establish periodic sync with the special AP?

always18 — Sun, 03 Nov 2024 02:59:27 GMT

There are 3 APs with different sid(1~3), It synced with sid_1 but not sid_2 .

Next is the log:

scan_recv:833_PAwR conn sample,sid:1,rssi:-49,type:1,addr:67:58:8D:2A:FF:03 (random)
Get better rssi ap:sid=1,rssi=-49,addr=0x3
Creating Periodic Advertising Sync
Waiting for periodic sync
scan_recv:840_PAwR conn sample,sid:2,rssi:-49,type:1,addr:7C:15:8E:D0:69:D3 (random)
scan_recv:841_PAwR conn sample,sid:3,rssi:-60,type:1,addr:45:59:25:CC:E7:0C (random)
scan_recv:840_PAwR conn sample,sid:2,rssi:-45,type:1,addr:7C:15:8E:D0:69:D3 (random)
Get better rssi ap:sid=2,rssi=-45,addr=0xD3
scan_recv:833_PAwR conn sample,sid:1,rssi:-47,type:1,addr:67:58:8D:2A:FF:03 (random)
scan_recv:840_PAwR conn sample,sid:2,rssi:-49,type:1,addr:7C:15:8E:D0:69:D3 (random)
scan_recv:841_PAwR conn sample,sid:3,rssi:-63,type:1,addr:45:59:25:CC:E7:0C (random)
scan_recv:840_PAwR conn sample,sid:2,rssi:-48,type:1,addr:7C:15:8E:D0:69:D3 (random)
scan_recv:833_PAwR conn sample,sid:1,rssi:-52,type:1,addr:67:58:8D:2A:FF:03 (random)
scan_recv:840_PAwR conn sample,sid:2,rssi:-49,type:1,addr:7C:15:8E:D0:69:D3 (random)
scan_recv:841_PAwR conn sample,sid:3,rssi:-55,type:1,addr:45:59:25:CC:E7:0C (random)
scan_recv:840_PAwR conn sample,sid:2,rssi:-47,type:1,addr:7C:15:8E:D0:69:D3 (random)
scan_recv:841_PAwR conn sample,sid:3,rssi:-56,type:1,addr:45:59:25:CC:E7:0C (random)
scan_recv:841_PAwR conn sample,sid:3,rssi:-60,type:1,addr:45:59:25:CC:E7:0C (random)
scan_recv:840_PAwR conn sample,sid:2,rssi:-49,type:1,addr:7C:15:8E:D0:69:D3 (random)
scan_recv:841_PAwR conn sample,sid:3,rssi:-59,type:1,addr:45:59:25:CC:E7:0C (random)
scan_recv:840_PAwR conn sample,sid:2,rssi:-50,type:1,addr:7C:15:8E:D0:69:D3 (random)
scan_recv:841_PAwR conn sample,sid:3,rssi:-61,type:1,addr:45:59:25:CC:E7:0C (random)
scan_recv:841_PAwR conn sample,sid:3,rssi:-61,type:1,addr:45:59:25:CC:E7:0C (random)
scan_recv:840_PAwR conn sample,sid:2,rssi:-42,type:1,addr:7C:15:8E:D0:69:D3 (random)
Get better rssi ap:sid=2,rssi=-42,addr=0xD3
scan_recv:833_PAwR conn sample,sid:1,rssi:-47,type:1,addr:67:58:8D:2A:FF:03 (random)
scan_recv:840_PAwR conn sample,sid:2,rssi:-43,type:1,addr:7C:15:8E:D0:69:D3 (random)
scan_recv:841_PAwR conn sample,sid:3,rssi:-62,type:1,addr:45:59:25:CC:E7:0C (random)
scan_recv:840_PAwR conn sample,sid:2,rssi:-42,type:1,addr:7C:15:8E:D0:69:D3 (random)
scan_recv:841_PAwR conn sample,sid:3,rssi:-55,type:1,addr:45:59:25:CC:E7:0C (random)
scan_recv:841_PAwR conn sample,sid:3,rssi:-56,type:1,addr:45:59:25:CC:E7:0C (random)
scan_recv:841_PAwR conn sample,sid:3,rssi:-57,type:1,addr:45:59:25:CC:E7:0C (random)
scan_recv:840_PAwR conn sample,sid:2,rssi:-43,type:1,addr:7C:15:8E:D0:69:D3 (random)
scan_recv:841_PAwR conn sample,sid:3,rssi:-61,type:1,addr:45:59:25:CC:E7:0C (random)
Synced to sid=1,addr_info_67:58:8D:2A:FF:03 (random),service_data_0 has 128 subevents,phy=2,reassembly.len=0:0 0 0.
Periodic sync established.
Stopped scanning

RE: How can ESL establish periodic sync with the special AP?

Hung Bui — Fri, 01 Nov 2024 15:14:19 GMT

Ah I see. I didn't spot that. But you don't have a timeout before you decide on which AP you want to sync to. From what I can see you will call k_sem_give(&sem_per_adv); on the first AP you scanned.

RE: How can ESL establish periodic sync with the special AP?

always18 — Fri, 01 Nov 2024 13:11:09 GMT

Hi,

As you can see in the code ,the AP with highest RSSI will be saved at AP_info, and it will be configure as the AP to be synced with.

You can try the demo, The bug will occurs.

RE: How can ESL establish periodic sync with the special AP?

Hung Bui — Fri, 01 Nov 2024 11:46:07 GMT

Hi Tan,
From what I can see in your code, what you were trying is to you will sync to the AP which has the RSSI >-80dBm. But I don't see how you reject an advertiser if the RSSI is <-80.
You should have another condition in addition to !per_adv_found for example something like this:

if ((!per_adv_found)&&(good_rssi)) {

// per_sid = info->sid;
// bt_addr_le_copy(&per_addr, info->addr);

per_sid = AP_info.sid;
bt_addr_le_copy(&per_addr, &(AP_info.addr));

per_adv_found = true;
k_sem_give(&sem_per_adv);
}
Where you assign good_rssi=1 when info->rssi > AP_info.rssi .

If you want to compare between APs and choose the best one, you would need to have a timeout, and only select the one with highest RSSI after the timeout.