What is the update frequency for RSSI acquisition

[Ziyao Zhou]

Hi

I am using hci_pwr_ctrl and want to get the RSS (read_conn_rssi(default_conn_handle, &rssi) in a very high frequency. However, I notice when I use 100Hz frequency, a series of RSSI are the same. I suspect that the RSSI is not updated correctly and needs to be updated at a certain frequency. Is this correct?

void modulate_tx_power(void *p1, void *p2, void *p3)
{
	int8_t txp_get = 0;
	uint8_t idx = 0;

	while (1) {
		if (!default_conn) {
			printk("Set Tx power level to %d\n", txpower[idx]);
			set_tx_power(BT_HCI_VS_LL_HANDLE_TYPE_ADV,
				     0, txpower[1]);  //一直0

			k_sleep(K_SECONDS(5));

			printk("Get Tx power level -> ");
			get_tx_power(BT_HCI_VS_LL_HANDLE_TYPE_ADV,
				     0, &txp_get);
			printk("TXP = %d\n", txp_get);

			idx = (idx+1) % DEVICE_BEACON_TXPOWER_NUM;
		} else {
			int8_t rssi = 0xFF;
			int8_t txp_adaptive;

			idx = 0;

			read_conn_rssi(default_conn_handle, &rssi);
			printk("Connected (%d) - RSSI = %d\n",
			       default_conn_handle, rssi);
			if (rssi > -70) {
				txp_adaptive = 0;//一直0
			} else if (rssi > -90) {
				txp_adaptive = -12;
			} else {
				txp_adaptive = -4;
			}
			printk("Adaptive Tx power selected = %d\n",
			       txp_adaptive);
			set_tx_power(BT_HCI_VS_LL_HANDLE_TYPE_CONN,
				     default_conn_handle, txp_adaptive);
			get_tx_power(BT_HCI_VS_LL_HANDLE_TYPE_CONN,
				     default_conn_handle, &txp_get);
			printk("Connection (%d) TXP = %d\n",
			       default_conn_handle, txp_get);

k_sleep(K_MSEC(10));
		}
	}
}

[Amanda Hsieh]

Yes, you are correct. See the PS:

After a signal level change, the RSSI will settle after approximately RSSI_SETTLE.

[Ziyao Zhou]

Thank you for your information, does that mean every 20us the RSSI will refresh?

By the way, what is the precision of RSSI, I heard from this ticket that the RSSI readout has +- 12dbm range from the exact number, is that correct?

[Amanda Hsieh]

Ziyao Zhou said:

does that mean every 20us the RSSI will refresh?

No, that's the required settling time after a signal level change. 

Ziyao Zhou said:

what is the precision of RSSI, I heard from this ticket that the RSSI readout has +- 12dbm range from the exact number, is that correct?

It's as my colleague explained in that post. 

[Ziyao Zhou]

Amanda Hsieh said:

No, that's the required settling time after a signal level change. 

Thank you for your information. So what is the refresh time for RSSI? Is it related to connection interval?

[Amanda Hsieh]

The RSSI refresh time is not directly related to the connection interval. The RSSI is measured continuously and filtered using a single-pole IIR filter. The key specifications for RSSI timing are: RSSI sampling time (RSSIPERIOD) from the RSSI_START task and RSSI settling time (RSSISETTLE) after a signal level change. See the nRF54L15 PS.