How to measure RSSI with NRF51822?

[Nicolas]

Hello!

I would like to measure RSSI for different channels so as to decide which channel is the least noisy one (and more particularly, which ones are already used). I cannot make it work, so I am asking you for help.

My code:

for(i=0; i<20; ++i) 
{
	NRF_RADIO->FREQUENCY = i;
	NRF_RADIO->EVENTS_READY = 0U;
	NRF_RADIO->TASKS_RXEN = 1U;
	while(NRF_RADIO->EVENTS_READY == 0U);
	NRF_RADIO->EVENTS_END = 0U;
	
	NRF_RADIO->TASKS_START = 1U;
	
	NRF_RADIO->EVENTS_RSSIEND = 0U;
	NRF_RADIO->TASKS_RSSISTART = 1U;
	while(NRF_RADIO->EVENTS_RSSIEND == 0U);
	
	rssi[i] = NRF_RADIO->RSSISAMPLE;
	
	NRF_RADIO->EVENTS_DISABLED = 0U;    
	NRF_RADIO->TASKS_DISABLE   = 1U;  // Disable the radio.

	while(NRF_RADIO->EVENTS_DISABLED == 0U)
	{
			// Do nothing.
	}
}

With this code, even if a module is continuously emitting signal, rssi is 90 (0x5A) for every channel. Am I missing something? Isn't it possible to measure RSSI without receiving any packet?

Best regards, Nicolas

[Håkon Alseth]

Try starting the TASKS_RSSISTART when you get an address match. I've posted an example that shows how to use the RSSI here, maybe that can help you out: devzone.nordicsemi.com/.../

[Nicolas]

The problem is that there is no address match because I don't wait for any answer, I just want to measure if there is already something on this carrier.

[Håkon Alseth]

I am sorry for misreading your question. Most likely, the RSSI of 90 is correct (actual value -90 dBm). However, note that the RSSI is only valid from -50 to -80 dBm with the resolution and accuracy given in the PS.

If you check the array, there will most likely be one or two samples that differ from the 0x5A value, especially if you change the RF channel in between every 20 samples.

[Erol CIRACI]

my code...

void radio_init() {

radio_configure();

NRF_RADIO->SHORTS |= RADIO_SHORTS_ADDRESS_RSSISTART_Msk; //RSSI enable

NRF_RADIO->EVENTS_READY = 0U; NRF_RADIO->TASKS_RXEN = 1; // Enable radio and wait for ready.

while (NRF_RADIO->EVENTS_READY == 0U) { }

}

uint8_t radio_rssi_get (void) {

uint8_t sample;

NRF_RADIO->EVENTS_RSSIEND = 0; NRF_RADIO->TASKS_RSSISTART = 1U;

while (NRF_RADIO->EVENTS_RSSIEND == 0) { } sample = (NRF_RADIO->RSSISAMPLE & RADIO_RSSISAMPLE_RSSISAMPLE_Msk);

NRF_RADIO->EVENTS_RSSIEND = 0;

return sample;

}

void main()

{

uint8_t rssi = 0; 

NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
NRF_CLOCK->TASKS_HFCLKSTART    = 1;
  
while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0) {} 

radio_init();

while(true) 
{
    rssi = radio_rssi_get();
}

}