RSSI Register Interpretation calculation formula explanation

That's doing two's complement to int16 conversation.

My code do not produce RSSI Values less than -80dBm obtained using the above formula is there any error in reading method or should I check hardware?(Custom HW)



//RF Settings
PKTCTRL1 = 0x00; // No address check. Append status bytes.
PKTCTRL0 = 0x44; // Variable packet length. Use CRC. Enable whitening
PKTLEN = 0x55; //64 old before fixed packet 85 packets
// Filter BW, data rate.

// Filter BW, data rate.

MDMCFG4 = 0x2D; // Data Rate = 250 kbps------>(0x6D-->270.8khz) (0x2D---->541.666667khz) binary value --> 0b101101 Band_width-->541666.66666666666666666666666667
MDMCFG3 = 0x3B; // Rx filter bandwidth = 542 kHz --> binary value --> 0b111011 59

// IF frequency
FSCTRL1 = 0x0C;
FSCTRL0 = 0x00;

// Modulation format, detection level
MDMCFG2 = 0x13; // 30/32 bits sync word detection. GFSK modulation. 0x03-->2_fsk 0x
MDMCFG1 = 0x43; // 8 byte preamble. No FEC.
MDMCFG0 = 0x3B; // Channel spacing: 250 kHz

// Deviation setting
DEVIATN = 0x64;


// Calibration synth
MCSM2 = 0x07;
MCSM1 = 0x30; // RXOFF_MOODE: Idle; TXOFF_MODE: Idle
MCSM0 = 0x08; // Manual FS calibration

// Frequency offset compensation configuration
FOCCFG = 0x1D;

// Bit synchronization
BSCFG = 0x1C;
AGCCTRL2 = 0xC7; .
AGCCTRL1 = 0x40;
AGCCTRL0 = 0xB0;


// Front end settings (from SmartRF04)
FREND1 = 0xB6;
FREND0 = 0x10;

// Synth calibration
FSCAL3 = 0xEF;
FSCAL2 = 0x2C;
FSCAL1 = 0x24;
FSCAL0 = 0x1F;


PA_TABLE0 = 0x84;


//RSSI Calculations


if (RSSI >= 128) // calculate RSSI in dBm
{ rssi_testing = (int) ((RSSI - 256) / 2) - RSSI_OFFSET;}
else
{ rssi_testing = (int) (RSSI / 2) - RSSI_OFFSET;}

rssisum += abs(rssi_testing ); // convert to asbsolute value

rssipkts++;
if (rssipkts > 20) {
rssipkts = 0;
rssiavg= rssisum / 21; // calculate the average RSSI value over the last 21 packets
rssisum = 0;
rssivalid= 1; // mark rssiavg as valid
}
if(rssivalid==1)
{

//check for RSSI here
if(rssiavg>80)
{
//Never happens



}


}

Would you be able to connect your board to a CCDebugger and test with SmartRF Studio?
Have you tried to do the measurement with a 50 ohm termination to be sure you are not picking up background noise?

No I have not tried with CCDebugger ,And when testing with two cc1110 modules when one is switched off the other doesn't show any RSSI reading hence it may not be the background noise am I right?.I suppose some problem with the way I read RSSI .I do so in a timer and lit an led when RSSI drops below the specified value .


int xdata rssi_value=0;//Variable declaration

void timer1(void) interrupt 9 //8khz timer
{

if (RSSI >= 128) // calculate RSSI in dBm
{ rssi_value= (int) ((RSSI - 256) / 2) - 77;}
else
{ rssi_value = (int) (RSSI / 2) - 77;}

if(rssi_value < -60) //for checking RSSI value below -60 dbm
{

led=1;



}else
{



led=0;


}




}

I'm a bit confused, under which conditions are you not able to get under -80 dBm?

What does "does not show any RSSI reading" mean? The above function return a number? Do you have a signal generator? If so, apply a tone with known level and see if the RSSI follows the level on the SG.

Any reason why you have
{rssi_dBm = (INT16)((INT16)( rssi_dec - 256) / 2) - rssi_offset;} //negative value

in your first post but
{ rssi_value= (int) ((RSSI - 256) / 2) - 77;}

in a couple of others? If you always get a limitation equal to the RSSI offset it sounds like you have a issue with the variables. I only checked the first code you posted in detail.