Rev1.0a | 26/50
CMT2300A
SAR
FILTER
CODE to dBm
CONVERT
SAR
ADC
RSSI AVG
FILTER
RSSI_CODE<7:0>
RSSI_DBM<7:0>
RSSI_AVG_MODE<2:0>
COMPARE to
RSSI_TRIG_TH<7:0>
RESULT
LATCH
LATCH
RSSI_DET_SEL<1:0>
RSSI_DET_SEL<1:0>
4.3.3 Sleep Timer
The CMT2300A integrates a sleep timer driven by 32 kHz low power oscillator (LPOSC). When this function is enabled, the timer
wakes the chip from sleep periodically. When the chip operates in a duty cycle mode, the sleep time can be configured from
0.03125 ms to 41922560 ms. Due to the low power oscillator frequency will change with the temperature and voltage drift, it will
be automatically calibrated during power on and will be periodically calibrated since then. These calibrations will keep the frequency
tolerance of the oscillator 1%.
4.3.4 Low Battery Detection
The chip sets up low voltage detection.When the chip is tuned to a certain frequency, the test is performed once.Frequency
tuning occurs when the chip jumps from the SLEEP/STBY state to the RFS/TFS/TX/RX state. The result can be read by the
LBD_VALUE register.
4.3.5 Received Signal Strength Indicator(RSSI)
RSSI is used to evaluate the signal strength inside the channel. The cascaded I/Q logarithmic amplifier amplifies the signal before it
is sent to the demodulator. The logarithmic amplifier of I channels and Q channel contains the received signal indicator, in which the
DC voltage is generated is proportional to the input signal strength. The output of RSSI is the sum of the values of the two
channels’
signals. The output has 80dB dynamic range above the sensitivity. After the RSSI output is sampled by the ADC and filtered by a
SAR FILTER and a RSSI AVG FILTER. The order of the average filter can be set by RSSI_AVG_MODE<2:0>. The code value is
translated into dBm value after filtering. Users can read the register RSSI_CODE<7:0> to obtain the RSSI code value, or
RSSI_DBM<7:0> to obtain the dBm value. By setting the register RSSI_DET_SEL<1:0> Users can determine whether the RSSI is
output to the MCU in real time, or latched at the instance when the preamble, sync, or the whole packet is received.
Also, CMT2300A allows the user to setup a threshold by RSSI_TRIG_TH<7:0> to compare with the real-time RSSI value. If the
RSSI is larger than the threshold it outputs logic 1, otherwise outputs logic 0. The output can be used as a source of the RSSI VLD
interrupt, of the receive time extending condition in the super- low power (SLP) mode
Figure 7. RSSI detection and comparison circuit
CMT2300A has done a certain degree of calibration before delivery. In order to obtain more accurate RSSI measurement results,
the user needs to recalibrate the RSSI circuit in their dedicated applications. For further information, please refer to the
“AN144-CMT2300AW RSSI Usage Guideline”.