Fix overflow at extremely low RSSI #15177
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Some LoRaWAN modem like SX1272 can receive downlink packets with RSSI level less than -127. So "int8_t" is not enough for store all possible RSSI values. For example, SX1272 has sensitivity at -137 dBm. Problem was manifested in the file "SX1272_LoRaRadio.cpp" at SX1272_LoRaRadio::handle_dio0_irq() method. When value of _rf_settings.lora_packet_handler.rssi_value calculated incorrect RSSI will be stored. Example case: Value readen from register REG_LR_PKTSNRVALUE _rf_settings.lora_packet_handler.snr_value equals -47. Value readen from register REG_LR_PKTRSSIVALUE equals 17. RSSI_OFFSET equals "-139", snr equals "-11". For case MODEM_LORA value calculated by formula: _rf_settings.lora_packet_handler.rssi_value = RSSI_OFFSET + rssi + (rssi >> 4) + snr; and result value will be "124" because of int8_t overflow so it's not correct value (too high). Correct value must be: -139 + 17 + (1) + (-11) = -132. Another motivation: at all other places int16_t type used to store RSSI value.
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This is a new version of previous PR:
#15079
Some LoRaWAN modem like SX1272 can receive downlink packets with RSSI level less than -127. So "int8_t" is not enough for store all possible RSSI values. For example, SX1272 has sensitivity at -137 dBm.
Problem was manifested in the file "SX1272_LoRaRadio.cpp" at SX1272_LoRaRadio::handle_dio0_irq() method.
When value of _rf_settings.lora_packet_handler.rssi_value calculated incorrect RSSI will be stored. Example case:
Value readen from register REG_LR_PKTSNRVALUE _rf_settings.lora_packet_handler.snr_value equals -47.
Value readen from register REG_LR_PKTRSSIVALUE equals 17.
RSSI_OFFSET equals "-139", snr equals "-11".
For case MODEM_LORA value calculated by formula:
_rf_settings.lora_packet_handler.rssi_value = RSSI_OFFSET + rssi + (rssi >> 4) + snr;
and result value will be "124" because of int8_t overflow so it's not correct value (too high).
Correct value must be:
-139 + 17 + (1) + (-11) = -132.
Another motivation: at all other places int16_t type used to store RSSI value.
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