Rainbow Electronics ATmega128RFA1 - Manual (Page 67)

67
8266A-MCU Wireless-12/09
ATmega128RFA1
Example:
Consider a 5 octet ACK frame. The MHR field consists of
0100 0000 0000 0000 0101 0110.
The leftmost bit (b
0
) is transmitted first in time. The FCS is in this case
0010 0111 1001 1110.
The leftmost bit (r
0
) is transmitted first in time.
9.5.2.3 Automatic FCS generation
The automatic FCS generation is performed with register bit TX_AUTO_CRC_ON = 1
(reset value). This allows the radio transceiver to autonomously compute the FCS. For
a frame with a frame length specified as N (3 ≤ N ≤ 127), the FCS is calculated on the
first N-2 octets in the Frame Buffer and the resulting FCS field is transmitted in place of
the last two octets from the Frame Buffer.
If the automatic FCS generation of the radio transceivers is enabled, the Frame Buffer
write access can be stopped right after MAC payload. There is no need to write FCS
dummy bytes.
In RX_AACK mode, when a received frame needs to be acknowledged, the FCS of the
ACK frame is always automatically generated by the radio transceiver, independent of
the TX_AUTO_CRC_ON setting.
Example:
A frame transmission of length five with TX_AUTO_CRC_ON set, is started with a
Frame Buffer write access of five bytes (the last two bytes can be omitted). The first
three bytes are used for FCS generation; the last two bytes are replaced by the
internally calculated FCS.
9.5.2.4 Automatic FCS check
An automatic FCS check is applied on each received frame with a frame length N ≥ 2.
The bit RX_CRC_VALID of register PHY_RSSI is set if the FCS of a received frame is
valid. The register bit is updated when issuing a TRX24_RX_END interrupt and remains
valid until a new frame reception causes the next TRX24_RX_END interrupt.
In RX_AACK mode, the radio transceiver rejects the frame and the TRX24_RX_END
interrupt is not issued if the FCS of the received frame is not valid.
In TX_ARET mode, the FCS and the sequence number of an ACK are automatically
checked. The ACK is not accepted if one of those is not correct.
9.5.3 Received Signal Strength Indicator (RSSI)
The Received Signal Strength Indicator is characterized by:
• Minimum RSSI level is -90 dBm (RSSI_BASE_VAL);
• Dynamic range is 81 dB;
• Minimum RSSI value is 0;
• Maximum RSSI value is 28.
9.5.3.1 Overview
The RSSI is a 5-bit value indicating the receive power in the selected channel in steps
of 3 dB. No attempt is made to distinguish IEEE 802.15.4 signals from others. Only the