Modem Modes of Operation
MC1321x Reference Manual, Rev. 1.6
Freescale Semiconductor
7-7
7.3.3.1
Packet Receive Mode
Receive Mode is the state where the transceiver is waiting for an incoming data frame. The advantage of
Packet Receive Mode is that it allows the MC1321x to receive the whole packet without intervention from
the microcontroller. The entire packet payload is stored in RX Packet RAM and the microcontroller fetches
the data after determining the length and validity of the RX packet. The disadvantage of Packet Mode is
that there is a significant latency to reading the RX data via a SPI recursive access, processing the frame
data and providing a response if required; the Streaming Mode is better suited to faster response time.
The MC1321x waits for preamble followed by a Start of Frame Delimiter. From there, the Frame Length
Indicator is used to determine length of the frame and calculate CRC. The receive function provides the
following frame information/data:
1. The frame payload data - accessed through rx_pkt_ram[15:0] RX_Pkt_RAM Register 01.
2. CRC valid status - reported by crc_valid, IRQ_Status Register 24, Bit 0.
3. Payload data length - reported by rx_pkt_latch[6:0], RX_Pkt_Latch Register 2D, Bits 6 - 0.
4. Link quality indicator (LQI) - this is a measure of the received energy that occurs during the
received frame. Once a preamble is detected, the received energy is measured over a 64
μ
s period
and stored in cca_final[7:0], RX_Pkt_Latch Register 2D, Bits 15 - 8.
NOTE
After a frame is received, the application must determine the validity of the
packet. Due to noise, it is possible for an invalid packet to be reported with
either of the following conditions:
a.) A valid CRC and a frame length of 0,1, or 2.
b.) Invalid CRC and invalid frame length.
The application software needs to verify that:
a.) The CRC is valid.
b.) The frame length is valid with a value of 3 or greater.
The following is a typical sequence for packet receive operation (not using a timer-based start):
NOTE
This sequence shows use of the TXTXEN signal to control a sequence. The
RXTXEN signal can be tied high and left high. The sequence will then start
based on writing of the field xcvr_seq[1:0].
1. The RX frequency must be programmed.
2. If not already low, the MCU sets RXTXEN low.
3. Control bits cleared (no Stream Mode and no timer):
a) tmr_trig_en = 0.
b) tx_strm = 0.
c) rx_strm = 0.
d) use_strm_mode = 0.
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