Modem Modes of Operation
MC1321x Reference Manual, Rev. 1.6
7-10
Freescale Semiconductor
The Stream Mode requires that the host MCU either supply a data word every 64 microseconds for a TX
sequence or read a data word every 64 microseconds for a RX sequence (with the exception shown being
in the note below). If this timing requirement is violated, a strm_data_err status will be issued causing an
interrupt request if enabled. The use of the interrupts is important to supporting stream data mode.
For TX Mode once the sequence has been initiated, an interrupt (tx_strm_irq bit) is generated for every
new data word required for the packet (excluding the CRC data). The MCU is required to write the data
word to TX_Pkt_RAM Register 02 within the 64 microsecond interval. Writing the TX word will clear the
interrupt request and thus saves an access to the IRQ_Status Register 24 to clear the IRQ. An interrupt will
also be generated to signal the completion of the TX operation via the tx_done_irq bit.
The RX Mode is slightly more complex. Once a RX sequence has been initiated and a RX packet is being
received, the first interrupt generated (rx_strm_irq bit) is for the MCU to read the RX Packet Length from
Register 2D. Reading Register 2D will clear the interrupt and saves a read of the IRQ_Status Register 24.
There will be a following interrupt (rx_strm_irq bit) for each received data word and the interrupt can be
cleared by reading the data from RX_Pkt_RAM Register 01. The data must be read within the 64
microsecond period, and no interrupt will be generated for the received CRC data. An interrupt will also
be generated for the completion of the RX operation via the rx_done_irq bit.
NOTE
•
There is one exception to the 64 microsecond response time. For the stream receive sequence and
an odd byte length packet, the last data transfer only has 8 bits (one byte) of valid data and the data
will only be available for 32 microseconds. If the packet length is even, the full 64 microseconds
is available
•
If the timing of the last data transfer on a stream receive is violated, a strm_data_err status will not
be issued and the user software must take this into account
These are described in more detail in the following sections.
7.3.4.1
Stream Receive Mode
The advantage of Stream Receive Mode is that it allows the microcontroller to fetch data from the
MC1321x as soon as the data arrives. As a result the MCU can begin processing frame information as it
arrives and provide a quicker turn-around time if a response is required. The disadvantage of Stream Mode
is that there is a significant amount of overhead required from the MCU to process incoming data on a
word-by-word basis.
NOTE
Similar to Packet Receive Mode, 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.
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