14.5.2 Frame Transmit Procedure

A frame transmission comprises of two actions, a write to Frame Buffer and the transmission of its contents. Both actions can be run in parallel if required by critical protocol timing.

The figure below illustrates the AT86RF212B frame transmit procedure, when writing and transmitting the frame consecutively. After a Frame Buffer write access, the frame transmission is initiated by asserting pin 11 (SLP_TR) or writing command TX_START to the TRX_CMD bits in the TRX_STATE register (TRX_STATE.TRX_CMD). The transceiver must be either in PLL_ON state for basic operating mode or TX_ARET_ON state for extended operating mode. The completion of the transaction is indicated by interrupt IRQ_3 (TRX_END).

Alternatively for time critical applications when the frame start transmission time needs to be minimized, a frame transmission task can be started first. Then it can be followed by the Frame Buffer write access event (populating PSDU data). This way the data to be transmitted is needs to be written in the transmit frame buffer as the transceiver initializes and begins SHR transmission.

By initiating a transmission, either by asserting pin 11 (SLP_TR) or writing a TX_START command to the TRX_CMD bits, the radio transceiver starts transmitting the SHR, which is internally generated.

Front end initialization takes one symbol period to settle PLL and ramp up the PA. SHR transmission takes another 40 symbol periods for BPSK or 10 symbol periods delay for O-QPSK. By this time the PHR must be available in the Frame Buffer. Furthermore, the SPI data rate must be higher than the PHY data rate to avoid a Frame Buffer underrun, which is indicated by IRQ_6 (TRX_UR).