Filling The Rx Fifo; Draining The Rx Fifo; Module Baud Rate And Clock Delay Generation - NXP Semiconductors freescale KV4 Series Reference Manual

Functional description
The POPNXTPTR field in the SR points to the RX FIFO entry that is returned when the
POPR is read. The POPNXTPTR contains the positive offset from RXFR0 in a number
of 32-bit registers. For example, POPNXTPTR equal to two means that the RXFR2
contains the received SPI data that will be returned when the POPR is read. The
POPNXTPTR field is incremented every time the POPR is read. The maximum value of
the field is equal to the maximum implemented RXFR number and it rolls over after
reaching the maximum.

44.5.2.5.1 Filling the RX FIFO

The RX FIFO is filled with the received SPI data from the shift register. While the RX
FIFO is not full, SPI frames from the shift register are transferred to the RX FIFO. Every
time an SPI frame is transferred to the RX FIFO, the RX FIFO Counter is incremented by
one.
If the RX FIFO and shift register are full and a transfer is initiated, the RFOF bit in the
SR is set indicating an overflow condition. Depending on the state of the ROOE bit in the
MCR, the data from the transfer that generated the overflow is either ignored or shifted in
to the shift register. If the ROOE bit is set, the incoming data is shifted in to the shift
register. If the ROOE bit is cleared, the incoming data is ignored.

44.5.2.5.2 Draining the RX FIFO

Host CPU or a DMA can remove (pop) entries from the RX FIFO by reading the module
POP RX FIFO Register (POPR). A read of the POPR decrements the RX FIFO Counter
by one. Attempts to pop data from an empty RX FIFO are ignored and the RX FIFO
Counter remains unchanged. The data, read from the empty RX FIFO, is undetermined.
When the RX FIFO is not empty, the RX FIFO Drain Flag (RFDF) in the SR is set. The
RFDF bit is cleared when the RX_FIFO is empty and the DMA controller indicates that a
read from POPR is complete or by writing a 1 to it.

44.5.3 Module baud rate and clock delay generation

The SCK frequency and the delay values for serial transfer are generated by dividing the
system clock frequency by a prescaler and a scaler with the option for doubling the baud
rate. The following figure shows conceptually how the SCK signal is generated.
SCK
System Clock
1 1+DBR
Prescaler
Scaler
Figure 44-27. Communications clock prescalers and scalers
KV4x Reference Manual, Rev. 2, 02/2015
Preliminary
1206 Freescale Semiconductor, Inc.