MC68HC55 Datasheet, PDF (25/40 Pages) Freescale Semiconductor, Inc – Two-Channel CMOS ASIC Device

MC68HC55 Technical Data
Enable (Disable) Function
When this FIFO is completely empty, the SPI can write four new values
to fill the FIFO without waiting for any action on the DSI side of the FIFO.
Values are “pushed” into the FIFO from the SPI and values are “popped”
after they have been serially sent out the DSI. When the FIFO is full,
additional attempts to write new data from the SPI side are ignored. The
host MCU should be sure the TFNFx status bit is set before writing more
data to the FIFO.
Reset, abort, or enable going to 0 cause asynchronous entry to the
TX_IDLE state which corresponds to the FIFO empty condition. The
push and pop pointers are cleared and X_FIFO_EMPTY is set to true.
X_FIFO_PUSH causes the push pointer to be incremented,
X_FIFO_EMPTY to be set to false, and the state to transition to
TX_NOT_EMPTY. The push request comes from the SPI transfer state
machine after a new value has been written into the FIFO.
From TX_NOT_EMPTY, several things can happen. Additional values
can be pushed into the FIFO, if the push pointer is the same as the pop
pointer minus one. This push fills the FIFO so the state advances to
TX_FULL. Each time a new data value is pushed into the FIFO, the push
pointer is incremented. From TX_NOT_EMPTY, values may also be
popped from the FIFO, freeing a stage for additional data. If the pop
pointer is the same as the push pointer minus one, the pop removes the
last value in the FIFO, so X_FIFO_EMPTY is set to true and the state
changes back to TX_IDLE. Each time a value is popped, the pop pointer
is incremented.
When the transmit FIFO is full, no additional data can be written into the
FIFO, so no new push requests will be generated. From TX_FULL, the
only valid change is caused by a pop which causes the pop pointer to
increment and the state goes back to TX_NOT_EMPTY. (Of course
reset, abort, or disable could cause the state to asynchronously change
to the TX_IDLE state).
MC68HC55
Functional Description
Technical Data
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