MC68306FC16 Datasheet, PDF (120/191 Pages) Motorola, Inc – Integrated EC000 Processor

register position of the FIFO. Thus, data flowing from the receiver to the CPU is quadruple
buffered.
In addition to the data byte, three status bits, PE, FE, and RB, are appended to each data
character in the FIFO; OE is not appended. By programming the ERR bit in the channel's
mode register (DUMR1), status is provided in character or block modes.
The RxRDY bit in the DUSR is set whenever one or more characters are available to be
read by the CPU. A read of the receiver buffer produces an output of data from the top of
the FIFO stack. After the read cycle, the data at the top of the FIFO stack and its
associated status bits are 'popped', and new data can be added at the bottom of the stack
by the receiver shift register. The FIFO-full status bit (FFULL) is set if all three stack
positions are filled with data. Either the RxRDY or FFULL bit can be selected to cause an
interrupt.
In the character mode, status provided in the DUSR is given on a character-by-character
basis and thus applies only to the character at the top of the FIFO. In the block mode, the
status provided in the DUSR is the logical OR of all characters coming to the top of the
FIFO stack since the last reset error command. A continuous logical OR function of the
corresponding status bits is produced in the DUSR as each character reaches the top of
the FIFO stack. The block mode is useful in applications where the software overhead of
checking each character's error cannot be tolerated. In this mode, entire messages are
received, and only one data integrity check is performed at the end of the message. This
mode allows a data-reception speed advantage, but does have a disadvantage since
each character is not individually checked for error conditions by software. If an error
occurs within the message, the error is not recognized until the final check is performed,
and no indication exists as to which character in the message is at fault.
In either mode, reading the DUSR does not affect the FIFO. The FIFO is 'popped' only
when the receive buffer is read. The DUSR should be read prior to reading the receive
buffer. If all three of the FIFO's receiver holding registers are full when a new character is
received, the new character is held in the receiver shift register until a FIFO position is
available. If an additional character is received during this state, the contents of the FIFO
are not affected. However, the character previously in the receiver shift register is lost, and
the OE bit in the DUSR is set when the receiver detects the start bit of the new
overrunning character.
To support control flow capability, the receiver can be programmed to automatically
negate and assert RTS≈. When in this mode, RTS≈ is automatically negated by the
receiver when a valid start bit is detected and the FIFO stack is full. When a FIFO position
becomes available, RTS≈ is asserted by the receiver. Using this mode of operation,
overrun errors are prevented by connecting the RTS≈ to the CTS≈ input of the transmitting
device.
If the FIFO stack contains characters and the receiver is disabled, the characters in the
FIFO can still be read by the CPU. If the receiver is reset, the FIFO stack and all receiver
status bits, corresponding output ports, and interrupt request are reset. No additional
characters are received until the receiver is re-enabled.
6-12
MC68306 USER'S MANUAL
MOTOROLA