Datasheet

14-32 MCF5407 User’s Manual
Operation
14.5.2.5 FIFO Stack in UART0
The FIFO stack is used in the UART’s receiver buffer logic. The stack consists of three
receiver holding registers. The receive buffer consists of the FIFO and a receiver shift
register connected to the RxD (see Figure 14-29). Data is assembled in the receiver shift
register and loaded into the top empty receiver holding 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, parity error (PE), framing error (FE), and
received break (RB), are appended to each data character in the FIFO; OE (overrun error)
is not appended. By programming the ERR bit in the channel’s mode register (UMR1n),
status is provided in character or block modes.
USRn[RxRDY] is set when at least one character is 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
the receiver shift register can add new data at the bottom of the stack. 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.
The two error modes are selected by UMR1n[ERR] as follows:
• In character mode (UMR1n[ERR] = 0, status is given in the USRn for the character
at the top of the FIFO.
• In block mode, the USRn shows a logical OR of all characters reaching the top of
the FIFO stack since the last
RESET ERROR STATUS command. Status is updated as
characters reach the top of the FIFO stack. Block mode offers a data-reception speed
advantage where the software overhead of error-checking each character cannot be
tolerated. However, errors are not detected until the check is performed at the end of
an entire message—the faulting character is not identified.
In either mode, reading the USRn does not affect the FIFO. The FIFO is popped only when
the receive buffer is read. The USRn should be read before reading the receive buffer. If all
three receiver holding registers are full, a new character is held in the receiver shift register
until space is available. However, if a second new character is received, the contents of the
the character in the receiver shift register is lost, the FIFOs are unaffected, and USRn[OE]
is set when the receiver detects the start bit of the new overrunning character.
To support flow control, the receiver can be programmed to automatically negate and assert
R
TS, in which case the receiver automatically negates RTS when a valid start bit is detected
and the FIFO stack is full. The receiver asserts R
TS when a FIFO position becomes
available; therefore, overrun errors can be prevented by connecting R
TS to the CTS input
of the transmitting device.