UART Modules
Freescale Semiconductor
32-22
•
In block mode, the USR
n
shows a logical OR of all characters reaching the top of the FIFO since
the last RESET ERROR STATUS command. Status is updated as characters reach the top of the
FIFO. 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 USR
n
does not affect the FIFO. The FIFO is popped only when the receive
buffer is read. The USR
n
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 character in the receiver shift register is lost, the
FIFOs are unaffected, and USR
n
[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 U
n
RTS, in
which case the receiver automatically negates U
n
RTS when a valid start bit is detected and the FIFO is
full. The receiver asserts U
n
RTS when a FIFO position becomes available; therefore, connecting U
n
RTS
to the U
n
CTS input of the transmitting device can prevent overrun errors.
NOTE
The receiver continues reading characters in the FIFO if the receiver is
disabled. If the receiver is reset, the FIFO, U
n
RTS control, all receiver status
bits, interrupts, and DMA requests are reset. No more characters are
received until the receiver is reenabled.
32.4.3
Looping Modes
The UART can be configured to operate in various looping modes. These modes are useful for local and
remote system diagnostic functions. The modes are described in the following paragraphs and in
Section 32.3, “Memory Map/Register Definition.”
The UART’s transmitter and receiver should be disabled when switching between modes. The selected
mode is activated immediately upon mode selection, regardless of whether a character is being received
or transmitted.
32.4.3.1
Automatic Echo Mode
In automatic echo mode, shown in
, the UART automatically resends received data bit by bit.
The local CPU-to-receiver communication continues normally, but the CPU-to-transmitter link is
disabled. In this mode, received data is clocked on the receiver clock and re-sent on U
n
TXD. The receiver
must be enabled, but the transmitter need not be.
Figure 32-21. Automatic Echo
Disabled
Disabled
Tx
Rx
CPU
U
n
RXD Input
U
n
RXD Input
U
n
TXD Output
Содержание MCF54455
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