Philips Semiconductors Product data sheet
SC28L91
3.3 V or 5.0 V Universal Asynchronous
Receiver/Transmitter (UART)
2004 Oct 21 24
MR2 – Mode Register 2
MR2 is accessed when the MR pointer points to MR2, which occurs after any access to MR1. Accesses to MR2 do not change the pointer.
ADDR BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
MR CHANNEL MODE Tx CONTROLS
RTS CTS
ENABLE Tx STOP BIT LENGTH
NOTE: Add 0.5 to binary codes 0–7 for 5 bit character lengths.
0x00 00 = Normal 0 = 0.563 4 = 0.813 8 = 1.563 C = 1.813
01 = Auto-Echo 0 = No 0 = No 1 = 0.625 5 = 0.875 9 = 1.625 D = 1.875
10 = Local loop 1 = Yes 1 = Yes 2 = 0.688 6 = 0.938 A = 1.688 E = 1.938
11 = Remote loop 3 = 0.750 7 = 1.000 B = 1.750 F = 2.000
NOTE:
Add 0.5 to values shown for 0–7 if channel is programmed for 5 bits/char.
MR2[7:6]— Mode Select
The channel of the UART can operate in one of four modes.
MR2[7:6] = 00 is the normal mode, with the transmitter and receiver
operating independently .
MR2[7:6] = 01 places the channel in the automatic echo mode,
which automatically retransmits the received data. The following
conditions are true while in automatic echo mode:
1. Received data is reclocked and retransmitted on the TxD output.
2. The receive clock is used for the transmitter.
3. The receiver must be enabled, but the transmitter needs not be
enabled.
4. The TxRDY and TxEMT status bits are inactive.
5. The received parity is checked, but is not regenerated for
transmission, i.e. transmitted parity bit is as received.
6. Character framing is checked, but the stop bits are retransmitted
as received.
7. A received break is echoed as received until the next valid start
bit is detected.
8. CPU to receiver communication continues normally, but the CPU
to transmitter link is disabled.
MR2[7:6] = 10 selects local loop back diagnostic mode. In this
mode:
1. The transmitter output is internally connected to the receiver
input.
2. The transmit clock is used for the receiver.
3. The TxD output is held High.
4. The RxD input is ignored.
5. The transmitter must be enabled, but the receiver need not be
enabled.
6. CPU to transmitter and receiver communications continue
normally.
MR2[7:6] = 11 selects remote loop back diagnostic mode. In this
mode:
1. Received data is reclocked and retransmitted on the TxD
out–put.
2. The receive clock is used for the transmitter.
3. Received data is not sent to the local CPU, and the error status
conditions are inactive.
4. The received parity is not checked and is not regenerated for
transmission, i.e., transmitted parity is as received.
5. The receiver must be enabled.
6. Character framing is not checked, and the stop bits are
retransmitted as received.
7. A received break is echoed as received until the next valid start
bit is detected.
The user must exercise care when switching into and out of the
various modes. The selected mode will be activated immediately
upon mode selection, even if this occurs in the middle of a received
or transmitted character. Likewise, if a mode is deselected the
device will switch out of the mode immediately.
An exception to this occurs when switching out of auto echo or
remote loop back modes. If the de-selection occurs just after the
receiver has sampled the stop bit (indicated in auto echo by
assertion of RxRDY) and the transmitter is enabled, then the
transmitter will remain in auto echo mode until the stop bit(s) have
been re-transmitted.
In most situations the above is rendered transparent by other
system considerations. However recall that the stop bit sequence
may be very long compared to bus cycles. If rapid reconfiguration of
the transmitter is desired in the above conditions the controlling
system should wait for the TxEMT bit to set or issue a Tx software
reset before reconfiguration begins.
MR2[5]— T ransmitter Request–to–Send Control
This bit controls the deactivation of the RTSN output (OP0) by the
transmitter. This output is normally asserted by setting OPR[0] and
negated by resetting OPR[0]. MR2[5] = 1 caused OPR[0] to be
reset automatically one bit time after the characters in the transmit
shift register and in the TxFIFO, if any, are completely transmitted
including the programmed number of stop bits, if the transmitter is
not enabled.
This feature can be used to automatically terminate the transmission
of a message as follows (“line turnaround”):
1. Program auto–reset mode: MR2[5] = 1.
2. Enable transmitter .
3. Asset RTSN: OPR[0] = 1.
4. Send message.
5. Disable transmitter after the last character is loaded into the
TxFIFO.