SC26C92 datasheet(17/31 Pages) PHILIPS

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Philips Semiconductors

Product specification

SC26C92

Dual universal asynchronous receiver/transmitter (DUART)

2000 Jan 31

MR2A – Channel A Mode Register 2

MR2A is accessed when the Channel A MR pointer points to MR2,

which occurs after any access to MR1A. Accesses to MR2A do not

change the pointer.

MR2A[7:6] – Channel A Mode Select

Each channel of the DUART can operate in one of four modes.

MR2A[7:6] = 00 is the normal mode, with the transmitter and

receiver operating independently.

MR2A[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 TxDA out-

put.

2. The receive clock is used for the transmitter. Data is received on

the rising edge of the RxC1x clock and retransmitted on the next

fall of the RxC!x clock.

3. The receiver must be enabled, but the transmitter need not be

enabled.

4. The Channel A TxRDY and TxEMT status bits are inactive.

5. The received parity is checked, but is not regenerated for trans-

mission, 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.

MR2A(7:6) = 10 selects the 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 TxDA output is held High.

4. The RxDA input is ignored.

5. The transmitter must be enabled, but the receiver need not be

enabled.

6. CPU to transmitter and receiver communications continue nor-

mally.

MR2A[7:6] = 11 selects a remote loop back diagnostic mode. In this

mode:

1. Received data is reclocked and retransmitted on the TxDA 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 retrans-

mitted as received.

7. A received break is echoed as received until the next valid start

bit is detected.

8. A delay of one bit time is seen at the remote receiver.

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

is switching out of autoecho or remote loopback modes: if the

de-selection occurs just after the receiver has sampled the stop bit

(indicated in autoecho by assertion of RxRDY), and the transmitter

is enabled, the transmitter will remain in autoecho mode until the

entire stop has been re-transmitted.

MR2A[5] – Channel A Transmitter Request-to-Send Control

This bit controls the deactivation of the RTSAN output (OP0) by the

transmitter. This output is normally asserted by setting OPR[0] and

negated by resetting OPR[0].

MR2A[5] = 1 caused OPR[0] to be reset automatically one bit time

after the characters in the Channel A 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:

1. Program auto-reset mode: MR2A[5] = 1.

2. Enable transmitter.

3. Asset RTSAN: OPR[0] = 1.

4. Send message.

5. Disable transmitter after the last character is loaded into the

Channel A TxFIFO. Tx status and Tx interrupts will be disabled

at this time.

6. The last character will be transmitted and OPR[0] will be reset

one bit time after the last stop bit, causing RTSAN to be negated.

In this mode, the meaning of “RTSAN” is that the transmission is

ended.

MR2A[4] – Channel A Clear-to-Send Control

If this bit is 0, CTSAN has no effect on the transmitter. If this bit is a

1, the transmitter checks the state of CTSAN (IPO) each time it is

ready to send a character. If IPO is asserted (Low), the character is

transmitted. If it is negated (High), the TxDA output remains in the

marking state and the transmission is delayed until CTSAN goes

low. Changes in CTSAN while a character is being transmitted do

not affect the transmission of that character..

MR2A[3:0] – Channel A Stop Bit Length Select

This field programs the length of the stop bit appended to the

transmitted character. Stop bit lengths of 9/16 to 1 and 1-9/16 to 2

bits, in increments of 1/16 bit, can be programmed for character

lengths of 6, 7, and 8 bits. For a character lengths of 5 bits, 1-1/16

to 2 stop bits can be programmed in increments of 1/16 bit. In all

cases, the receiver only checks for a ‘mark’ condition at the center

of the stop bit position (one–half bit time after the last data bit, or

after the parity bit if enabled is sampled).

If an external 1X clock is used for the transmitter, MR2A[3] = 0

selects one stop bit and MR2A[3] = 1 selects two stop bits to be

transmitted.

MR0B – Channel B Mode Register 0

MR0B is accessed when the Channel B MR pointer points to MR1.

The pointer is set to MR0 by RESET or by a ‘set pointer’ command

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