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SCC68681 Datasheet(PDF) 11 Page - NXP Semiconductors |
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SCC68681 Datasheet(HTML) 11 Page - NXP Semiconductors |
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11 / 29 page  Philips Semiconductors Product data SCC68681 Dual asynchronous receiver/transmitter (DUART) 2004 Apr 06 11 MR1A – Channel A Mode Register 1 MR1A is accessed when the Channel A MR pointer points to MR1. The pointer is set to MR1 by RESET or by a ‘set pointer’ command applied via CRA. After reading or writing MR1A, the pointer will point to MR2A. MR1A[7] – Channel A Receiver Request-to-Send Control This bit controls the deactivation of the RTSAN output (OP0) by the receiver. This output is normally asserted by setting OPR[0] and negated by resetting OPR[0]. MR1A[7] = 1 causes RTSAN to be negated upon receipt of a valid start bit if the Channel A FIFO is full. However, OPR[0] is not reset and RTSAN will be asserted again when an empty FIFO position is available. This feature can be used for flow control to prevent overrun in the receiver by using the RTSAN output signal to control the CTSN input of the transmitting device. MR1A[6] – Channel A Receiver Interrupt Select This bit selects either the Channel A receiver ready status (RxRDY) or the Channel A FIFO full status (FFULL) to be used for CPU interrupts. It also causes the selected bit to be output on OP4 if it is programmed as an interrupt output via the OPCR. MR1A[5] – Channel A Error Mode Select This bit select the operating mode of the three FIFOed status bits (FE, PE, received break) for Channel A. In the ‘character’ mode, status is provided on a character-by-character basis; the status applies only to the character at the top of the FIFO. In the ‘block’ mode, the status provided in the SR for these bits is the accumulation (logical-OR) of the status for all characters coming to the top of the FIFO since the last ‘reset error’ command for Channel A was issued. MR1A[4:3| – Channel A Parity Mode Select If ‘with parity’ or ‘force parity’ is selected a parity bit is added to the transmitted character and the receiver performs a parity check on incoming data MR1A[4:3] = 11 selects Channel A to operate in the special multidrop mode described in the Operation section. MR1A[2] – Channel A Parity Type Select This bit selects the parity type (odd or even) if the ‘with parity’ mode is programmed by MR1A[4:3], and the polarity of the forced parity bit if the ‘force parity’ mode is programmed. It has no effect if the ‘no parity’ mode is programmed. In the special multidrop mode it selects the polarity of the A/D bit. MR1A[1:0] – Channel A Bits Per Character Select This field selects the number of data bits per character to be transmitted and received. The character length does not include the start, parity, and stop bits. 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 re-transmits the received data. The following conditions are true while in automatic echo mode: 1. Received data is re-clocked and re-transmitted on the TxDA output. 2. The receive clock is used for the transmitter. 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 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. Two diagnostic modes can also be configured. MR2A[7:6] = 10 selects local loopback 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 normally. The second diagnostic mode is the remote loopback mode, selected by MR2A[7:6] = 11. In this mode: 1. Received data is re-clocked and re-transmitted on the TxDA output. 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 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 CAUTION: When the transmitter controls the OP pin (usually used for the RTSN signal) the meaning of the pin is not RTSN at all! Rather, it signals that the transmitter has finished the transmission (i.e., end of block). This bit allows deactivation of the RTSN output by the transmitter. This output is manually asserted and negated by the appropriate |
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