HCS08 datasheet(217/316 Pages) NXP

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Serial Communications Interface (S08SCIV4)

MC9S08SG8 MCU Series Data Sheet, Rev. 8

Freescale Semiconductor

217

14.3.5.2

Stop Mode Operation

During all stop modes, clocks to the SCI module are halted.

In stop1 and stop2 modes, all SCI register data is lost and must be re-initialized upon recovery from these

two stop modes. No SCI module registers are affected in stop3 mode.

The receive input active edge detect circuit is still active in stop3 mode, but not in stop2. . An active edge

on the receive input brings the CPU out of stop3 mode if the interrupt is not masked (RXEDGIE = 1).

Note, because the clocks are halted, the SCI module will resume operation upon exit from stop (only in

stop3 mode). Software should ensure stop mode is not entered while there is a character being transmitted

out of or received into the SCI module.

14.3.5.3

Loop Mode

When LOOPS = 1, the RSRC bit in the same register chooses between loop mode (RSRC = 0) or

single-wire mode (RSRC = 1). Loop mode is sometimes used to check software, independent of

connections in the external system, to help isolate system problems. In this mode, the transmitter output is

internally connected to the receiver input and the RxD pin is not used by the SCI, so it reverts to a

general-purpose port I/O pin.

14.3.5.4

Single-Wire Operation

When LOOPS = 1, the RSRC bit in the same register chooses between loop mode (RSRC = 0) or

single-wire mode (RSRC = 1). Single-wire mode is used to implement a half-duplex serial connection.

The receiver is internally connected to the transmitter output and to the TxD pin. The RxD pin is not used

and reverts to a general-purpose port I/O pin.

In single-wire mode, the TXDIR bit in SCIC3 controls the direction of serial data on the TxD pin. When

TXDIR = 0, the TxD pin is an input to the SCI receiver and the transmitter is temporarily disconnected

from the TxD pin so an external device can send serial data to the receiver. When TXDIR = 1, the TxD pin

is an output driven by the transmitter. In single-wire mode, the internal loop back connection from the

transmitter to the receiver causes the receiver to receive characters that are sent out by the transmitter.

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