HCS08 datasheet(176/290 Pages) MOTOROLA

176 / 290 page

Serial Communications Interface (SCI) Module

MC9S08GB/GT

176

MOTOROLA

If no new character is waiting in the transmit data buffer after a stop bit is shifted out the TxD pin, the

transmitter sets the transmit complete flag and enters an idle mode, with TxD high, waiting for more

characters to transmit.

Writing 0 to TE does not immediately release the pin to be a general-purpose I/O pin. Any transmit activity

that is in progress must first be completed. This includes data characters in progress, queued idle

characters, and queued break characters.

11.5.2 Send Break and Queued Idle

The SBK control bit in SCIxC2 is used to send break characters which were originally used to gain the

attention of old teletype receivers. Break characters are a full character time of logic 0 (including a 0 where

the stop bit would be normally). Normally, a program would wait for TDRE to become set to indicate the

last character of a message has moved to the transmit shifter, then write 1 and then write 0 to the SBK bit.

This action queues a break character to be sent as soon as the shifter is available. If SBK is still 1 when the

queued break moves into the shifter (synchronized to the baud rate clock), an additional break character is

queued. If the receiving device is another Motorola SCI, the break characters will be received as 0s in all

eight (or nine) data bits and a framing error (FE = 1).

When idle-line wakeup is used, a full character time of idle (logic 1) is needed between messages to wake

up any sleeping receivers. Normally, a program would wait for TDRE to become set to indicate the last

character of a message has moved to the transmit shifter, then write 0 and then write 1 to the TE bit. This

action queues an idle character to be sent as soon as the shifter is available. As long as the character in the

shifter does not finish while TE = 0, the SCI transmitter never actually releases control of the TxD pin. If

there is a possibility of the shifter finishing while TE = 0, set the general-purpose I/O controls so the pin

that is shared with TxD is an output driving a logic 1. This ensures that the TxD line will look like a normal

idle line even if the SCI loses control of the port pin between writing 0 and then 1 to TE.

11.6 Receiver Functional Description

In this section, the receiver block diagram (Figure 11-4) is used as a guide for the overall receiver

functional description. Next, the data sampling technique used to reconstruct receiver data is described in

more detail. Finally, two variations of the receiver wakeup function are explained.

11.6.1 Receiver Block Diagram

Figure 11-4 shows the receiver portion of the SCI.

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