SERIAL COMMUNICATION INTERFACE (SCI)
6 - 20
PORT C
MOTOROLA
forced high (idle). Data present in the SCI transmit data register (STX) will not be trans-
mitted. STX may be written and TDRE will be cleared, but the data will not be transferred
into the shift register. TE does not inhibit TDRE or transmit interrupts. TE is cleared by a
hardware and software reset.
Setting TE will cause the transmitter to send a preamble of 10 or 11 consecutive ones (de-
pending on WDS). This procedure gives the programmer a convenient way to ensure that
the line goes idle before starting a new message. To force this separation of messages
by the minimum idle line time, the following sequence is recommended:
1. Write the last byte of the first message to STX
2. Wait for TDRE to go high, indicating the last byte has been transferred to the
transmit shift register
3. Clear TE and set TE back to one. This queues an idle line preamble to imme-
diately follow the transmission of the last character of the message (including
the stop bit)
4. Write the first byte of the second message to STX
In this sequence, if the first byte of the second message is not transferred to the STX prior
to the finish of the preamble transmission, then the transmit data line will simply mark idle
until STX is finally written.
6.3.2.1.9
SCR Idle Line Interrupt Enable (ILIE) Bit 10
When ILIE is set, the SCI interrupt occurs when IDLE is set. When ILIE is clear, the IDLE
interrupt is disabled. ILIE is cleared by hardware and software reset.
An internal flag, the shift register idle interrupt (SRIINT) flag, is the interrupt request to the
interrupt controller. SRIINT is not directly accessible to the user.
When a valid start bit has been received, an idle interrupt will be generated if both IDLE
(SCI Status Register bit 3) and ILIE equals one. The idle interrupt acknowledge from the
interrupt controller clears this interrupt request. The idle interrupt will not be asserted
again until at least one character has been received. The result is as follows:
1. The IDLE bit shows the real status of the receive line at all times.
2. Idle interrupt is generated once for each idle state, no matter how long the idle
state lasts.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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