mc68hc916y3 Freescale Semiconductor, Inc, mc68hc916y3 Datasheet - Page 262

• Manufacturer Part Number: mc68hc916y3
• Description: Mc68hc16y3 16 Bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

11.4.3.6 Receiver Operation
11-30
MOTOROLA
Both TDRE and TC have associated interrupts. The interrupts are enabled by the
transmit interrupt enable (TIE) and transmission complete interrupt enable (TCIE) bits
in SCCR1. Service routines can load the last byte of data in a sequence into SCDR,
then terminate the transmission when a TDRE interrupt occurs.
The RE bit in SCCR1 enables (RE = 1) and disables (RE = 0) the receiver. The
receiver contains a receive serial shifter and a parallel receive data register (RDR) lo-
cated in the SCI data register (SCDR). The serial shifter cannot be directly accessed
by the CPU16. The receiver is double-buffered, allowing data to be held in RDR while
other data is shifted in.
Receiver bit processor logic drives a state machine that determines the logic level for
each bit-time. This state machine controls when the bit processor logic is to sample
the RXD pin and also controls when data is to be passed to the receive serial shifter.
A receive time clock is used to control sampling and synchronization. Data is shifted
into the receive serial shifter according to the most recent synchronization of the re-
ceive time clock with the incoming data stream. From this point on, data movement is
synchronized with the MCU system clock. Operation of the receiver state machine is
detailed in the QSM Reference Manual (QSMRM/AD).
The number of bits shifted in by the receiver depends on the serial format. However,
all frames must end with at least one stop bit. When the stop bit is received, the frame
is considered to be complete, and the received data in the serial shifter is transferred
to RDR. The receiver data register flag (RDRF) is set when the data is transferred.
Noise errors, parity errors, and framing errors can be detected while a data stream is
being received. Although error conditions are detected as bits are received, the noise
flag (NF), the parity flag (PF), and the framing error (FE) flag in SCSR are not set until
data is transferred from the serial shifter to RDR.
RDRF must be cleared before the next transfer from the shifter can take place. If
RDRF is set when the shifter is full, transfers are inhibited and the overrun error (OR)
flag in SCSR is set. OR indicates that RDR needs to be serviced faster. When OR is
set, the data in RDR is preserved, but the data in the serial shifter is lost. Because
framing, noise, and parity errors are detected while data is in the serial shifter, FE, NF,
and PF cannot occur at the same time as OR.
When the CPU16 reads SCSR and SCDR in sequence, it acquires status and data,
and also clears the status flags. Reading SCSR acquires status and arms the clearing
mechanism. Reading SCDR acquires data and clears SCSR.
When RIE in SCCR1 is set, an interrupt request is generated whenever RDRF is set.
Because receiver status flags are set at the same time as RDRF, they do not have
separate interrupt enables.
Freescale Semiconductor, Inc.
For More Information On This Product,
QUEUED SERIAL MODULE
Go to: www.freescale.com
MC68HC16Y3/916Y3
USER’S MANUAL