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MCF51QE128 MCU Series Reference Manual, Rev. 3
Freescale Semiconductor 309
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15.3.3.2.2 Address-Mark Wakeup
When wake is set, the receiver is configured for address-mark wakeup. In this mode, RWU is cleared
automatically when the receiver detects a logic 1 in the most significant bit of a received character (eighth
bit when M is cleared and ninth bit when M is set).
Address-mark wakeup allows messages to contain idle characters, but requires the msb be reserved for use
in address frames. The logic 1 msb of an address frame clears the RWU bit before the stop bit is received
and sets the RDRF flag. In this case, the character with the msb set is received even though the receiver
was sleeping during most of this character time.
15.3.4 Interrupts and Status Flags
The SCI system has three separate interrupt vectors to reduce the amount of software needed to isolate the
cause of the interrupt. One interrupt vector is associated with the transmitter for TDRE and TC events.
Another interrupt vector is associated with the receiver for RDRF, IDLE, RXEDGIF, and LBKDIF events.
A third vector is used for OR, NF, FE, and PF error conditions. Each of these ten interrupt sources can be
separately masked by local interrupt enable masks. The flags can be polled by software when the local
masks are cleared to disable generation of hardware interrupt requests.
The SCI transmitter has two status flags that can optionally generate hardware interrupt requests. Transmit
data register empty (TDRE) indicates when there is room in the transmit data buffer to write another
transmit character to SCIxD. If the transmit interrupt enable (TIE) bit is set, a hardware interrupt is
requested when TDRE is set. Transmit complete (TC) indicates that the transmitter is finished transmitting
all data, preamble, and break characters and is idle with TxD at the inactive level. This flag is often used
in systems with modems to determine when it is safe to turn off the modem. If the transmit complete
interrupt enable (TCIE) bit is set, a hardware interrupt is requested when TC is set. Instead of hardware
interrupts, software polling may be used to monitor the TDRE and TC status flags if the corresponding
TIE or TCIE local interrupt masks are cleared.
When a program detects that the receive data register is full (RDRF = 1), it gets the data from the receive
data register by reading SCIxD. The RDRF flag is cleared by reading SCIxS1 while RDRF is set and then
reading SCIxD.
When polling is used, this sequence is naturally satisfied in the normal course of the user program. If
hardware interrupts are used, SCIxS1 must be read in the interrupt service routine (ISR). Normally, this is
done in the ISR anyway to check for receive errors, so the sequence is automatically satisfied.
The IDLE status flag includes logic that prevents it from getting set repeatedly when the RxD line remains
idle for an extended period of time. IDLE is cleared by reading SCIxS1 while IDLE is set and then reading
SCIxD. After IDLE has been cleared, it cannot become set again until the receiver has received at least
one new character and has set RDRF.
If the associated error was detected in the received character that caused RDRF to be set, the error flags
— noise flag (NF), framing error (FE), and parity error flag (PF) — are set at the same time as RDRF.
These flags are not set in overrun cases.