SCC Asynchronous HDLC Mode and IrDA
MPC885 PowerQUICC Family Reference Manual, Rev. 2
25-8
Freescale Semiconductor
Table 25-6
describes reception errors.
25.13 SCC Asynchronous HDLC Registers
The following sections describe the SCC registers when in asynchronous HDLC mode.
25.13.1 Asynchronous HDLC Event Register (SCCE)/Asynchronous HDLC
Mask Register (SCCM)
The SCC event register (SCCE) is used as the asynchronous HDLC event register to generate interrupts
and report events recognized by the asynchronous HDLC channel. When an event is recognized, the
asynchronous HDLC controller sets the corresponding SCCE bit. Interrupts can be masked by clearing the
appropriate bit in the asynchronous HDLC mask register (SCCM). SCCE bits, shown in
Figure 25-4
, are
cleared by writing ones—writing zeros has no effect. Unmasked SCCE bits must be cleared before the
CPM clears the internal interrupt request.
Table 25-6. Receive Errors
Error Description
Overrun
All SCCs have 16-byte Rx FIFOs. Overrun occurs when the CP cannot keep up with the data rate
or the SDMA channel cannot write the received data to memory. The previous data byte and frame
status are lost. The controller closes the buffer and sets RxBD[OV] and SCCE[RXF]. The receiver
then looks for the next frame.
CD Lost during
Frame
Reception
The channel stops receiving frames, closes the buffer, and sets SCCE[RXF] and RxBD[CD]. This
error has highest priority. The rest of the frame is lost and other errors are not checked in that frame.
The receiver then searches for the next frame once CD is reasserted.
Abort Sequence When an abort sequence (0x7D, 0x7E for PPP; 0x7D, 0xC1 for IrLAP) is detected, the channel
closes the buffer by setting SCCE[RXF] and RxBD[AB]. CRC error status is not checked on aborted
frames. If no frame is being received, the next BD is opened and then closed with RxBD[AB] set.
CRC
The channel writes the received cyclic redundancy check to the buffer, closes the buffer, and sets
SCCE[RXF] and RxBD[CR]. After receiving this error, the receiver prepares to receive the next
frame.
Break Sequence
Received
The receiver detected the first character in a break sequence. The channel closes the buffer and
sets SCCE[RXF] and RxBD[BRK]. CRC error status is not checked. SCCE[BRKS] is set when the
first break of a sequence is found; SCCE[BRKE] is set when an idle bit is received after a break
sequence.
0
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Field
—
GLR
GLT
—
IDL
—
BRKE BRKS
TXE
RXF
BSY
TXB
RXB
Reset
0
R/W
R/W
Addr
0xA30 (SCCE2)/0xA34 (SCCM2) 0xA50 (SCCE3)/0xA54 (SCCM3); 0xA70 (SCCE4)/0xA74 (SCCM4)
Figure 25-4. Asynchronous HDLC Event Register (SCCE)/Asynchronous HDLC Mask Register (SCCM)
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