Scc Hdlc Receive Buffer Descriptor (Rxbd) - Freescale Semiconductor MPC850 User Manual

SCC HDLC Receive Buffer Descriptor (RxBD)

Table 23-6. PSMR HDLC Field Descriptions (Continued)
Bits Name
6 RTE Retransmit enable.
0 No retransmission.
1 Automatic frame retransmission is enabled. Particularly useful in the HDLC bus protocol and ISDN
applications where multiple HDLC controllers can collide. Note that retransmission occurs only if a
lost CTS occurs on the first or second buffer of the frame.
7 — Reserved, should be cleared.
8 FSE Flag sharing enable. Valid only if GSMR_H[RTSM] = 1. Can be modified on-the-fly.
0 Normal operation.
1 If NOF[0–3] = 0b0000, a single shared flag is sent between back-to-back frames. Other values of
NOF[0–3] are decremented by 1. Useful in signaling system #7 applications.
9 DRT Disable receiver while transmitting.
0 Normal operation.
1 As the SCC sends data, the receiver is disabled and gated by the internal RTS. This helps if the
HDLC channel is on a multidrop line and the SCC does not need to receive its own transmission.
10 BUS HDLC bus mode.
0 Normal HDLC operation.
1 HDLC bus operation is selected. See Section 23.14, "HDLC Bus Mode with Collision Detection."
11 BRM HDLC bus RTS mode. Valid only if BUS = 1. Otherwise, it is ignored.
0 Normal RTS operation during HDLC bus mode. RTS is asserted on the first bit of the Tx frame and
negated after the first collision bit is received.
1 Special RTS operation during HDLC bus mode. RTS is delayed by one bit with respect to the
normal case, which helps when the HDLC bus protocol is being run locally and sent over a
long-distance line at the same time. The one-bit delay allows RTS to be used to enable the
transmission line buffers so that the electrical effects of collisions are not sent over the transmission
line.
12 MFF Multiple frames in Tx FIFO. The receiver is not affected.
0 Normal operation. The Tx FIFO must never contain more than one HDLC frame. The CTS lost
status is reported accurately on a per-frame basis.
1 The Tx FIFO can hold multiple frames, but lost CTS may not be reported on the buffer/frame it
occurred on. This can improve performance of HDLC transmissions of small back-to-back frames or
when the number of flags between frames should be limited.
13–15 — Reserved, should be cleared.
23.9 SCC HDLC Receive Buffer Descriptor (RxBD)
The CPM uses the RxBD, shown in Figure 23-4, to report on data received for each buffer.
0 1
E
Offset + 0 —
Offset + 2
Offset + 4
Offset + 6
Figure 23-4. SCC HDLC Receive Buffer Descriptor (RxBD)
2 3 4 5
W I CM
L F
MPC850 Family User's Manual
Description
6 7 8 9 10
— DE — LG
Data Length
Rx Buffer Pointer
11 12 13 14
NO AB CR OV
15
CD