SCC HDLC Mode
MPC885 PowerQUICC Family Reference Manual, Rev. 2
23-4
Freescale Semiconductor
Figure 23-2
shows 16- and 8-bit address recognition.
Figure 23-2. HDLC Address Recognition
0x46
MFLR
Hword
Max frame length register. The HDLC compares the incoming HDLC frame’s length
with the user-defined limit in MFLR. If the limit is exceeded, the rest of the frame is
discarded and RxBD[LG] is set in the last BD of that frame. At the end of the frame
the SCC reports frame status and frame length in the last RxBD. The MFLR is
defined as all in-frame bytes between the opening and closing flags.
0x48
MAX_CNT
Hword
Maximum length counter. A temporary down-counter used to track frame length.
0x4A
RFTHR
Hword
Received frames threshold. Used to reduce potential interrupt overhead when
each in a series of short HDLC frames causes an SCCE[RXF] event. Setting
RFTHR determines the frequency of RXF interrupts, which occur only when the
RFTHR limit is reached. Provide enough empty RxBDs for the number of frames
specified in RFTHR.
0x4C
RFCNT
Hword
Received frames count. RFCNT is a down-counter used to implement RFTHR.
0x4E
HMASK
Hword
Mask register (HMASK) and four address registers (HADDR
n) for address
recognition. The SCC reads the frame address from the HDLC receiver, compares
it with the HADDRs, and masks the result with HMASK. Setting an HMASK bit
enables the corresponding comparison bit, clearing a bit masks it. When a match
occurs, the frame address and data are written to the buffers. When no match
occurs and a frame is error-free, the nonmatching address received counter
(NMARC) is incremented.
The eight low-order bits of HADDR
n should contain the first address byte after the
opening flag. For example, to recognize a frame that begins 0x7E (flag), 0x68,
0xAA, using 16-bit address recognition, HADDR
n should contain 0xAA68 and
HMASK should contain 0xFFFF. For 8-bit addresses, clear the eight high-order
HMASK bits. See Figure 23-2..
0x50
HADDR1
Hword
0x52
HADDR2
Hword
0x54
HADDR3
Hword
0x56
HADDR4
Hword
0x58
TMP
Hword
Temporary storage.
0x5A
TMP_MB
Hword
Temporary storage.
1
From SCC base. SCC base = IMMR + 0x3D00 (SCC2) or 0x3E00 (SCC3) or 0x3F00 (SCC4)
Table 23-1. HDLC-Specific SCC Parameter RAM Memory Map (continued)
Offset
1
Name
Width
Description
Flag
0x7E
etc.
Flag
0x7E
Address
0x68
Address
0xAA
Control
0x44
etc.
Address
0x55
Control
0x44
16-Bit Address Recognition
8-Bit Address Recognition
0x00FF
HMASK
0xXX55
HADDR1
0xXX55
HADDR2
0xXX55
HADDR3
0xXX55
HADDR4
0xFFFF
HMASK
0xAA68
HADDR1
0xFFFF
HADDR2
0xAA68
HADDR3
0xAA68
HADDR4
Recognizes one 16-bit address (HADDR1) and
the 16-bit broadcast address (HADDR2)
Recognizes a single 8-bit address (HADDR1)
Summary of Contents for PowerQUICC MPC870
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