MC68000 MOTOROLA [Motorola, Inc], MC68000 Datasheet - Page 191

• Manufacturer Part Number: MC68000
• Manufacturer: MOTOROLA [Motorola, Inc]

ENTER HUNT MODE Command
4.5.12.6 HDLC Address Recognition
Each HDLC controller has five 16-bit registers for address recognition: one mask register
and four address registers (HMASK, HADDR1, HADDR2, HADDR3, and HADDR4). The
HDLC controller reads the frame's address from the HDLC receiver, checks it against the
four address register values, and then masks the result with the user-defined HMASK. A one
in HMASK represents a bit position for which address comparison should occur; a zero rep-
resents a masked bit position. Upon an address match, the address and the data following
are written into the data buffers. When the addresses are not matched and the frame is error
free, the nonmatching address received counter (NMARC) is incremented.
Examples of 16- and 8-bit HDLC address recognition are shown in Figure 4-25.
4.5.12.7 HDLC Maximum Frame Length Register (MFLR)
The HDLC controller checks the length of an incoming HDLC frame against the user-defined
value given in this 16-bit register. If this limit is exceeded, the remainder of the incoming
HDLC frame is discarded, and the LG (Rx frame too long) bit is set in the last BD belonging
to that frame. The HDLC controller waits to the end of the frame and reports the frame status
MOTOROLA
After a hardware or software reset and the enabling of the channel by its SCC mode reg-
ister, the channel is in the receive enable mode and will use the first BD in the table.
The ENTER HUNT MODE command is generally used to force the HDLC receiver to abort
reception of the current frame, generate an RXB interrupt (if enabled) as the buffer is
closed, and enter the hunt mode. In the hunt mode, the HDLC controller continually scans
the input data stream for the flag sequence. After receiving the command, the current re-
ceive buffer is closed, and the CRC is reset. Further frame reception will use the next BD.
If an enabled receiver has been disabled by clearing ENR in the SCC mode register), the
ENTER HUNT MODE command must be given to the channel before setting ENR again.
Subsequent frames will then be received, starting with the next BD.
FLAG
$7E
RECOGNIZES ONE 16-BIT ADDRESS (HADDR1) AND
ADDRESS
THE 16-BIT BROADCAST ADDRESS (HADDR2).
HADDR1
HADDR2
HADDR3
HADDR4
For 8-bit addresses, mask out the eight high-order bits in the
HMASK register.
HMASK
$68
16-BIT ADDRESS RECOGNITION
Figure 4-25. HDLC Address Recognition Examples
ADDRESS
$AA
$FFFF
$AA68
$AA68
$AA68
$FFFF
CONTROL
MC68302 USER’S MANUAL
$44
NOTE
ETC.
FLAG
$7E
RECOGNIZES A SINGLE 8-BIT ADDRESS (HADDR1).
HADDR2
HADDR3
HADDR4
HADDR1
HMASK
Communications Processor (CP)
8-BIT ADDRESS RECOGNITION
ADDRESS
$55
$XX55
$XX55
$XX55
$XX55
$00FF
CONTROL
$44
ETC.
4-71