Communications Processor (CP)
MOTOROLA
MC68302 USER’S MANUAL
4-35
# Initialized by the user (M68000 core).
##
Modified by the CP following a CP or system reset.
Certain parameter RAM values need to be initialized by the user before the SCC is enabled.
Those values not so designated are initialized/written by the CP. Once initialized, most pa-
rameter RAM values will not need to be accessed in user software since most of the activity
is centered around the transmit and receive buffer descriptors, not the parameter RAM.
However, if the parameter RAM is accessed by the user, the following should be noted. The
parameter RAM can be read at any time. The parameter RAM values related to the SCC
transmitted can only be written 1) whenever the ENT bit in the SCM is zero or 2) after a
STOP TRANSMIT command and before a RESTART TRANSMIT command. The parame-
ter RAM values related to the SCC receiver can only be written 1) whenever the ENR bit in
the SCM is zero or 2) if the receiver has previously been enabled, after the ENTER HUNT
MODE command and before the ENR bit is set. See 4.5.10 Disabling the SCCs for a dis-
cussion of when the SCC registers may be changed.
The registers (see Table 4-6) that typically need to be accessed by the user are described
in the following paragraphs.
4.5.6.1 Data Buffer Function Code Register (TFCR, RFCR)
This register defines the address space of the receive (RFCR) and transmit (TFCR) data
buffers. These registers must be initialized if the SCC is used.
NOTE
The value of the function code register for any channel may be
equal to that of any other, but do not initialize FC2–FC0 with the
value “111” which causes a conflict with the interrupt acknowl-
edge cycle to occur.
Table 4-6. SCC Parameter RAM Memory Map
Address
Name
Width
Description
SCC Base + 80 #
SCC Base + 81 #
SCC Base + 82 #
SCC Base + 84 ##
SCC Base + 86 ##
SCC Base + 87 ##
SCC Base + 88
SCC Base + 8C
SCC Base + 8E
SCC Base + 90 ##
SCC Base + 92 ##
SCC Base + 93 ##
SCC Base + 94
SCC Base + 98
SCC Base + 9A
SCC Base + 9C
SCC Base + BF
RFCR
TFCR
MRBLR
RBD#
TBD#
Byte
Byte
Word
Word
Byte
Byte
2 Words
Word
Word
Word
Byte
Byte
2 Words
Word
Word
Rx Function Code
Tx Function Code
Maximum Rx Buffer Length
Rx Internal State
Reserved
Rx Internal Buffer Number
Rx Internal Data Pointer
Rx Internal Byte Count
Rx Temp
Tx Internal State
Reserved
Tx Internal Buffer Number
Tx Internal Data Pointer
Tx Internal Byte Count
Tx Temp
First Word of Protocol-Specific Area
Last Word of Protocol-Specific Area
7
0
6
5
4
3
0
2
0
1
0
0
0
FC2
FC1
FC0