Communications Processor (CP)
4-114
MC68302 USER’S MANUAL
MOTOROLA
Data Length
The data length is the number of octets that the DDCMP controller should transmit from
this BD's data buffer. It is never modified by the CP. The data length should be greater
than zero.
Tx Buffer Pointer
This pointer, which contains the address of the associated data buffer, may be even or
odd. The buffer may reside in either internal or external memory.
NOTE
For correct operation of the function codes, the upper 8 bits of
the pointer must be initialized to zero.
4.5.14.12 DDCMP Event Register
The SCC event register (SCCE) is referred to as the DDCMP event register when the SCC
is configured for DDCMP. It is an 8-bit register used to report events recognized by the DDC-
MP channel and to generate interrupts. On recognition of an event, the DDCMP controller
sets its corresponding bit in this register. Interrupts generated by this register may be
masked in the DDCMP mask register.
The DDCMP event register is a memory-mapped register that may be read at any time. A
bit is cleared by writing a one (writing zero does not affect a bit's value). More than one bit
may be cleared at a time. All unmasked bits must be cleared before the CP will clear the
internal interrupt request. This register is cleared at reset.
CTS—Clear-To-Send Status Changed
A change in the status of the CTS line was detected on the DDCMP channel. The SCC
status register may be read to determine the current status.
CD—Carrier Detect Status Changed
A change in the status of the CD line was detected on the DDCMP channel. The SCC sta-
tus register may be read to determine the current status.
Bit 5—Reserved for future use.
TXE—Tx Error
An error (CTS lost or underrun) occurred on the transmitter channel.
RBK—Receive Block
A complete block has been received on the DDCMP channel. A block is defined as recep-
tion of a complete header, a complete message, or a receiver error condition.
BSY—Busy Condition
A data byte was received and discarded due to lack of buffers. The receiver will enter hunt
mode automatically.
7
6
5
4
3
2
1
0
CTS
CD
—
TXE
RBK
BSY
TX
RBD