MC68000 MOTOROLA [Motorola, Inc], MC68000 Datasheet - Page 157

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

4.5.6.5 Other General Parameters
Additional parameters are listed in Table 4-2. These parameters do not need to be accessed
by the user in normal operation, and are listed only because they may provide helpful infor-
mation for experienced users and for debugging.
The Rx and Tx internal data pointers are updated by the SDMA channels to show the next
address in the buffer to be accessed.
The Tx byte count is a down-count value that is initialized with the Tx BD data length and
decremented with every byte read by the SDMA channels. The Rx byte count is a down-
count value that is initialized with the MRBLR value and decremented with every byte written
by the SDMA channels.
The Rx internal state, Tx internal state, Rx temp, Tx temp, and reserved areas are for RISC
use only.
4.5.7 SCC Initialization
The SCCs require a number of registers and parameters to be configured after a power-on
reset. The following is a proper sequence for initializing the SCCs, regardless of the protocol
used.
MOTOROLA
1. If SCC2 or SCC3 is used, write the parallel port A and B control registers (PACNT and
2. Write SIMODE to configure the serial channels physical interface for the three SCCs
PBCNT) to configure pins as parallel I/O lines or peripheral functions as needed (see
3.3 Parallel I/O Ports).
1.
2.
3.
The Rx byte count, Rx internal data pointer, and RBD# can be
used to extract data out of a receive buffer before the buffer is
completely full. However, the use of this technique is not recom-
mended unless no other solution to the application requirement
can be found! The IMP was specifically designed to eliminate the
need for this technique by allowing a programmable receive
buffer size for each SCC, by closing buffers immediately upon
an error, and by closing a receive buffer after a user-program-
mable line idle period in the case of UART mode. Having consid-
ered these capabilities, a user desiring to extract data from a
partially full data buffer should note the following cautions:
The Rx byte count and Rx internal data pointer may not be
valid before the first byte has been written to the buffer or af-
ter the last byte has been written to the buffer.
The parameters, Rx byte count, Rx internal data pointer,
and RBD#, are not updated simultaneously.
The RBD# and the empty bit of the Rx BD are not updated
simultaneously.
MC68302 USER’S MANUAL
NOTE
Communications Processor (CP)
4-37