Communication Processor Module
16-172
MPC823 USER’S MANUAL
MOTOROLA
SCC2
COMMUNICATION
16
PROCESSOR
MODULE
The TOD bit does not need to be set if a new TX buffer descriptor is added to the circular
queue or if other TX buffer descriptors in that queue have not finished transmitting. Once
they are finished, however, the new TX buffer descriptors are processed. The first bit of the
frame will typically be clocked out 5-6 bit times after the TOD bit has been set to 1.
TOD—Transmit on Demand
0 = Normal operation.
1 = The RISC microcontroller gives high priority to the current TX buffer descriptor and
does not wait the normal polling time to check that the TX buffer descriptor’s R bit
has been set. Instead, it begins transmitting the frame. This bit is automatically
cleared after one serial clock.
Bits 1–15—Reserved
These bits are reserved and should be set to 0.
16.9.6 SCC2 Buffer Descriptor Operation
Data associated with the serial communication controller channel transmitter and receiver
is stored in buffers and each buffer is referenced by a buffer descriptor that can be located
anywhere in internal memory. The MPC823 internal memory has enough space for 224
buffer descriptors (BDs) to be shared between the universal serial bus, serial
communication controller, serial managment controllers, serial peripheral interface, and I2C
controller. However, you must define how the buffer descriptors will be allocated to the serial
channel’s transmitter or receiver. You can select 100 buffer descriptors for the SCC2
receiver or 20 buffer descriptors for the transmitter.
The buffer descriptor table forms a circular queue with a programmable length. You can
program the start address of each channel buffer descriptor table in the internal memory. In
addition, you can allocate the parameter area of an unused channel to the other used
channels as buffer descriptor tables or actual buffers. See
Figure 16-63 for details.
TODR
BIT
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
FIELD
TOD
RESERVED
RESET
00
R/W
R/W
ADDR
0xA2C
Note: The communication processor module sets all the status bits in these buffer
descriptors. You should clear all the status bits before submitting the buffer
descriptor to the communication processor module. For example, the parity error
bit is only set when a parity error occurs.