D-56
MPC561/MPC563 Reference Manual
MOTOROLA
Serial Input/Output Port (SIOP)
D.20.3 SIOP Function Performance
Like all TPU3 functions, the performance limit of the SIOP function depends, because of
the operational nature of the scheduler, on the service time (latency) associated with other
active TPU3 channels. Where two channels are used for a uni-directional system and no
other TPU3 channels are active, the maximum baud rate is approximately 230 at a bus
speed of 16.77 MHz. A three-channel bidirectional system under the same conditions has
a maximum baud rate of approximately 200. When more TPU3 channels are active, these
performance figures will be degraded; however, the scheduler assures that the worst-case
latency in any TPU3 application can be closely approximated. TPU3 reference manual
guidelines and information given in the SIOP-state timing table should be used to perform
an analysis on any proposed TPU3 application that appears to approach the TPU’s
performance limits.
D.20.3.1 XFER_SIZE Greater Than 16
XFER_SIZE is normally programmed to be in the 1- to 16-bit range to match the size of
SIOP_DATA, and has thus been shown as a 5-bit value in the host interface diagram.
However, the TPU3 actually uses all 16 bits of the XFER_SIZE parameter when loading
BIT_COUNT. In some unusual circumstances this can be used to an advantage. If an input
device is producing a data stream of greater than 16 bits then manipulation of XFER_SIZE
will allow selective capturing of the data. In clock-only mode, the extended XFER_SIZE
can be used to generate up to 0xFFFF clocks.
D.20.3.2 Data Positioning
As stated above, the TPU3 does not “justify” the data position in SIOP_DATA. Therefore,
in the case of a byte transfer, the data output will be sourced from one byte and the data
input will shift into the other byte. This is true for all data sizes except 16 bits, in which case
the full SIOP_DATA register is used for both data output and input.
Table D-4. SIOP State Timing 1
1 Execution times do not include the time slot transition time (TST = 10 or 14 RCPU clocks).
State Number and Name
Max. RCPU Clock Cycles
Number of RAM Accesses by TPU3
S1 SIOP_INIT
HSQ = X0
X1
28
38
7
S2 DATA_OUT
HSQ = X0
X1
14
24
4
S3 DATA_IN
HSQ = 0X
1X
14
28
4
6