
28. Serial Communications Interface (SCIc, SCId)
28.4
Multi-Processor Communications Function
Using the multi-processor communication functions enables to transmit and receive data by sharing a communication
line between multiple processors by using asynchronous serial communication in which the multi-processor bit is added.
In multi-processor communication, a unique ID code is allocated to each receiving station. Serial communication cycles
consist of an ID transmission cycle to specify the receiving station and a data transmission cycle to transmit data to the
specified receiving station. The multi-processor bit is used to distinguish between the ID transmission cycle and the data
transmission cycle. When the multi-processor bit is set to 1, it indicates the ID transmission cycle and when the multi-
processor bit is set to 0, it indicates the data transmission cycle.
Figure 28.14 shows an example of communication
between processors by using a multi-processor format. First, a transmitting station transmits communication data in
which the multi-processor bit set to 1 is added to the ID code of the receiving station. Next, the transmitting station
transmits the communication data in which the multi-processor bit set to 0 is added to the transmission data. Upon
receiving the communication data in which the multi-processor bit is set to 1, the receiving station compares the received
ID with the ID of the receiving station itself and if the two match, receives the communication data that is subsequently
transmitted. If the received ID does not match with the ID of the receiving station, the receiving station skips the
communication data until again receiving the communication data in which the multi-processor bit is set to 1.
For supporting this function, the SCI provides the MPIE bit in SCR. When the MPIE bit is set to 1, transfer of receive
data from the RSR to the RDR, detection of a reception error, and setting the respective status flags ORER and FER in
SSR are disabled until reception of data in which the multi-processor bit is set to 1. Upon receiving a reception character
in which the multi-processor bit is set to 1, the MPBT bit in SSR is set to 1 and the MPIE bit in SCR is automatically
cleared, thus returning to a normal reception operation. During this time, an RXI interrupt is generated if the RIE bit in
SCR is set.
When the multi-processor format is specified, specification of the parity bit is disabled. Apart from this, there is no
difference from the operation in the normal asynchronous mode. A clock which is used for the multi-processor
communication is also the same as the clock used in the normal asynchronous mode.
Figure 28.14
An Example of Communication using the Multi-Processor Format
(Example of Transmission of Data AAh to Receiving Station A)
Transmitting
station
Receiving
station A
(ID = 01)
Receiving
station B
Receiving
station C
Receiving
station D
Communication line
Serial data
ID transmission cycle =
specification of a receiving station
Data transmission cycle = data
transmission to the receiving
station specified by ID
(MPB = 0)
01h
AAh
MPB: Multi-processor bit
(ID = 02)
(ID = 03)
(ID = 04)
(MPB = 1)