
R8C/3GM Group
24. Synchronous Serial Communication Unit (SSU)
R01UH0284EJ0100 Rev.1.00
Aug 09, 2011
24.4.2
Data Transmission
synchronous serial communication unit operates as described below (The data transfer length can be set from 8
to 16 bits using the SSBR register).
When synchronous serial communication unit is set as a master device, it outputs a synchronous clock and data.
When synchronous serial communication unit is set as a slave device, it outputs data synchronized with the
input clock.
When the TE bit is set to 1 (transmit enabled) before writing the transmit data to the SSTDR register, the TDRE
bit is automatically set to 0 (data not transferred from registers SSTDR to SSTRSR) and the data is transferred
from registers SSTDR to SSTRSR.
After the TDRE bit is set to 1 (data transferred from registers SSTDR to SSTRSR), transmission starts. When
the TIE bit in the SSER register is set to 1, the TXI interrupt request is generated. When one frame of data is
transferred while the TDRE bit is set to 0, data is transferred from registers SSTDR to SSTRSR and
transmission of the next frame is started. If the 8th bit is transmitted while the TDRE bit is set to 1, the TEND
bit in the SSSR register is set to 1 (the TDRE bit is set to 1 when the last bit of the transmit data is transmitted)
and the state is retained. The TEI interrupt request is generated when the TEIE bit in the SSER register is set to
1 (transmit-end interrupt request enabled). The SSCK pin is fixed “H” after transmit-end.
Transmission cannot be performed while the ORER bit in the SSSR register is set to 1 (overrun error). Confirm
that the ORER bit is set to 0 before transmission.
Figure 24.5
Example of Synchronous Serial Communication Unit Operation for Data Transmission
(Clock Synchronous Communication Mode, 8-Bit SSU Data Transfer Length)
SSCK
b0
SSO
SSUMS = 0 (clock synchronous communication mode), CPHS = 0 (data change at
odd numbers), CPOS = 0 (“H” when clock stops), and BS3 to BS0 = 1000b (8 bits)
b1
b7
b0
b1
b7
1 frame
TDRE bit in
SSSR register
TEND bit in
SSSR register
TEI interrupt request
generation
Write data to SSTDR register
Processing
by program
1 frame
TXI interrupt request generation
BS0 to BS3: Bits in SSBR register
CPHS, CPOS: Bits in SSMR register
SSUMS: Bit in SSMR2 register