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SE R IAL I/ O
7751 Group User’s Manual
7–18
7.3 Clock synchronous serial I/O mode
7.3.1 Transfer clock (synchronizing clock)
Data transfer is performed synchronously with the transfer clock. For the transfer clock, the user can select
whether to generate the transfer clock internally or to input it from an external.
The transfer clock is generated by operation of the transmit control circuit. Accordingly, even when performing
only reception, set the transmit enable bit to “1,” and set dummy data in the UARTi transmit buffer register
in order to make the transmit control circuit active.
(1)
Generating transfer clock internally
The count source selected with the BRG count source select bits is divided by the BRGi, and its
BRGi output is further divided by 2. This is the transfer clock. The transfer clock is output from the
CLK
i
pin.
[Setting relevant registers]
Select an internal clock (bit 3 at addresses 30
16
, 38
16
= “0”).
Select the BRGi’s count source (bits 0 and 1 at addresses 34
16
, 3C
16
)
Set “divide value – 1” (= n; 00
16
to FF
16
) to the BRGi (addresses 31
16
, 39
16
).
Transfer clock frequency =
Enable transmission (bit 0 at addresses 35
16
, 3D
16
= “1”).
Set data to the UARTi transmit buffer register (addresses 32
16
, 3A
16
)
[Pin’s state]
A transfer clock is output from the CLK
i
pin.
Serial data is output from the TxDi pin. (Dummy data is output when performing only reception.)
(2)
Inputting transfer clock from an external
A clock input from the CLK
i
pin is the transfer clock.
[Setting relevant registers]
Select an external clock (bit 3 at addresses 30
16
, 38
16
= “1”).
Enable transmission (bit 0 at addresses 35
16
, 3D
16
= “1”).
Set data to the UARTi transmit buffer register (addresses 32
16
, 3A
16
).
[Pin’s state]
A transfer clock is input from the CLK
i
pin.
Serial data is output from the TxD
i
pin. (Dummy data is output when performing only reception.)
f
i
2 (n+1)
f
i
:
Frequency of BRGi’s count source (f
2/
f
4
, f
16/
f
32
, f
64/
f
128
, f
512/
f
1024
)