Shift Clock - Fujitsu MB90390 Series Hardware Manual

22.4.1

Shift Clock

There are two modes of shift clock: internal or external shift clock. These two modes
are selected by setting the SMCS. To switch the modes, ensure that serial I/O transfer is
stopped. To check whether the serial I/O transfer is stopped, read the BUSY bit.
■ Internal Shift Clock Mode
In internal shift clock mode, data transfer is based on the internal clock. As a synchronization timing
output, a shift clock of 50% duty ratio can be output from the SCK4 pin. Data is transferred at one bit per
clock. The transfer frequency and speed is expressed as follows:
Transfer frequency [Hz] =
"A" is the division ratio indicated by the SMD bits of SMCS. The value can be 2
φ is the machine frequency.
■ External Shift Clock Mode
In external shift clock mode, the data transfer is based on the external clock supplied via the SCK4 pin.
Data is transferred at one bit per clock.
The transfer speed can be between DC and 1/(5 machine cycles). For example, the transfer speed can be up
to 2 MHz when 1 machine cycle is equal to 0.1 μs. The external clock frequency has a maximum value of
2 MHz.
A data bit can also be transferred by software, which is enabled as described below.
Select external shift clock mode, and write "0" to the SCOE bit of SMCS. Then, write "1" to the direction
register for the port sharing the SCK4 pin, and place the port in output mode. Then, when "1" and "0" are
written to the data register (PDR) of the port, the port value output via the SCK4 pin is fetched as the
external clock and transfer starts. Ensure that the shift clock starts from "H".
Note:
The SMCS or SDR must not be written to during serial I/O operation.
Transfer speed [s] =
A div
CHAPTER 22 SERIAL I/O
A div
1 2 3 4 5 6
, 2 , 2 , 2 , 2 , 2 or 2
7
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