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(1) Serial clock
a.
Clock source
SBI0CR1<SCK2:0> is used to select the following functions:
Internal clock
In internal clock mode one of seven frequencies can be selected. The serial
clock signal is output to the outside on the SCK pin. The SCK pin goes high
when data transfer starts. When the device is writing (in transmit mode) or
reading (in receive mode), data cannot follow the serial clock rate, so an
automatic wait function is executed which automatically stops the serial clock
and holds the next shift operation until reading or writing has been completed.
1
SCK pin output
a
SO pin output
0
Write
a
transmitted data
Figure 3.10.23 Automatic Wait Function
External clock (<SCK2:0> = 111)
An external clock input via the SCK pin is used as the serial clock. In order to
ensure the integrity of shift operations, both the high and low-level serial clock
pulse widths shown below must be maintained. The maximum data transfer
frequency is 2.1 MHz (when fc = 33 MHz).
SCK pin
Figure 3.10.24 Maximum Data Transfer Frequency when External Clock Input Used
Automatic wait function
2
3
7
8
a
a
a
a
a
1
2
5
6
7
t
t
SCKL
SCKH
t
, t
> 8/fc
SCKL
SCKH
91C824-175
1
2
6
7
8
b
b
b
b
b
b
c
0
1
4
5
6
7
b
c
TMP91C824
1
2
3
c
c
0
1
2
2008-02-20
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