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FUNCTIONAL DESCRIPTION
2.7 Serial I/O
2.7.2 Serial I/O register (address 00DD
The serial I/O register is serial-parallel conversion register used for data transfer. This register consists of
8-bit and can be used as both transmit and receive register.
Serial I/O register is assigned to address 00DD
Although data transfer is performed bit by bit, it is possible to specify whether the data is transferred
beginning with most-significant-bit (MSB) or least-significant-bit (LSB) by using bit 5 of the serial I/O mode
register.
(1) When bit 5 of the serial I/O mode register is "0"
Receive: Data is received bit by bit beginning with the MSB (bit 7) of the serial I/O register.
Transmit: Data is transmitted bit by bit beginning with the LSB (bit 0) of the serial I/O register.
(2) When bit 5 of the serial I/O mode register is "1"
Receive: Data is received bit by bit beginning with the LSB (bit 0) of the serial I/O register.
Transmit: Data is transmitted bit by bit beginning with the MSB (bit 7) of the serial I/O register.
2.7.3 Clock source generating circuit
The clock source generating circuit can select oscillation frequency divided by 4, 16, 32, and 64 as the
internal clock. Also, it can select an external clock (the external clock is selected immediately after reset).
Bit 2 of the serial I/O mode register specifies internal clock or external clock. When bit 2 of the serial
I/O mode register is set to "0," an external clock is
selected, when "1," an internal clock is selected.
When selecting an internal clock, set the division
of oscillation frequency by bits 0 and 1 of the serial
I/O mode register.
Oscillation frequency divided by 4:
set both bits 1 and 0 to "0."
Oscillation frequency divided by 16:
set bit 1 = "0" and bit 0 = "1."
Oscillation frequency divided by 32:
set bit 1 = "1" and bit 0 = "0."
Oscillation frequency divided by 64:
set both bit 1 and 0 to "1."
The contents of bits 0 and 1 of the serial I/O mode register are invalid, when selecting an external clock.
2.7.4 Serial input/output common transmission/reception mode
Pin P2
/S
can also be used as the serial I/O data input pin when the serial input pin selection bit (bit
1
OUT
6 of the serial I/O mode register; address 00DC
bit of port P2 direction register to input mode.)
With this function, pin P2
P2
/S
0
CLK
P2
/S
1
OUT
P2
/S
2
Note: To start receiving, set "FF
Fig. 2.7.3 Serial input/output common transfer mode block diagram
2-42
16
/S
can also be used as general-purpose input port P2
2
IN
Input or output
"1"
SM6
"0"
IN
" to the serial I/O register
16
7220 Group User's Manual
)
.
16
Table 2.7.1 Clock source selection
Serial I/O clock
External clock
Internal
clock
) is set to "1." (It is not necessary to set the corresponding
16
Transmit mode
Serial I/O register (8)
Receive mode
Port P2
data
2
Serial I/O mode register
Bit 2
0
f(X
)/4
IN
f(X
)/16
IN
1
f(X
)/32
IN
f(X
)/64
IN
.
2
Clock
SM: Serial I/O register
Bit 1
Bit 0
0 or 1 (Invalid)
0
0
0
1
1
0
1
1
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