NEC 78K0S/KB1+ User Manual page 61

(3) External clock input
The P121 pin is used as the X1 pin to input an external clock, and therefore it cannot be used as an I/O port pin.
The P122 pin can be used as an I/O port pin.
The system clock oscillation is selected by the option byte. For details, refer to CHAPTER 18 OPTION BYTE.
Generation of reset signal sets port 12 to the input mode.
Figures 4-12 and 4-13 show the block diagrams of port 12.
WR
PU
RD
WR
PORT
WR
PM
PU12: Pull-up resistor option register 12
P12: Port register 12
PM12: Port mode register 12
RD: Read signal
WR××: Write signal
CHAPTER 4 PORT FUNCTIONS
Figure 4-12. Block Diagram of P120 and P123
PU12
PU120, PU123
P12
Output latch
(P120, P123)
PM12
PM120, PM123
User's Manual U17446EJ3V1UD
V
DD
P-ch
P120, P123
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