Operation Of Cpu Clock Generator - NEC 78K0S/KB1+ User Manual

5.5

Operation of CPU Clock Generator

A clock (f ) is supplied to the CPU from the system clock (f
CPU
oscillators.
• High-speed internal oscillator:
• Crystal/ceramic oscillator:
• External clock input circuit:
The system clock oscillator is selected by the option byte. For details of the option byte, refer to CHAPTER 18
OPTION BYTE.
(1) High-speed internal oscillator
When the high-speed internal oscillation is selected by the option byte, the following is possible.
• Shortening of start time
If the high-speed internal oscillator is selected as the oscillator, the CPU can be started without having to wait
for the oscillation stabilization time of the system clock. Therefore, the start time can be shortened.
• Improvement of expandability
If the high-speed internal oscillator is selected as the oscillator, the X1 and X2 pins can be used as I/O port
pins. For details, refer to CHAPTER 4 PORT FUNCTIONS.
Figures 5-8 and 5-9 show the timing chart and status transition diagram of the default start by the high-speed
internal oscillation.
Remark When the high-speed internal oscillation is used, the clock accuracy is ±5%.
Figure 5-8. Timing Chart of Default Start by High-Speed Internal Oscillation
V
DD
RESET
H
Internal reset
System clock
CPU clock
Note Operation stop time is 277
78
CHAPTER 5 CLOCK GENERATORS
Internally oscillates a clock of 8 MHz (TYP.).
Oscillates a clock of 1 to 10 MHz.
Supplies a clock of 1 to 10 MHz to X1 pin.
(a)
Option byte is read.
System clock is selected.
Note
(Operation stops )
μ μ
s (MIN.), 544 s (TYP.), and 1.075 ms (MAX.).
User's Manual U17446EJ3V1UD
) oscillated by one of the following three types of
X
(b)
High-speed internal oscillation clock
PCC = 02H, PPCC = 02H