NEC NEC PD78081(A), User Manual

Page: 101 / 274

78
CHAPTER 5 CLOCK GENERATOR
5.6.2 CPU clock switching procedure
This section describes CPU clock switching procedure.
Figure 5-7. CPU Clock Switching
(1) The CPU is reset by setting the RESET signal to low level after power-on. After that, when reset is released
by setting the RESET signal to high level, main system clock starts oscillation. At this time, oscillation
stabilization time (2
17
/f
X
) is secured automatically.
After that, the CPU starts executing the instruction at the minimum speed of the main system clock (12.8
µ
s when
operated at 5.0 MHz).
(2) After the lapse of a sufficient time for the V
DD
voltage to increase to enable operation at maximum speeds,
the processor clock control register (PCC) and oscillation mode selection register (OSMS) are rewritten and
the maximum-speed operation is carried out.
V
DD
RESET
CPU Clock
Wait (26.2 ms : 5.0 MHz)
Internal Reset Operation
Minimum
Speed
Operation Maximum Speed
Operation