CHAPTER 5 CLOCK GENERATOR
Preliminary User’s Manual U16846EJ1V0UD
96
(2) Improvement of performance
Because the CPU can be started without waiting for the high-speed system clock oscillation stabilization time, the
total performance can be improved.
A timing diagram of the CPU default start using Ring-OSC is shown in Figure 5-13.
Figure 5-13. Timing Diagram of CPU Default Start Using Ring-OSC
Ring-OSC clock
(f
R
)
CPU clock
High-speed
system clock (f
XP
)
Operation
stopped: 17/f
R
High-speed system clock oscillation stabilization time:
2
11
/f
XP
to 2
16
/f
XP
Note
RESET
Ring-OSC clock
High-speed system clock
Switched by software
Note Check using the oscillation stabilization time counter status register (OSTC). Waiting for the oscillation
stabilization time is not required when the external RC oscillation clock is selected as the high-speed
system clock by the option byte. Therefore, the CPU clock can be switched without reading the OSTC
value.
(a) When the RESET signal is generated, bit 0 of the main clock mode register (MCM) is set to 0 and the Ring-
OSC clock is set as the CPU clock. However, a clock is supplied to the CPU after 17 clocks of the Ring-OSC
clock have elapsed after RESET release (or clock supply to the CPU stops for 17 clocks). During the
RESET period, oscillation of the high-speed system clock and Ring-OSC clock is stopped.
(b) After RESET release, the CPU clock can be switched from the Ring-OSC clock to the high-speed system
clock using bit 0 (MCM0) of the main clock mode register (MCM) after the high-speed system clock
oscillation stabilization time has elapsed. At this time, check the oscillation stabilization time using the
oscillation stabilization time counter status register (OSTC) before switching the CPU clock. The CPU clock
status can be checked using bit 1 (MCS) of MCM.
(c) Ring-OSC can be set to stopped/oscillating using the Ring-OSC mode register (RCM) when “Can be stopped
by software” is selected for the Ring-OSC by the option byte, if the high-speed system clock is used as the
CPU clock. Make sure that MCS is 1 at this time.
(d) When Ring-OSC is used as the CPU clock, the high-speed system clock can be set to stopped/oscillating
using the main OSC control register (MOC). Make sure that MCS is 0 at this time.
(e) Select the high-speed system clock oscillation stabilization time (2
11
/f
XP
, 2
13
/f
XP
, 2
14
/f
XP
, 2
15
/f
XP
, 2
16
/f
XP
) using
the oscillation stabilization time select register (OSTS) when releasing STOP mode while the high-speed
system clock is being used as the CPU clock. In addition, when releasing STOP mode while RESET is
released and the Ring-OSC clock is being used as the CPU clock, check the high-speed system clock
oscillation stabilization time using the oscillation stabilization time counter status register (OSTC).