Datasheet

78K0/Ix2 CHAPTER 5 CLOCK GENERATOR
R01UH0010EJ0500 Rev.5.00 172
Feb 28, 2012
Cautions 1. If the rise of the voltage to reach 2.7 V after turning on the power is more gradual than 0.5 V/ms
(MIN.), perform one of the following operations.
 Input a low level to the RESET pin until 2.7 V is reached after turning on the power.
 Set the LVI default start function to operate (LVISTART = 1) using the option byte and perform a
wait processing until the supply voltage rises from 1.91 V (TYP.) to 2.7 V (see Figure 5-13).
When a low level has been input to the RESET pin until the voltage reaches 2.7 V, the CPU
operates with the same timing as <2> and thereafter in Figure 5-12, after the reset has been
released by the RESET pin.
2. It is not necessary to wait for the oscillation stabilization time when an external clock input from
the EXCLK pin is used.
Remark While the microcontroller is operating, a clock that is not used as the CPU clock can be stopped via software
settings. The internal high-speed oscillation clock and high-speed system clock can be stopped by
executing the STOP instruction (refer to (4) in 5.6.1 Example of controlling high-speed system clock,
and (3) in 5.6.2 Example of controlling internal high-speed oscillation clock).
Figure 5-13. Clock Generator Operation When Power Supply Voltage Is Turned On
(When LVI Default Start Function Enabled Is Set (Option Byte: LVISTART = 1))
0 V
1.91 V (TYP.)
Internal high-speed
oscillation clock (f
IH )
CPU clock
High-speed
system clock (f
XH )
(when X1 oscillation
selected)
Internal reset signal
Power supply
voltage (V
DD )
Internal high-speed
oscillation clock
High-speed system clock
Switched by
software
<6>
<1>
<3>
<2>
<5>
X1 clock
oscillation stabilization time:
2
8
/f X to 2
18
/f X
Note
Starting X1 oscillation
is set by software.
2.7 V
<4>
Reset processing (12 to 51 μs)
Waiting for oscillation
accuracy stabilization
(102 to 407 μs)
<1> When the power is turned on, an internal reset signal is generated by the power-on-clear (POC) circuit.
<2> When the power supply voltage exceeds 1.91 V (TYP.), the reset is released and the internal high-speed
oscillator automatically starts oscillation.
<3> After the reset is released and reset processing is performed, the CPU starts operation on the internal high-speed
oscillation clock.
<4> Wait for the specified time using software or wait using the LVI function until the supply voltage rises from 1.91 V
(TYP.) to 2.7 V.
<5> Set the start of oscillation of the X1 clock via software (refer to (1) in 5.6.1 Example of controlling high-speed
system clock).
<6> When switching the CPU clock to the X1 clock, wait for the clock oscillation to stabilize, and then set switching via
software (refer to (3) in 5.6.1 Example of controlling high-speed system clock).