CHAPTER 8 CLOCK GENERATION FUNCTION
206
User’s Manual U14492EJ3V0UD
8.3 Input Clock Selection
The clock generator consists of an oscillator and a PLL synthesizer. For example, connecting a 5.0 MHz crystal
resonator or ceramic resonator to pins X1 and X2 enables a 50 MHz internal system clock (f
XX
) to be generated when
the multiplier is 10. Also, an external clock can be input directly to the oscillator. In this case, the clock signal should
be input only to pin X1 (pin X2 should be left open). Two basic operation modes are provided for the clock generator.
These are PLL mode and direct mode. The operation mode is selected by the CKSEL pin. The input to this pin is
latched on reset.
CKSEL
Operating Mode
0
PLL mode
1
Direct mode
Caution
The input level for the CKSEL pin must be fixed. If it is switched during operation, a
malfunction may occur.
8.3.1 Direct mode
In direct mode, an external clock is divided by two and the divided clock is supplied as the internal system clock.
The maximum frequency that can be input in direct mode is 50 MHz. The V850E/IA1 is mainly used in application
systems in which operates at relatively low frequencies.
Caution
In direct mode, an external clock must be input (an external resonator should not be
connected).
8.3.2 PLL mode
In PLL mode, an external resonator is connected or external clock is input and multiplied by the PLL synthesizer.
The multiplied PLL output is divided by the division ratio specified by the clock control register (CKC) to generate a
system clock that is 10, 5, 2.5, or 1 times the frequency (f
X
) of the external resonator or external clock.
After reset, an internal system clock (f
XX
) that is 1 time the frequency (1
×
f
X
) of the input clock frequency (f
X
) is
generated.
When a frequency that is 10 times (10
×
f
X
) the input clock frequency (f
X
) is generated, a system with low noise and
low power consumption can be realized because a frequency of up to 50 MHz is obtained based on a 5 MHz external
resonator or external clock.
In PLL mode, if the clock supply from an external resonator or external clock source stops, operation of the internal
system clock (f
XX
) based on the self-propelled frequency of the clock generator’s internal voltage controlled oscillator
(VCO) continues. In this case, f
XX
is undefined. However, do not devise an application method expecting to use this
self-propelled frequency.
Example:
Clocks when PLL mode (f
XX
= 10
×
f
X
) is used
Internal System Clock Frequency (f
XX
)
External Resonator or External Clock Frequency (f
X
)
50.000 MHz
5.0000 MHz
40.000 MHz
4.0000 MHz
Содержание V850E/IA1 mPD703116
Страница 2: ...2 User s Manual U14492EJ3V0UD MEMO...