E202075_HD404889 Datasheet, PDF (63/202 Pages) Renesas Technology Corp – Low-Voltage AS Microcomputers with On-Chip LCD Circuit

HD404889/HD404899/HD404878/HD404868 Series
Internal Oscillator Circuit
Figure 22 shows the clock pulse generator circuit. As shown in table 21, a ceramic oscillator or crystal
oscillator can be connected to OSC1 and OSC2, and a 32.768 kHz crystal oscillator can be connected to X1
and X2. External clock operation is possible for the system oscillator. Set bit 1 (SSR1) of the system clock
select register (SSR: $004) according to the frequency of the oscillator connected to OSC1 and OSC2
(figure 24).
Note: If the setting of bit 1 in the system clock select register does not match the frequency of the system
oscillator, the subsystem using 32.768 kHz oscillation will not operate correctly.
OSC2
System
oscillator
fOSC _{1/4 or 1/32} fcyc _Timing
division tcyc generation
circuit*
circuit
OSC1
X2
Sub
system
clock
oscillator
fx _{1/8 or 1/4} fSUB _Timing
division tsubcyc generator
circuit*
circuit
X1
1/8
division
circuit
fW ^Timing
twcyc ^generation
circuit
LSON
øCPU
System
clock
selection
circuit _øPER
CPU
•ROM
•RAM
• Registers, flags
•I/O
Peripheral
functions
Interrupts
TMA3 bit
Time
base
øCLK
clock
selection
circuit
Timer A
interrupts
Notes: The division ratio can be selected by setting bit 0 or bit 2 in the system clock select register
(SSR:$004).
Figure 22 Clock Pulse Generator Circuit
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