ST6255B View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
ST6255B
STMicroelectronics
'ST6255B' PDF : 74 Pages View PDF
ST62T55B ST62T65B/E65B
3 CLOCKS, RESET, INTERRUPTS AND POWER SAVING MODES
3.1 CLOCK SYSTEM
The MCU features a Main Oscillator which can be
driven by an external clock, or used in conjunction
with an AT-cut parallel resonant crystal or a suita-
ble ceramic resonator, or with an external resistor
(RNET).
8 illustrates various possible oscillator configura-
tions using an external crystal or ceramic resonator,
an external clock input, an external resistor (RNET).
CL1 an CL2 should have a capacitance in the range
12 to 22 pF for an oscillator frequency in the 4-8 MHz
range.
A programmable divider is provided in order to adjust
the internal clock of the MCU to the best power con-
sumption and performance trade-off.
The internal MCU clock frequency (fINT) drives di-
rectly the AR TIMER while it is divided by 12 to
drive the TIMER, the A/D converter and the
Watchdog timer, and by 13 to drive the CPU core,
as may be seen in 9.
With an 8MHz oscillator frequency, the fastest ma-
chine cycle is therefore 1.625µs.
A machine cycle is the smallest unit of time needed
to execute any operation (for instance, to increment
the Program Counter). An instruction may require
two, four, or five machine cycles for execution.
3.1.1 Main Oscillator
The oscillator configuration may be specified by se-
lecting the appropriate option. When the CRYSTAL/
RESONATOR option isselected,it mustbeusedwith
a quartz crystal, a ceramic resonator or an external
signal provided on the OSCinpin. When theRCNET-
WORK option is selected, the system clock is gen-
erated by an external resistor.
Figure 8. Oscillator Configurations
CRYSTAL/RES ONATOR CLOCK
CRYSTA L/RESONATOR option
ST6xxx
OSCin
OSCout
CL1n
CL2
EXTE RNAL CLOCK
CRYSTAL /RESONATOR option
ST6xxx
OSCin
OSCout
NC
RC NETWORK
RC NETWORK option
ST6xxx
OSCin
NC
OSCout
RNET
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