参数资料
型号: ST62E25CF1
厂商: STMICROELECTRONICS
元件分类: 微控制器/微处理器
英文描述: 8-BIT, UVPROM, 8 MHz, MICROCONTROLLER, CDIP28
封装: WINDOWED, SIDE BRAZED, CERAMIC, DIP-28
文件页数: 6/70页
文件大小: 1088K
代理商: ST62E25CF1
14/70
ST62T15C/T25C/E25C
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). In addition, a Low Frequency Auxiliary Os-
cillator (LFAO) can be switched in for security rea-
sons, to reduce power consumption, or to offer the
benefits of a back-up clock system.
The Oscillator Safeguard (OSG) option filters
spikes from the oscillator lines, provides access to
the LFAO to provide a backup oscillator in the
event of main oscillator failure and also automati-
cally limits the internal clock frequency (fINT) as a
function of VDD, in order to guarantee correct oper-
ation. These functions are illustrated in Figure 9,
Figure 10, Figure 11 and Figure 12.
Figure 8 illustrates various possible oscillator con-
figurations using an external crystal or ceramic res-
onator, an external clock input, an external resistor
(RNET), or the lowest cost solution using only the
LFAO. CL1 an CL2 should have a capacitance in the
range 12 to 22 pF for an oscillator frequency in the
4-8 MHz range.
The internal MCU clock frequency (fINT) 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 Figure 11.
With an 8MHz oscillator frequency, the fastest ma-
chine cycle is therefore 1.625s.
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/
RESONATORoptionisselected,itmustbeusedwith
a quartz crystal, a ceramic resonator or an external
signalprovided on the OSCinpin.When the RCNET-
WORK option is selected, the system clock is gen-
erated by an external resistor.
The main oscillator can be turned off (when the
OSG ENABLED option is selected) by setting the
OSCOFF bit of the ADC Control Register. The
Low Frequency Auxiliary Oscillator is automatical-
ly started.
Figure 8. Oscillator Configurations
INTEGRATED CLOCK
CRYSTAL/RESONATOR option
OSG ENABLED option
OSCin
OSCout
CL1n
CL2
ST6xxx
CRYSTAL/RESONATOR CLOCK
CRYSTAL/RESONATOR option
OSCin
OSCout
ST6xxx
EXTERNAL CLOCK
CRYSTAL/RESONATOR option
NC
OSCin
OSCout
ST6xxx
NC
OSCin
OSCout
RNET
ST6xxx
RC NETWORK
RC NETWORK option
NC
14
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