ST6380 datasheet(18/82 Pages) STMICROELECTRONICS

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ST6380, ST6381, ST6382, ST6383, ST6388, ST6389

3 CLOCKS, RESET, INTERRUPTS AND POWER SAVING MODES

3.1 ON-CHIP CLOCK OSCILLATOR

The internal oscillator circuit is designed to require

a minimum of external components. A crystal

quartz, a ceramic resonator, or an external signal

(provided to the OSCin pin) may be used to gener-

ate a system clock with various stability/cost trade-

offs. The typical clock frequency is 8MHz. Please

note that different frequencies will affect the oper-

ation of those peripherals (D/As, SPI) whose refer-

ence frequencies are derived from the system

clock.

The different clock generator connection schemes

are shown in Figure 10 and 11. One machine cycle

takes 13 oscillator pulses; 12 clock pulses are

needed to increment the PC while and additional

13th pulse is needed to stabilize the internal latch-

es during memory addressing. This means that

with a clock frequency of 8MHz the machine cycle

is 1.625µSec.

The crystal oscillator start-up time is a function of

many variables: crystal parameters (especially

RS), oscillator load capacitance (CL), IC parame-

ters, ambient temperature, and supply voltage.It

must be observed that the crystal or ceramic leads

and circuit connections must be as short as possi-

ble. Typical values for CL1 and CL2 are in the

range of 15pF to 22pF but these should be chosen

based on the crystal manufacturers specification.

Typical input capacitance for OSCin and OSCout

pins is 5pF.

The oscillator output frequency is internally divided

by 13 to produce the machine cycle and by 12 to

produce the Timers and the Watchdog clock. A

byte cycle is the smallest unit needed to execute

any operation (i.e., increment the program coun-

ter). An instruction may need two, four, or five byte

cycles to be executed (See Table 6).

Table 6. Instruction Timing with 8MHz Clock

Figure 10. Clock Generator Option 1

Figure 11. Clock Generator Option 2

Figure 12. OSCin, OSCout Diagram

Instruction Type

Cycles

Execution

Time

Branch if set/reset

5 Cycles

8.125

µs

Branch & Subroutine Branch

4 Cycles

6.50

µs

Bit Manipulation

4 Cycles

6.50

µs

Load Instruction

4 Cycles

6.50

µs

Arithmetic & Logic

4 Cycles

6.50

µs

Conditional Branch

2 Cycles

3.25

µs

Program Control

2 Cycles

3.25

µs

OSCin

OSCout

CL1

CL2

ST6xxx

CRYSTAL/RESONATOR CLOCK

VA0016B

OSCin

OSCout

ST6xxx

EXTERNAL CLOCK

VA0015C

VA00462

OSCout

OSCin, OSCout (QUARTZ PINS)

OSCin

VDD

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