ST63156 datasheet(27/86 Pages) STMICROELECTRONICS

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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

Table 7. Intructions Timing with 8MHz Clock

Figure 23. Clock Generator Option (1)

Figure 24. Clock Generator Option (2)

Figure 25. OSCin, OSCout Diagram

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 opera-

tion of those peripherals (D/As, SPI, 62.5 kHz

OUT) whose reference frequencies are derived

from the system clock.

The different clock generator options connection

methods are shown in Figures 24 and 25. One ma-

chine cycle takes 13 oscillator pulses; 12 clock

pulses are needed to increment the PC while and

additional 13th pulse is needed to stabilize the in-

ternal latches during memory addressing. This

means that with a clock frequency of 8MHz the ma-

chine cycle is 1.625

µSec.

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 internallydivided

by 13 to produce the machine cycle and by 12 to

produce the Timer and the Watchdog clock. A byte

cycle is the smallest unit needed to execute any

operation (i.e., increment the program counter). An

instruction may need two, four, or five byte cycles

to be executed (See Table 7).

ON-CHIP CLOCK OSCILLATOR

ST63140,142,126,156

23/82

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