ST6391 datasheet(28/68 Pages) STMICROELECTRONICS

28 / 68 page

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 operation

of those peripherals (D/As, SPI) whose reference fre-

quencies are derived from the system clock.

The different clock generator options connection

methods are shown in Figures 25 and 26. 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 inter-

nal latches during memory addressing. This means

that with a clock frequency of 8MHz the machine cy-

cle is 1.625

µs.

The crystal oscillator start-up time is a function of

many variables: crystal parameters (especially RS),

oscillator load capacitance (CL), IC parameters, am-

bient temperature, and supply voltage.It must be ob-

served that the crystal or ceramic leads and circuit

connections must be as short as possible. Typical

values for CL1 and CL2 are in the range of 15pF to

22pF but these should be chosen based on the crys-

tal manufacturers specification. Typical input capaci-

tance 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).

Instruc tion 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 25. Clock Generator Option (1)

Figure 26. Clock Generator Option (2)

Figure 27. OSCin, OSCout Diagram

ST6391,92,93,95,97,99

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