COP8TAB5 datasheet(17/54 Pages) NSC

17 / 54 page

10.0 Functional Description (Continued)

10.6.3 External Oscillator

The External Oscillator mode can be selected by program-

ming Option Bit 3 to 0 and Option Bit 4 to 0. CKI can be

driven by an external clock signal provided it meets the

specified duty cycle, rise and fall times, and input levels.

G7/CKO is available as a general purpose input G7 and/or

Halt control. Figure 13 shows the external oscillator connec-

tion diagram.

10.6.4 Clock Prescaler

The device is equipped with a programmable clock prescaler

which allows the user to dynamically adjust the clock speed,

and thus the power dissipation, to the processing needs of

the application. By merely writing an eight-bit value to the

CLKPS register, the user can divide the input oscillator clock

by an integer multiple (1 — 256) and reduce the CPU clock

frequency. The format of the CLKPS Register is shown in

Table 5. The value written to the CLKPS register is one less

than the desired divider. A value of 0 (zero) written to the

CLKPS register yields a CPU clock equal to the input clock

frequency. A value of 255 written to the CLKPS register

yields a CPU clock with a period equal to 256 input clock

periods.

TABLE 5. Clock Prescale Register (CLKPS)

CLKPS

Bit 7

Bit 0

With On-Chip Bias Resistor

With External Bias Resistor

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20091725

FIGURE 12. Crystal Oscillator

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FIGURE 13. External Oscillator

With External Frequency Control Resistor (R/C+R)

With Fully On-Chip R/C Oscillator.

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FIGURE 14. R/C Oscillator

COP8TAB5 Datasheet(PDF) 17 Page - National Semiconductor (TI)