COP8FG datasheet(16/59 Pages) NSC

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5.0 Functional Description (Continued)

5.8 OSCILLATOR CIRCUITS

There are four clock oscillator options available: Crystal Os-

cillator with or without on-chip bias resistor, R/C Oscillator

with on-chip resistor and capacitor, and External Oscillator.

The oscillator feature is selected by programming the ECON

Table 1.

TABLE 1. Oscillator Option

ECON4

ECON3

Oscillator Option

External Oscillator

Crystal Oscillator without Bias Resistor

R/C Oscillator

Crystal Oscillator with Bias Resistor

5.8.1 Crystal Oscillator

The crystal Oscillator mode can be selected by programming

ECON Bit 4 to 1. CKI is the clock input while G7/CKO is the

clock generator output to the crystal. An on-chip bias resistor

connected between CKI and CKO can be enabled by pro-

gramming ECON Bit 3 to 1 with the crystal oscillator option

selection. The value of the resistor is in the range of 0.5M to

2M (typically 1.0M).

Table 2 shows the component values re-

quired for various standard crystal values. Resistor R2 is

only used when the on-chip bias resistor is disabled.

Figure

13 shows the crystal oscillator connection diagram.

TABLE 2. Crystal Oscillator Configuration,

A = 25˚C, VCC =5V

R1 (k

Ω)R2 (MΩ)

C1 (pF)

C2 (pF)

CKI Freq.

(MHz)

5.6

100

100–156

0.455

5.8.2 External Oscillator

The External Oscillator mode can be selected by program-

ming ECON Bit 3 to 0 and ECON 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 14 shows the external oscillator connection

diagram.

5.8.3 R/C Oscillator

The R/C Oscillator mode can be selected by programming

ECON Bit 3 to 1 and ECON Bit 4 to 0. In R/C oscillation

mode, CKI is left floating, while G7/CKO is available as a

general purpose input G7 and/or HALT control. The R/C con-

trolled oscillator has on-chip resistor and capacitor for maxi-

mum R/C oscillator frequency operation. The maximum fre-

quency is 5 MHz ± 35% for V

CC between 4.5V to 5.5V and

temperature range of −40˚C to +85˚C. For max frequency

operation, the CKI pin should be left floating. For lower fre-

quencies, an external capacitor should be connected be-

tween CKI and either V

CC or GND. Immunity of the R/C os-

cillator to external noise can be improved by connecting one

half the external capacitance to V

CC and one half to GND.

PC board trace length on the CKI pin should be kept as short

as possible.

Table 3 shows the oscillator frequency as a

function of external capacitance on the CKI pin.

Figure 15

shows the R/C oscillator configuration.

TABLE 3. R/C Oscillator Configuration,

−40˚C to +85˚C, V

CC = 4.5V to 5.5V,

OSC Freq. Variation of ± 35%

External

Capacitor (pF)*

R/C OSC Freq

(MHz)

Instr. Cycle

(µs)

2.0

2.5

5.0

125

6100

32 kHz

312.5

* Assumes 3-5 pF board capacitance.

With On-Chip Bias Resistor

DS101116-17

Without On-Chip Bias Resistor

DS101116-18

FIGURE 13. Crystal Oscillator

COP8FG Datasheet(PDF) 16 Page - National Semiconductor (TI)

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