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SX20AC/SX28AC

9.2 External RC Mode

The external RC oscillator mode provides a cost-effective

approach for applications that do not require a precise

operating frequency. In this mode, the RC oscillator fre-

quency is a function of the supply voltage, the resistor (R)

and capacitor (C) values, and the operating temperature.

In addition, the oscillator frequency will vary from unit to

unit due to normal manufacturing process variations. Fur-

thermore, the difference in lead frame capacitance

between package types also affects the oscillation fre-

quency, especially for low C values. The external R and

C component tolerances contribute to oscillator fre-

quency variation as well.
Figure 9-3 shows the external RC connection diagram.

The recommended R value is from 3k

to 100k. For R

values below 2.2k

, the oscillator may become unstable,

or may stop completely. For very high R values (such as

1 M

), the oscillator becomes sensitive to noise, humid-

ity, and leakage.
Although the oscillator will operate with no external

capacitor (C = 0pF), it is recommended that you use val-

ues above 20 pF for noise immunity and stability. With no

or small external capacitance, the oscillation frequency

can vary significantly due to variation in PCB trace or

package lead frame capacitances.

9.3 Internal RC Mode

The internal RC mode uses an internal oscillator, so the

device does not need any external components. At 4

MHz, the internal oscillator provides typically +/8%

accuracy over the allowed temperature range. The inter-

nal clock frequency can be divided down to provide one

of eight lower-frequency choices by selecting the desired

value in the FUSE Word register. The frequency range is

from 31.25 KHz to 4 MHz. The default operating fre-

quency of the internal RC oscillator may not be 4 MHz.

This is due to the fact that the SX device requires trim-

ming to obtain 4 MHz operation. The parts shipped out of

the factory are not trimmed. The device relies on the pro-

gramming tool provided by the third party vendors to sup-

port trimming.

10.0 REAL TIME CLOCK (RTCC) /

WATCHDOG TIMER

The device contains an 8-bit Real Time Clock/Counter

(RTCC) and an 8-bit Watchdog Timer (WDT). An 8-bit

programmable prescaler extends the RTCC to 16 bits. If

the prescaler is not used for the RTCC, it can serve as a

postscaler for the Watchdog Timer. Figure 10-1 shows

the RTCC and WDT block diagram.

10.1 RTCC

RTCC is an 8-bit real-time timer that is incremented once

each instruction cycle or from a transition on the RTCC

pin. The on-board prescaler can be used to extend the

RTCC counter to 16 bits.
To select the internal clock source, bit 5 of the OPTION

mented at each instruction cycle unless the prescaler is

selected to increment the counter.
To select the external clock source, bit 5 of the OPTION

incremented with each valid signal transition at the RTTC

pin. By using bit 4 of the OPTION register, the transition

can be programmed to be either a falling edge or rising

edge. Setting the control bit selects the falling edge to

increment the counter. Clearing the bit selects the rising

edge.
The RTCC generates an interrupt as a result of an RTCC

rollover from 0FF to 000. There is no interrupt pending bit

to indicate the overflow occurrence. The RTCC register

must be sampled by the program to determine any over-

flow occurrence.

Figure 9-3. RC Oscillator Mode

Internal

Circuitry

OSC2

OSC1

SX Device