SX28ACSLASHSS datasheet(26/51 Pages) PARALLAX

26 / 51 page

Parallax SX20AC/SX28AC

www.parallax.com

Page 26 of 51

Rev 1.6 11/20/2006

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.

The RTCC counter can be clocked by the internal

instruction cycle clock or by an external clock source

presented at the RTCC pin.

To select the internal clock source, bit 5 of the OPTION

incremented 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 RTCC

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

overflow occurrence.

10.2. Watchdog Timer

The watchdog logic consists of a Watchdog Timer which

shares the same 8-bit programmable prescaler with the

RTCC. The prescaler actually serves as a postscaler if

used in conjunction with the WDT, in contrast to its use

as a prescaler with the RTCC.

10.3. The Prescaler

The 8-bit prescaler may be assigned to either the RTCC or

the WDT through the PSA bit (bit 3 of the OPTION

WDT. If assigned to the WDT, the WDT clocks the

prescaler and the prescaler divide rate is selected by the

PS0, PS1, and PS2 bits located in the OPTION register.

Clearing the PSA bit assigns the prescaler to the RTCC.

Once assigned to the RTCC, the prescaler clocks the

RTCC and the divide rate is selected by the PS0, PS1, and

PS2 bits in the OPTION register. The prescaler is not

mapped into the data memory, so run-time access is not

possible. The prescaler cannot be assigned to both the

RTCC and WDT simultaneously.

Figure 10-1

RTCC and WDT Block Diagram

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