Samsung KS57C2308 Real-Time and Watch-Time Measurement, Using a System or Subsystem Clock Source

TIMERS and TIMER/COUNTERS KS57C2308/P2308/C2316/P2316

WATCH TIMER

OVERVIEW

The watch timer is a multi-purpose timer which consists of three basic components:

—8-bit watch timer mode register (WMOD)

—Clock selector

—Frequency divider circuit

Watch timer functions include real-time and watch-time measurement and interval timing for the main and subsystem clock. It is also used as a clock source for the LCD controller and for generating buzzer (BUZ) output.

Real-Time and Watch-Time Measurement

To start watch timer operation, set bit 2 of the watch timer mode register (WMOD.2) to logic one. The watch timer starts, the interrupt request flag IRQW is automatically set to logic one, and interrupt requests commence in 0.5-second intervals.

Since the watch timer functions as a quasi-interrupt instead of a vectored interrupt, the IRQW flag should be cleared to logic zero by program software as soon as a requested interrupt service routine has been executed.

Using a System or Subsystem Clock Source

The watch timer can generate interrupts based on the system clock frequency or on the subsystem clock. When the zero bit of the WMOD register is set to "1", the watch timer uses the subsystem clock signal (fxt) as its source; if WMOD.0 = "0", the system clock (fxx) is used as the signal source, according to the following formula:

Watch timer clock (fw) =	System clock (fxx)	= 32.768 kHz (fxx = 4.19 MHz)
	128

This feature is useful for controlling timer-related operations during stop mode. When stop mode is engaged, the main system clock (fx) is halted, but the subsystem clock continues to oscillate. By using the subsystem clock as the oscillation source during stop mode, the watch timer can set the interrupt request flag IRQW to "1", thereby releasing stop mode.

Clock Source Generation for LCD Controller

The watch timer supplies the clock frequency for the LCD controller (fLCD). Therefore, if the watch timer is disabled, the LCD controller does not operate.

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