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S3C9228/P9228 (Preliminary Spec) BASIC TIMER
10-3
BASIC TIMER FUNCTION DESCRIPTION
Watchdog Timer Function
You can program the basic timer overflow signal (BTOVF) to generate a reset by setting BTCON.7–BTCON.4 to
any value other than “1010B”. (The “1010B” value disables the watchdog function.) A reset clears BTCON to
“00H”, automatically enabling the watchdog timer function. A reset also selects the CPU clock (as determined by
the current CLKCON register setting), divided by 4096, as the BT clock.
A reset whenever a basic timer counter overflow occurs. During normal operation, the application program must
prevent the overflow, and the accompanying reset operation, from occurring. To do this, the BTCNT value must
be cleared (by writing a "1" to BTCON.1) at regular intervals.
If a system malfunction occurs due to circuit noise or some other error condition, the BT counter clear operation
will not be executed and a basic timer overflow will occur, initiating a reset. In other words, during normal
operation, the basic timer overflow loop (a bit 7 overflow of the 8-bit basic timer counter, BTCNT) is always
broken by a BTCNT clear instruction. If a malfunction does occur, a reset is triggered automatically.
Oscillation Stabilization Interval Timer Function
You can also use the basic timer to program a specific oscillation stabilization interval following a reset or when
stop mode has been released by an external interrupt.
In stop mode, whenever a reset or an internal and an external interrupt occurs, the oscillator starts. The BTCNT
value then starts increasing at the rate of fxx/4096 (for reset), or at the rate of the preset clock source (for an
internal and an external interrupt). When BTCNT.3 overflows, a signal is generated to indicate that the
stabilization interval has elapsed and to gate the clock signal off to the CPU so that it can resume normal
operation.
In summary, the following events occur when stop mode is released:
1. During stop mode, a power-on reset or an internal and an external interrupt occurs to trigger the stop mode
release and oscillation starts.
2. If a power-on reset occurred, the basic timer counter will increase at the rate of fxx/4096. If an internal and an
external interrupt is used to release stop mode, the BTCNT value increases at the rate of the preset clock
source.
3. Clock oscillation stabilization interval begins and continues until bit 3 of the basic timer counter overflows.
4. When a BTCNT.3 overflow occurs, normal CPU operation resumes.