
W925E/C240
8-bit CID Microcontroller
Revision : A6
-41-
Release Date : 2002/7/2
The Watchdog timer will de disabled by a power-on/fail reset. The Watchdog timer reset does not
disable the watchdog timer, but will restart it. In general, software should restart the timer to put it
into a known state.
The control bits that support the Watchdog timer are discussed below.
WATCHDOG CONTROL
WDIF: WDCON.3 - Watchdog Timer Interrupt flag. This bit is set whenever the time-out occurs in
the watchdog timer. If the Watchdog interrupt is enabled (EIE.5), then an interrupt will
occur (if the global interrupt enable is set and other interrupt requirements are met).
Software or any reset can clear this bit.
WTRF: WDCON.2 - Watchdog Timer Reset flag. This bit is set whenever a watchdog reset
occurs. This bit is useful for determined the cause of a reset. Software must read it, and
clear it manually. A Power-fail reset will clear this bit. If EWT = 0, then this bit will not be
affected by the watchdog timer.
EWT: WDCON.1 - Enable Watchdog timer Reset. This bit when set to 1 will enable the
Watchdog timer reset function. Setting this bit to 0 will disable the Watchdog timer reset
function, but will leave the timer running
RWT: WDCON.0 - Reset Watchdog Timer. This bit is used to clear the Watchdog timer and to
restart it. This bit is self-clearing, so after the software writes 1 to it the hardware will
automatically clear it. If the Watchdog timer reset is enabled, then the RWT has to be set
by the user within 512 clocks of the time-out. If this is not done then a Watchdog timer
reset will occur.
CLOCK CONTROL
WD1,WD0: CKCON.7, CKCON.6 - Watchdog Timer Mode select bits. These two bits select the
time-out interval for the watchdog timer. The reset time is longer 512 clocks time
than the interrupt time-out value.
The default Watchdog time-out is 2
12
clocks, which is the shortest time-out period. The EWT,
WDIF and RWT bits are protected by the Timed Access procedure. This prevents software from
accidentally enabling or disabling the watchdog timer. More importantly, it makes it highly
improbable that errant code can enable or disable the watchdog timer.
6.9
Serial Port 1
The P4.0 and P4.1 can be used as a 8-bit serial input/output port1. P4.0 is the serial port 1 clock
I/O pin and P4.1 is the serial port 1 data I/O pin. The serial port 1 is controlled by SCON1 register
which is described as below.
SF1: Serial port 1 interrupt flag. When 8-bits data is transceived completely, SF1 is set by
hardware. SF1 is cleared when serial interrupt1 routine is executed or cleared by software.
REN1: Set REN1 from 0 to 1 to start the serial port1 to receive 8-bit serial data.
SFQ: SFQ=0 Serial clock output frequency is equal to f
OSC
/2
SFQ=1 Serial clock output frequency is equal to f
OSC
/ 256
SEDG: SEDG=0 Serial data latched at falling edge of clock, SCLK=Low initially.
SEDG=1 Serial data latched at rising edge of clock, SCLK=High initially.
CLKIO: CLKIO=0 P4.0(SCLK) work as output mode
CLKIO=1 P4.0(SCLK) work as input mode
SIO:
Any instruction causes a write to SBUF1 will start the transmission of serial port 1. As the REN1 is
from 0 to 1, the serial port 1 begins to receive a byte into SBUF1 in the frequency of the serial
clock. REN1 could be cleared by software after receive function begins. The LSB is transmitted/
SIO=0 P4.0 & P4.1 work as normal I/O pin
SIO=1 P4.0 & P4.1 work as Serial port1 function