ST7263-EMU2 STMicroelectronics, ST7263-EMU2 Datasheet - Page 34

• Manufacturer Part Number: ST7263-EMU2
• Description: MCU, MPU & DSP Development Tools ST7 Emulator Board
• Manufacturer: STMicroelectronics

Specifications of ST7263-EMU2

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

ST7263
WATCHDOG TIMER (Cont’d)
5.3.3 Functional Description
The counter value stored in the CR register (bits
T6:T0), is decremented every 49,152 machine cy-
cles, and the length of the timeout period can be
programmed by the user in 64 increments.
If the watchdog is activated (the WDGA bit is set)
and when the 7-bit timer (bits T6:T0) rolls over
from 40h to 3Fh (T6 becomes cleared), it initiates
a reset cycle pulling low the reset pin for typically
500ns.
The application program must write in the CR reg-
ister at regular intervals during normal operation to
prevent an MCU reset. The value to be stored in
the CR register must be between FFh and C0h
(see
MHz)):
– The WDGA bit is set (watchdog enabled)
– The T6 bit is set to prevent generating an imme-
– The T5:T0 bits contain the number of increments
Table 14. Watchdog Timing (f
Notes: Following a reset, the watchdog is disa-
bled. Once activated it cannot be disabled, except
by a reset.
The T6 bit can be used to generate a software re-
set (the WDGA bit is set and the T6 bit is cleared).
5.3.3.1 Using Halt Mode with the WDG
The HALT instruction stops the oscillator. When
the oscillator is stopped, the WDG stops counting
and is no longer able to generate a reset until the
microcontroller receives an external interrupt or a
reset.
If an external interrupt is received, the WDG re-
starts counting after 4096 CPU clocks. If a reset is
generated, the WDG is disabled (reset state).
Recommendations
– Make sure that an external event is available to
– Before executing the HALT instruction, refresh
34/109
diate reset
which represents the time delay before the
watchdog produces a reset.
wake up the microcontroller from Halt mode.
the WDG counter, to avoid an unexpected WDG
Max
Min
Table 14 . Watchdog Timing (fCPU = 8
CR Register
initial value
FFh
C0h
WDG timeout period
CPU
393.216
= 8 MHz)
6.144
(ms)
– When using an external interrupt to wake up the
– For the same reason, reinitialize the level sensi-
– The opcode for the HALT instruction is 0x8E. To
– As the HALT instruction clears the I bit in the CC
5.3.4 Interrupts
None.
5.3.5 Register Description
CONTROL REGISTER (CR)
Read /Write
Reset Value: 0111 1111 (7Fh)
Bit 7 = WDGA Activation bit .
This bit is set by software and only cleared by
hardware after a reset. When WDGA = 1, the
watchdog can generate a reset.
0: Watchdog disabled
1: Watchdog enabled
Bit 6:0 = T[6:0] 7-bit timer (MSB to LSB).
These bits contain the decremented value. A reset
is produced when it rolls over from 40h to 3Fh (T6
becomes cleared).
WDGA
reset immediately after waking up the microcon-
troller.
microcontroller, reinitialize the corresponding I/O
as “Input Pull-up with Interrupt” before executing
the HALT instruction. The main reason for this is
that the I/O may be wrongly configured due to ex-
ternal interference or by an unforeseen logical
condition.
tiveness of each external interrupt as a precau-
tionary measure.
avoid an unexpected HALT instruction due to a
program counter failure, it is advised to clear all
occurrences of the data value 0x8E from memo-
ry. For example, avoid defining a constant in
ROM with the value 0x8E.
register to allow interrupts, the user may choose
to clear all pending interrupt bits before execut-
ing the HALT instruction. This avoids entering
other peripheral interrupt routines after executing
the external interrupt routine corresponding to
the wake-up event (reset or external interrupt).
7
T6
T5
T4
T3
T2
T1
T0
0