HT48R50A-1/HT48C50-1
WS2
WS1
WS0
Division Ratio
OSC). Also, the frequencies of the internal RC oscillator
can be 3.2MHz, 1.6MHz, 800kHz and 400kHz (depends
on the options).
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
1:1
1:2
The WDT oscillator is a free running on-chip RC oscillator,
and no external components are required. Even if the sys-
tem enters the power down mode, the system clock is
stopped, but the WDT oscillator still works within a period
of 65ms at 5V. The WDT oscillator can be disabled by op-
tions to conserve power.
1:4
1:8
1:16
1:32
1:64
1:128
Watchdog Timer - WDT
WDTS (09H) Register
The WDT clock source is implemented by a dedicated
RC oscillator (WDT oscillator), RTC clock or instruction
clock (system clock divided by 4), determines the op-
tions. This timer is designed to prevent a software mal-
function or sequence from jumping to an unknown
location with unpredictable results. The Watchdog
Timer can be disabled by options. If the Watchdog Timer
is disabled, all the executions related to the WDT result
in no operation. The RTC clock is enabled only in the in-
ternal RC+RTC mode.
The WDT overflow under normal operation will initialize
²chip reset² and set the status bit ²TO². But in the HALT
mode, the overflow will initialize a ²warm reset² and only
the Program Counter and SP are reset to zero. To clear
the contents of WDT (including the WDT prescaler),
three methods are adopted; external reset (a low level to
RES), software instruction and a ²HALT² instruction.
The software instruction include ²CLR WDT² and the
other set - ²CLR WDT1² and ²CLR WDT2². Of these
two types of instruction, only one can be active depend-
ing on the option - ²CLR WDT times selection option². If
the ²CLR WDT² is selected (i.e. CLRWDT times equal
one), any execution of the ²CLR WDT² instruction will
clear the WDT. In the case that ²CLR WDT1² and ²CLR
WDT2² are chosen (i.e. CLRWDT times equal two),
these two instructions must be executed to clear the
WDT; otherwise, the WDT may reset the chip as a result
of time-out.
Once the internal WDT oscillator (RC oscillator with a
period of 65ms at 5V normally) is selected, it is first di-
vided by 256 (8-stage) to get the nominal time-out pe-
riod of 17ms at 5V. This time-out period may vary with
temperatures, VDD and process variations. By invoking
the WDT prescaler, longer time-out periods can be real-
ized. Writing data to WS2, WS1, WS0 (bit 2,1,0 of the
WDTS) can give different time-out periods. If WS2,
WS1, and WS0 are all equal to 1, the division ratio is up
to 1:128, and the maximum time-out period is 2.1s at 5V
seconds. If the WDT oscillator is disabled, the WDT
clock may still come from the instruction clock and oper-
ates in the same manner except that in the HALT state
the WDT may stop counting and lose its protecting pur-
pose. In this situation the logic can only be restarted by
external logic. The high nibble and bit 3 of the WDTS are
reserved for user's defined flags, which can be used to
indicate some specified status.
Power Down Operation - HALT
The HALT mode is initialized by the ²HALT² instruction
and results in the following...
·
The system oscillator will be turned off but the WDT
oscillator remains running (if the WDT oscillator is se-
lected).
·
·
The contents of the on chip RAM and registers remain
unchanged.
If the device operates in a noisy environment, using the
on-chip RC oscillator (WDT OSC) or 32kHz crystal oscilla-
tor (RTC OSC) is strongly recommended, since the HALT
will stop the system clock.
WDT and WDT prescaler will be cleared and re-
counted again (if the WDT clock is from the WDT os-
cillator).
S
y
s
t
e
m
C
l
o
c
k
/
4
W
D
T
P
r
e
s
c
a
l
e
r
R
T
C
O
S
C
O
p
t
i
o
n
8
-
b
i
t
C
o
u
n
t
e
r
7
-
b
i
t
C
o
u
n
t
e
r
S
e
l
e
c
t
W
D
T
O
S
C
8
-
t
o
-
1
M
U
X
W
S
0
~
W
S
2
W
D
T
T
i
m
e
-
o
u
t
Watchdog Timer
Rev. 2.00
11
March 8, 2006
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
