Holtek HT46R22, Handbook

Page: 84 / 162

counter to give longer watchdog time-outs. In the case of the HT46R47/HT46C47 devices, this divi-
sion ratio is fixed by an internal counter which gives a 2
15
fixed division ratio. In the case of the
other devices, the division ratio can be varied by selecting different configuration options to give a
2
12
to 2
15
division ratio range. As the clear instruction only resets the last stage of the counter
chain, for this reason the actual division ratio and corresponding WDT time-out can vary by a fac-
tor of two. The exact division ratio depends upon the residual value in the WDT counter before the
clear instruction is executed. As an example, if a WDT time out value of 2
12
(4096) is chosen in the
configuration options, the actual time out value can range from f
S
/2
12
to f
S
/2
13
, where f
S
represents
the WDT clock source. As mentioned earlier this clock source can come from either the internal
WDT oscillator or from the system clock divided by four.
If the instruction clock is used as the clock source, it should be noted that when the system enters
the power-down mode, then the instruction clock is stopped and the WDT will lose its protecting
purposes. In such cases, the system can only be restarted via external logic. For systems that op-
erate in noisy environments, using the internal WDT oscillator is strongly recommended.
Under normal program operation, the WDT time-out will initialize a
²
chip reset
²
and set the status
bit
²
TO
²
. However, if the system is in the power-down mode, only a WDT time-out reset from
²
HALT
²
will be initialized which will only reset the Program Counter and SP. Three methods can be adopted
to clear the contents of the WDT. The first is an external hardware reset (a low level on the RES pin),
the second is via software instructions and the third is via a
²
HALT
²
instruction. There are two meth-
ods of using software instructions to clear the Watchdog Timer, one of which must be chosen by con-
figuration option. The first option is to use the single
²
CLR WDT
²
instruction while the second is to
use the two commands
²
CLR WDT1
²
and
²
CLR WDT2
²
. For the first option, a simple execution of
²
CLR WDT
²
will clear the WDT while for the second option, both
²
CLR WDT1
²
and
²
CLR WDT2
²
must both be executed to successfully clear the WDT. Note that for this second option, if
²
CLR
WDT1
²
is used to clear the WDT, successive executions of this instruction will have no effect, only
the execution of a
²
CLR WDT2
²
instruction will clear the WDT. Similarly after the
²
CLR WDT2
²
in-
struction has been executed, only a successive
²
CLR WDT1
²
instruction can clear the Watchdog
Timer.
Note 1. The 4-to-1 configuration option to select f
S
/2
12
, f
S
/2
13
,f
S
/2
14
or f
S
/2
15
is not applicable in the
HT46R47/HT46C47, which has a fixed f
S
/2
15
division ratio.
2. Because only the last stage of counter chain is cleared by instructions, the WDT time-out period
varies. As an example, the selected value of 2
16
/f
S
may range from 2
16
/f
S
to 2
15
/f
S.
Chapter 1 Hardware Structure
75
C o n f i g .
O p t i o n
S e l e c t
W D T S o u r c e S e l e c t
C o n t r o l
L o g i c
C L R W D T 1 F l a g
C L R W D T 2 F l a g
1 o r 2 I n s t r u c t i o n s
f
S Y S
/ 4
W D T O S C O u t p u t
8 - b i t D i v i d e r
f
S
C L R
W D T T i m e - o u t
( 2
1 3
/ f
S
, 2
1 4
/ f
S
, 2
1 5
/ f
S
o r 2
1 6
/ f
S
)
¸
2
7 - b i t P r e s c a l e r
f
S
/ 2
8
f
S
/ 2
1 2
, f
S
/ 2
1 3
, f
S
/ 2
1 4
o r f
S
/ 2
1 5
C o n f i g O p t i o n
Watchdog Timer