HT48R30A-1 데이터 시트보기 (PDF) - Holtek Semiconductor
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HT48R30A-1 Datasheet PDF : 39 Pages
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HT48R30A-1/HT48C30-1
Watchdog Timer - WDT
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.
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 operates in the same
manner except that in the HALT state the WDT may stop
counting and lose its protecting purpose. 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 de-
fined flags, which can be used to indicate some specified
status.
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.
WS2
0
0
0
0
1
1
1
1
WS1
0
0
1
1
0
0
1
1
WS0
0
1
0
1
0
1
0
1
Division Ratio
1:1
1:2
1:4
1:8
1:16
1:32
1:64
1:128
WDTS (09H) Register
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.
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.
· WDT and WDT prescaler will be cleared and re-
counted again (if the WDT clock is from the WDT os-
cillator).
· All of the I/O ports maintain their original status.
· The PDF flag is set and the TO flag is cleared.
The system can leave the HALT mode by means of an
external reset, an interrupt, an external falling edge sig-
nal on port A or a WDT overflow. An external reset
causes a device initialization and the WDT overflow per-
forms a ²warm reset². After the TO and PDF flags are
examined, the reason for chip reset can be determined.
The PDF flag is cleared by system power-up or execut-
ing the ²CLR WDT² instruction and is set when execut-
ing the ²HALT² instruction. The TO flag is set if the WDT
time-out occurs, and causes a wake-up that only resets
the Program Counter and SP; the others remain in their
original status.
S y s te m C lo c k /4
R TC O SC
W DT
O SC
O p tio n
S e le c t
8 - b it C o u n te r
W D T P r e s c a le r
7 - b it C o u n te r
8 -to -1 M U X
W S 0~W S 2
W D T T im e - o u t
Watchdog Timer
Rev. 2.00
11
April 24, 2009
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