HT36FA
·
·
·
All I/O ports maintain their original status.
TO PDF
RESET Conditions
The PDF flag is set and the TO flag is cleared.
0
u
0
1
1
0
u
1
u
1
RES reset during power-up
RES reset during normal operation
RES wake-up HALT
The HALT pin will output a high level signal to disable
the external ROM.
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. By examining the TO and PDF
flags, the cause for a chip reset can be determined. The
PDF flag is cleared when there is a system power-up or by
executing the ²CLR WDT² instruction and it is set when a
²HALT² instruction is executed. The TO flag is set if the
WDT time-out occurs, and causes a wake-up that only re-
sets the Program Counter and SP, the others remain in
their original status.
WDT time-out during normal operation
WDT wake-up HALT
Note: ²u² stands for ²unchanged²
To guarantee that the system oscillator has started and
stabilized, the SST (System Start-up Timer) provides an
extra-delay of 1024 system clock pulses during system
power up or when the system awakes from a HALT
state.
When a system power-up occurs, the SST delay is
added during the reset period. But when the reset co-
mes from the RES pin, the SST delay is disabled. Any
wake-up from HALT will enable the SST delay.
The port A wake-up and interrupt methods can be con-
sidered as a continuation of normal execution. Each bit
in port A can be independently selected to wake-up the
device by mask option. Awakening from an I/O port stim-
ulus, the program will resume execution of the next in-
struction. If awakening from an interrupt, two sequences
may occur. If the related interrupts is disabled or the in-
terrupts is enabled but the stack is full, the program will
resume execution at the next instruction. If the interrupt
is enabled and the stack is not full, a regular interrupt re-
sponse takes place.
The functional units chip reset status are shown below.
Program Counter
Interrupt
000H
Disable
Clear
Prescaler
Clear. After master reset,
WDT begins counting
WDT
Timer Counter (0/1)
Input/output ports
Stack Pointer
Off
Input mode
Points to the top of stack
Once a wake-up event occurs, it takes 1024 tSYS (sys-
tem clock period) to resume to normal operation. In
other words, a dummy cycle period will be inserted after
the wake-up. If the wake-up results from an interrupt ac-
knowledge, the actual interrupt subroutine will be de-
layed by one more cycle. If the wake-up results in next
instruction execution, this will execute immediately after
a dummy period has finished. If an interrupt request flag
is set to ²1² before entering the HALT mode, the
wake-up function of the related interrupt will be disabled.
V
D
D
R
E
S
Reset Circuit
To minimize power consumption, all I/O pins should be
carefully managed before entering the HALT status.
V
D
D
R
E
S
t
S S T
Reset
S
S
T
T
i
m
e
-
o
u
t
There are 3 ways in which a reset can occur:
C
h
i
p
R
e
s
e
t
·
·
·
RES reset during normal operation
RES reset during HALT
Reset Timing Chart
H
A
L
T
WDT time-out reset during normal operation
W
a
r
m
R
e
s
e
t
W
T
D
T
e
W
D
T
The WDT time-out during HALT is different from other
chip reset conditions, since it can perform a ²warm re -
set² that just resets the Program Counter and SP, leav-
ing the other circuits to maintain their state. Some regis-
ters remain unchanged during any other reset
conditions. Most registers are reset to the ²initial condi-
tion² when the reset conditions are met. By examining
the PDF and TO flags, the program can distinguish be-
tween different ²chip resets².
i
m
-
o
u
t
R
e
s
e
t
R
E
S
C
o
l
d
S
S
T
R
e
s
e
t
1
0
-
s
t
a
g
e
O
S
C
I
R
i
p
p
l
e
C
o
u
n
t
e
r
P
o
w
e
r
-
o
n
D
e
t
e
c
t
i
n
g
Reset Configuration
Rev. 1.10
11
March 13, 2006
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
