ht827a0 Holtek Semiconductor Inc., ht827a0 Datasheet - Page 16

• Manufacturer Part Number: ht827a0
• Description: 8-bit Microcontroller With Voice Rom
• Manufacturer: Holtek Semiconductor Inc.

set can then be determined after examining the
TO and PD flags. The PD flag is cleared when the
system powers up or executes the ²CLR WDT² in-
struction and set when the ²HALT² instruction is
executed. The TO flag is set if the WDT time-out
occurs, and causes a wake-up that resets only the
PC and SP. The others maintain their original
statuses.
The port A wake-up and interrupt methods can
be considered as a continuation of normal exe-
cution. Each bit in port A can be independently
selected to wake up the device by mask option.
Awakening from an I/O port stimulus, the pro-
gram resumes execution of the next instruc-
tion. However, if it is awakening from an
interrupt, two sequences may happen. The pro-
gram will resume execution at the next instruc-
tion if the related interrupt is disabled or the it
is enabled but the stack is full. Nonetheless, if
the interrupt is enabled and the stack is not
full, a regular interrupt response takes place.
Once the wake-up event occurs, and the system
clock comes from a crystal, it takes 1024 t
(system clock period) to resume a normal opera-
tion. In other words, the HT827A0 will insert a
dummy period after the wake-up. If the system
clock, on the other hand, is from an RC type of
oscillator, it will continue operation. The actual
interrupt subroutine execution will be delayed
by one or more cycles if the wake-up results
from an interrupt acknowledgment. On the
other hand, it will be executed immediately af-
ter the dummy period is finished if the wake-up
results in the next instruction execution.
To minimize power consumption, all the I/O
pins should be carefully managed before enter-
ing the HALT mode.
Reset
There are three ways in which a reset can occur:
·
·
·
The WDT time-out during HALT is different
from other chip reset conditions, since it can
perform a ²warm reset² that resets only the PC
and SP, leaving the other circuits to remain in
RES reset during normal operation
RES reset during HALT
WDT time-out reset during a normal operation
SYS
16
their original states. Some registers will re-
main unchanged during reset conditions. Most
registers are reset to the ²initial condition² once
the reset conditions are met. The program can
distinguish between different ²chip resets² by
examining the PD flag and TO flag.
Note: ²u² means ²unchanged²
To guarantee that the crystal oscillator is
started and stabilized, XST (Crystal Start-up
Timer) provides an extra-delay by an OSC
mask option. The extra-delay delays 1024 sys-
tem clock pulses when the system awakes from
the HALT state or from system power-up and
the RES transforms low to high. XST is auto-
matically selected if the crystal oscillator is in-
voked. On the other hand, it is disabled when
the RC oscillator is chosen. The XST delay is
added after XST is chosen and awakening from
the HALT state or the system powers up.
The reset duration comes only from RES if an
RC oscillator is selected. An extra delay, on the
other hand, is added during the power-up pe-
riod and any wakeup from HALT only if a crys-
tal oscillator is chosen instead.
The HT827A0 provides another useful feature for
purposes of testing and synchronization. Re-
leasing RES high will start execution if RES
keeps low long enough.
TO PD
1
1
0
u
0
u
u
1
0
1
WDT wake-up HALT
RES reset during power-up
RES reset during normal
operation
RES wake-up HALT
WDT time-out during normal
operation
RESET Conditions
March 15, 2000
HT827A0