HT48E70 datasheet(13/44 Pages) HOLTEK

13 / 44 page

HT48E70

Rev. 1.00

September 16, 2005

· 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.

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 options. Awakening from an I/O port stimulus,

the program will resume execution of the next instruc-

tion. If it awakens from an interrupt, two sequence may

occur. If the related interrupt is disabled or the interrupt

is enabled but the stack is full, the program will resume

execution at the next instruction. If the interrupt is en-

abled and the stack is not full, the regular interrupt re-

sponse takes place. If an interrupt request flag is set to

²1² before entering the HALT mode, the wake-up func-

tion of the related interrupt will be disabled. Once a

wake-up event occurs, it takes 1024 tSYS (system clock

period) to resume normal operation. In other words, a

dummy period will be inserted after a wake-up. If the

wake-up results from an interrupt acknowledge signal,

the actual interrupt subroutine execution will be delayed

by one or more cycles. If the wake-up results in the next

instruction execution, this will be executed immediately

after the dummy period is finished.

To minimize power consumption, all the I/O pins should

be carefully managed before entering the HALT status.

Reset

There are three ways in which a reset can occur:

· RES reset during normal operation

· RES reset during HALT

· WDT time-out reset during normal operation

The WDT time-out reset during HALT mode is different

from other chip reset conditions, since it can perform a

²warm reset² that resets only the Program Counter and

SP, leaving the other circuits in their original state. Some

registers remain unchanged during other reset condi-

tions. Most registers are reset to the

²initial condition²

when the reset conditions are met. By examining the

PDF and TO flags, the program can distinguish be-

tween different

²chip resets².

TO PDF

RESET Conditions

RES reset during power-on

RES reset during normal operation

RES wake-up from HALT mode

WDT time-out reset during normal opera-

tion

WDT wake-up from HALT mode

Note:

²u² stands for ²unchanged²

To guarantee that the system oscillator is started and

stabilized, the SST (System Start-up Timer) provides an

extra-delay of 1024 system clock pulses when the sys-

tem reset (power-up, WDT time-out or RES reset) or the

system awakes from the HALT state.

When a system reset occurs, the SST delay is added

during the reset period. Any wake-up from HALT will en-

able the SST delay.

R E S

V D D

1 0 0 k W

1 0 k W

0 . 1 m F *

0 . 0 1 m F *

Reset Circuit

Note:

²*² Make the length of the wiring, which is con-

nected to the RES pin as short as possible, to

avoid noise interference.

W a r m R e s e t

W D T

H A L T

C o l d

R e s e t

R E S

S y s t e m R e s e t

S S T

1 0 - b i t R i p p l e

C o u n t e r

O S C 1

Reset Configuration

t S S T

R E S

V D D

S S T T i m e - o u t

C h i p R e s e t

Reset Timing Chart

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