2.1.2 Power Down (PD) Mode
An instruction that sets PD/PCON.1 cause that to be the last instruction executed before going into the Power-
down mode. In the Power-Down mode, the on-chip oscillator and the Flash memory are stopped in order to
minimize power consumption. Only the power-on circuitry will continue to draw power during Power-Down.
The contents of on-chip RAM and SFRs are maintained. The only way to wake-up from power-down mode
is hardware reset. The power-down mode can be woken-up by RESET pin, external interrupt /INT0~/INT3,
RXD pin, T0 pin, T1 pin . When it is woken-up by RESET, the program will execute from the address 0x0000.
Be carefully to keep RESET pin active for at least 10ms in order for a stable clock. If it is woken-up from I/O,
the CPU will rework through jumping to related interrupt service routine. Before the CPU rework, the clock is
blocked and counted until 32768 in order for denouncing the unstable clock. To use I/O wake-up, interrupt-related
registers have to be enabled and programmed accurately before power-down is entered. Pay attention to have at
least one “NOP” instruction subsequent to the power-down instruction if I/O wake-up is used. When terminating
Power-down by an interrupt, the wake up period is internally timed. At the negative edge on the interrupt pin,
Power-Down is exited, the oscillator is restarted, and an internal timer begins counting. The internal clock will be
allowed to propagate and the CPU will not resume execution until after the timer has reached internal counter full.
After the timeout period, the interrupt service routine will begin. To prevent the interrupt from re-triggering, the
interrupt service routine should disable the interrupt before returning. The interrupt pin should be held low until
the device has timed out and begun executing. The user should not attempt to enter (or re-enter) the power-down
mode for a minimum of 4 us until after one of the following conditions has occured: Start of code execution(after
any type of reset), or Exit from power-down mode.
The following example C program demostrates that power-down mode be woken-up by external interrupt .
/*------------------------------------------------------------------*/
/* --- STC MCU International Limited -------------------------------*/
/* --- STC89xx Series MCU wake up Power-Down mode Demo -----------------------*/
/* --- Mobile: (86)13922805190 -------------------------------------*/
/* --- Fax: 86-755-82944243 ----------------------------------------*/
/* --- Tel: 86-755-82948412 ----------------------------------------*/
/* --- Web: www.STCMCU.com -----------------------------------------*/
/* If you want to use the program or the program referenced in the */
/* article, please specify in which data and procedures from STC */
/*------------------------------------------------------------------*/
#include <reg51.h>
#include <intrins.h>
sbit Begin_LED = P1^2;
//Begin-LED indicator indicates system start-up
unsigned char Is_Power_Down = 0;
//Set this bit before go into Power-down mode
sbit Is_Power_Down_LED_INT0
= P1^7; //Power-Down wake-up LED indicator on INT0
sbit Not_Power_Down_LED_INT0
= P1^6; //Not Power-Down wake-up LED indicator on INT0
sbit Is_Power_Down_LED_INT1
= P1^5; //Power-Down wake-up LED indicator on INT1
sbit Not_Power_Down_LED_INT1
= P1^4; //Not Power-Down wake-up LED indicator on INT1
sbit Power_Down_Wakeup_Pin_INT0 = P3^2; //Power-Down wake-up pin on INT0
sbit Power_Down_Wakeup_Pin_INT1 = P3^3; //Power-Down wake-up pin on INT1
sbit Normal_Work_Flashing_LED
= P1^3; //Normal work LED indicator
void Normal_Work_Flashing (void);
void INT_System_init (void);
void INT0_Routine (void);
void INT1_Routine (void);
