Analysis of low-power standby mode and power-down mode of single-chip microcomputer

In a battery-powered system, sometimes in order to reduce battery power consumption, it is necessary to use low-power mode when the program is not running. There are two types of low-power modes: standby mode and power-off mode.

The low power mode is controlled by the power control register PCON, which we mentioned in the previous lesson. The power control register is an 8-bit register defined bit by bit. Its format is as follows

PCON is mainly a special register set for power control of CHMOS type microcontroller. The unit address is 87H and its structure format is as follows:

PCON Power Management Register Structure

PCOND7D6D5D4D3D2D1D0 bit sign SMOD

GF1GF0PDIDL

PCON flag bit description:

In CHMOS type microcontrollers, except for the SMOD bit, all other bits are dummy. SMOD is the serial port baud rate multiplication bit. When SMOD = 1, the serial port baud rate is doubled. The system reset defaults to SMOD = 0.

Definition of each bit: SMOD: This bit is related to serial port communication.

SMOD=0; When serial port mode is 1, 2, 3, the baud rate is normal.

SMOD=1; When serial port mode is 1, 2, or 3, the baud rate is doubled.

GF1, GF0: Two general working flags that users can use freely.

PD: Power-down mode setting bit.

PD=0 The microcontroller is in normal working state.

PD=1 The MCU enters the Power Down mode and can be awakened by an external interrupt or hardware reset mode. After entering the Power Down mode, the external crystal oscillator stops oscillating, the CPU , timer , and serial port all stop working, and only the external interrupt works.

IDL: Idle mode setting bit.

IDL=0 The microcontroller is in normal working state.

IDL=1 The MCU enters the idle mode. Except for the CPU, all other parts continue to work. In the idle mode, it can be awakened by any interrupt or hardware reset.

SMOD is the baud rate multiplication bit used in serial communication. GF1 is the general flag bit 1 and GF0 is the general flag bit 0. PD is the power-down mode bit. PD=1 enters the power-down mode. IDL is the standby mode bit. IDL=1 enters the standby mode. In other words, as long as an instruction is executed to make the PD bit or IDL bit 1, then how does the microcontroller enter or exit the power-down working mode and standby working mode? Let's introduce it.

1 Standby mode

Entering standby mode

When the instruction is used to make the IDL of the PCON register = 1, the system enters the standby mode. At this time, the CPU stops working, but the clock signal is still provided to the RAM, timer, interrupt system and serial port. At the same time, the stack pointer SP, program counter PC, program status word PSW, accumulator ACC and all general registers are frozen. The current consumption of the microcontroller is reduced from 24mA to 3.7mA, which can save power consumption.

Exiting standby mode

To exit the standby mode, you can introduce an interrupt and arrange a RETI instruction in the interrupt program. We don't know what an interrupt is yet. Of course, it doesn't matter. In fact, the standby mode is similar to the sleep mode when we use the computer.

2 Power-off mode

Entering standby mode

When the instruction is used to make the PD of the PCON register = 1, the power-down mode is entered. At this time, all operations of the microcontroller stop, and only the data in the internal RAM is retained. In the power-down mode, the power supply can be reduced to 2V and the power consumption is only 50uA. At this time, the display and hard disk are also turned off.

Exiting standby mode

The only way to exit the power-down mode is to reset. However, the reset should be performed after the power supply voltage returns to normal. The reset time should be greater than 1mS. The power supply voltage cannot be reduced before entering the power-down mode. Therefore, a reliable microcontroller circuit is better to have a power detection circuit. Obviously, the power-down mode and the standby mode are two different low-power working modes. The former can reduce power consumption when there is no external event trigger, while the latter is used when the program stops running.

KEIL MCU C language standby mode implementation:

PCON=0X02; //MCU enters standby mode (power-down mode)