Exploring STC 8051 Microcontrollers – Coding

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Wakeup Timer and Low Power Mode

Wakeup timer is an interesting feature that is not seen in most microcontrollers. It allows us to automatically wakeup a microcontroller from sleep or low power mode after a fixed interval. It is especially useful in cases where we need to take periodic short measurements and then keep our device in low power state to conserve power. A good example is a refrigerator temperature controller. The temperature controller would periodically check interior temperature and decide when to operate the compressor for cooling. Since the compressor would work for short times, the control system of the refrigerator need not to be kept active all the times. Periodic short measurements are all that are needed. This, in turn, would help in energy saving. This tactic becomes even more vital if refrigerator of the discussion is battery-operated.    

Code

 #include "STC8xxx.h"
 #include "BSP.h"
  
 void setup(void);
  
     
 void main(void)
 {
     unsigned char i = 0;
     setup();
     
     while(1)
     {
       WKT_disable;
       
       for(i = 0; i <= 15; i++)  
       {
         P55_toggle;
         delay_ms(100);
       }
       
       WKT_enable;
       Go_to_Power_Down_State;
     };
 }
  
 void setup(void)
 {
     CLK_set_sys_clk(IRC_24M, 2, MCLK_SYSCLK_no_output, MCLK_out_P54);
     
     P55_open_drain_mode;
     
     WKT_set_interval(9000);
 }

Schematic

Explanation

In this example, two features of STC8A8k64S4A12 have been shown. Firstly, low power mode is demoed and secondly, the wakeup timer.

First, let’s see how the wakeup timer is set. Wakeup timer is clocked with internal 32kHz oscillator. This keeps it independent from the system clock. However, the internal 32KHz oscillator is not very accurate and so there could be some deviations. To set the wakeup timer, all we need to do is to load its internal 15-bit counter with some value so that we get our desired wakeup time. The wakeup time is calculated according to the following formula:

In the code, the counter value is set to 9000. This would roughly give a wake-up time of 4.5s once the core goes to low-power state. 

 WKT_set_interval(9000);

Note that although the counter is loaded the wake-up time is not started. This is so because we want to wake the micro after it enters low-power mode.

In the main loop, the wake-up timer is disabled. The onboard LED is flashed a few times to indicate that the code in the main loop is running. After flashing the LED, the wake-up timer is started and low-power mode is entered. 

 WKT_disable;
       
 for(i = 0; i <= 15; i++)  
 {
    P55_toggle;
    delay_ms(100);
 }
       
 WKT_enable;
 Go_to_Power_Down_State;

Entering low power mode is easy. Just add the following command:

 Go_to_Power_Down_State;

After entering low-power mode, the LED seems to freeze and there seems no activity at all for some time. After about 4.5s, the LED repeats flashing just like before. This time the LED flashing not only indicates main loop code execution but it also indicates that the wake timer ran and woke up the CPU core from low-power sleep mode.

Demo

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