Exploring STC 8051 Microcontrollers – Coding

Analogue Comparator

Analogue comparator is not typically found in many microcontrollers but STC8A8K64S4A12 packs one analogue comparator. Though this comparator has limited options, it is still very useful and very easy to use. It can be used for low battery/voltage detection, for comparing voltage levels, etc. The comparator can also be used to build simple switch mode power supplies and achieve feedback for various tasks.

The comparator block shows that P3.6 and P3.7 GPIO pins can be used as positive and negative inputs to the comparator unit. Alternatively, we can also use internal 1.344V voltage reference source or ADC input. The comparator output can be optionally filtered both using analogue and digital techniques. Lastly, we can retrieve comparator output via GPIO pins, flag and interrupts.


 #include "STC8xxx.h"
#include "BSP.h"
void setup(void);
void main(void)
        if(CMP_get_comp_status != 0x00)
void setup(void)
    CLK_set_sys_clk(IRC_24M, \ 
                    2, \
                    MCLK_SYSCLK_no_output, \
    CMP_setup(CMP_positive_input_P37, \
              CMP_negative_input_P36, \
              CMP_output_disable, \
              CMP_result_positive_output, \
              CMP_enable_analog_filtering, \



Analogue comparator is set up very easily by choosing the positive and negative input sources, output state, result state and filtering values. After setting all these, the comparator needs to be enabled.

CMP_setup(CMP_positive_input_P37, \
         CMP_negative_input_P36, \
         CMP_output_disable, \
         CMP_result_positive_output, \
         CMP_enable_analog_filtering, \

Here, we used the GPIO pins P3.6 and P3.7 as physical inputs to the comparator unit. We disabled the physical comparator output. The comparator will give output when positive input is larger than negative input. Lastly, 0.1µs analogue filtering is used along with some digital filtering by the addition of some latency. These filtering ensure that short-lived false signals are ignored.

Inside the main loop, comparator’s result flag status is continuously monitored. P5.5 onboard LED is turned on or off according to this flag’s status.

 if(CMP_get_comp_status != 0x00)


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  • Hi, I am using STC MCU since 10 years. Tech support is ZERO. but they are low cost, very stable. Now I have a problem when the chip that I used is obsolete. Now start to use STC8C2K64S4-28I-LQFP32 but no stc8Cxx.h file, I am using stc8Hxx.h file which compiles but in some stage freeze, the existing firmware. With stc8hxx.h file I can compile STC8F2K64S4-28I-LQFP32 and works not bad
    I wrote them many times for the stc8Cxx.h file never got answer. Where Can I find that file?
    Thank you

  • Hi. Can you explain how to use I2C in the slave mode ?

  • Thanks for these tutorials. I’m getting back into STCmicro coding now, having left them alone for the past several years. Back then I only used the STC89C52RC (and C54RD) but this time I’m also using the more powerful STC15 and STC8 types. Your blogs provide a wealth of useful information.

  • Hello,

    You have done great job with all these tutorials. I am an electronics engineer trying to learn some new stuff. I am located in Greece , Europe and I would like to purchase the development board that you are using and download some datasheets in English if possible but I cannot find them anywhere. Could you please help me?

  • i always get excited when you release new tutorials ,you are really doing a great job i wish i could write code and develop libraries like you.

  • Well, this is very nice and thorough tutorial indeed, many thanks!
    Unfortunately I doubt there is good any reason to learn the STC platform beyond curiosity.
    The STC 8051, although pretty evolved from the original 8051 ISA, does not offer anything crucial to justify the relatively high price of these micros and development tools along with certain cumbersomeness of this ancient platform.
    They simply can not compete even with the legacy Cortex M0 in any way. I am even not aware about any affordable debugger/emulator for them.
    All in all, I would never recommend anybody to start learning/using any 8051 without some very good reason to do so.

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