Application of internal analog comparator in microcontroller

Generally speaking, microcontrollers with internal A/D converters are relatively expensive, and generally only have 8 to 10-bit resolution, which is obviously not applicable when high resolution is required; while ordinary microcontrollers do not have A/D converters at all. D converter. With the development of modern electronic technology, some microcontrollers with small size and built-in analog comparators have appeared, such as ATMAL's AT89C2051, ZILOG's Z86E04, MICROCHIP's PIC16C620, etc. When these microcontrollers are used, the port connecting the comparator is generally only used for ordinary purposes. I/O is used, but its built-in analog comparator is rarely used. Taking AT89C2051 as an example, let's talk about the new method of using the built-in analog comparator of the microcontroller to form an A/D converter.

1 Hardware conversion circuit
   AT89C2051 is one of the MCS51 microcontroller series. Although it has only 20 pins, it integrates the standard core of the 51 series microcontroller, including 2k program memory, 128-byte data memory, and two 16-bit timer counters. , a standard full-duplex UART and an accurate analog comparator that were not available in previous products. Figure 1 is a circuit schematic diagram of a dual-integrating A/D converter using the analog comparator of AT89C2051. Among them: the structure of the built-in analog comparator is shown in the dotted line in the figure. The positive and negative input terminals of the comparator are connected to P1.0 and P1.1 respectively. These are two open-drain outputs without pull-up resistors. and input port. When writing "1" to P1.0 and P1.1, M1 and M2 are cut off, which is equivalent to the digital part of P1.0 and P1.1 being left floating. At this time, the input of the comparator is not affected by the output of the microcontroller port. Impact; Since P1.0 and P1.1 have strong current sinking capabilities, when "0" is written, P1.0 and P1.1 can sink 20mA of current, and the saturation voltages of M1 and M2 are very low. This feature can be used to completely discharge the integrating capacitor. The output end of the comparator is connected to P3.6 inside the microcontroller, and the output result of the comparator can be obtained by reading P3.6. Therefore, using the built-in comparator of AT89C2051 and a small number of peripheral devices, a dual-integrating A/D converter can be formed. In Figure 1, I ₀ is a constant current source, its current is about 0.5 ~ 2mA, C _f is the integrating capacitor, the selection of C _f and I ₀ depends on the number of digits in A/D conversion, Vref is the reference voltage, generally Take half of the maximum analog input voltage, U2 is an analog switch, channel 0 is connected to the reference voltage, and channels 1 to 7 are connected to the analog input, that is, the A/D converter has 7 input channels.

　

2. Conversion process:
   When the constant current source integrates the capacitor, the voltage on the integrating capacitor is linearly proportional to time. In this way, the timing counter inside the microcontroller can be used to measure the integration time of the reference voltage and the analog input voltage respectively, and then through the operation of the CPU to get the conversion result. The ports P1.2 to P1.4 of the microcontroller can be used to output the analog switch channel selection address. The timing counter T0 can be set to mode 1, 16-bit timing status, used to measure the integration time. The conversion process can be divided into 5 steps:
    The first step is to discharge the integrating capacitor, mainly writing "0" to P1.1, using its characteristics of large sink current to discharge C _f , and at the same time, the timer counter T0 is cleared.
    The second step is the reference voltage integration, that is, the analog switch selects channel 0, which is equivalent to connecting Vref to the positive input terminal of the comparator, writing "1" to P1.1, and starting the timer counter at the same time. In this way, I ₀ begins to respond to C _f Integration; the program loops to read the P3.6 status to detect the output result of the comparator. When the integrated voltage on the integrating capacitor is slightly greater than (because the comparator has extremely high gain, it can be approximately regarded as equal to) the reference voltage, compare The output of the device is inverted, and P3.6 jumps from high to low. After the program detects this jump, it stops the timer counter and saves the timer counter result T _ref at this time . At this time, it can be obtained from the relationship between the constant current source and the capacitor integral: Vref = (I ₀ T _ref )/C _f
　　third The first step is to discharge the integrating capacitor, that is, repeat the first step to discharge C _f and clear the timer counter T0.
    The fourth step is to integrate the input voltage. At this time, the analog switch can select one of channels 1 to 7, which is equivalent to connecting the analog input voltage Vx to the positive input terminal of the comparator. Repeat the second step to integrate the input voltage, and then the integral can be obtained. Time Tx, therefore, V _x = (I ₀ T _x )/C _f The integrated voltage waveform on
　　the integrating capacitor C _{f in the above 4 steps is shown in Figure 2.}

    The fifth step is to obtain the A/D conversion result through CPU operation. By dividing the second step result by the fourth step result, we can get:
    Vref/Vx=Tref/Tx

    After transformation:
    Vx=(VrefTx)/Tref

    The above formula is the result of A/D conversion.
    It can be seen from the above formula that the A/D conversion result Vx is only related to Vref, Tx, and Tref and has nothing to do with I ₀ and C _f . this point is very important. Because it means that the error caused by the temperature drift of the constant current source and the integrating capacitor is suppressed during the conversion process, thereby ensuring the stability of the A/D converter. This is also the advantage of the integral A/D converter. . Theoretically, the accuracy of the A/D converter only depends on the stability of the reference voltage and the accuracy of the microcontroller timing counter, both of which are relatively easy to guarantee. Of course, this refers to the situation when the constant current source is an ideal constant current source. In fact, the characteristics of the constant current source determine the nonlinear error of the A/D converter. Therefore, in situations with higher requirements, one with good linearity should be selected. Constant current source integrated circuits, such as LM334, etc., and when the requirements are not high, the constant current source circuit composed of discrete components as shown in Figure 3 can be used to achieve A/D conversion. At this time, the integrating capacitor can choose a polyester capacitor with a small temperature coefficient, etc.

    It should be noted that since the microcontroller uses port query to detect the output result of the comparator, the port query command of the MCS51 series microcontroller needs to occupy 2 machine cycles, and the timing counter needs to add 1 to each machine cycle. Therefore, when using it, it is possible that when the output of the comparator is inverted, the program cannot stop the timer counter immediately, but has to wait until the next machine cycle runs to the CLRTR0 statement. In this way, the value of the timing counter is always an even number. If this problem is not dealt with in the program, it will cause errors. The solution is to increase the integration time of I ₀ and C _f , that is, double the integration time, and then divide the readings of the timing counter, Tref and Tx, by 2. In this way, the error of adding 1 more to the timing counter can be removed. Get the correct result. In other words, if you want to get 12-bit resolution, you need a 13-bit timing counter reading. Although this algorithm sacrifices the 1-bit resolution of the timing counter and lengthens the conversion cycle, it ensures the accuracy of the measurement results. For microcontrollers with analog comparator output transition interrupts, such as Z86E08, etc., the above process will be simpler. You can use a program to set the interrupt when the comparator output is inverted, and stop the timer counter at the beginning of the interrupt subroutine. The actual integration time is obtained by subtracting the number of machine cycles from the comparator interrupt to the execution of the interrupt subroutine.
    Since the timing counter of AT89C2051 is 16 bits, the resolution of the A/D converter can actually reach 15 bits. Adjusting the values ​​of I ₀ and C _f can change the resolution of the A/D converter. The relationship between the values ​​of I ₀ and C _{f and the resolution and the clock frequency of the microcontroller should satisfy the following formula:}