51 single chip microcomputer - ADC explanation (a/d conversion, d/a conversion)

51 MCU must learn series

51 Single chip learning —— Interrupt the system

51 Single chip learning —— Memory details （ Program memory 、 Intraslice RAM、 expand RAM、EEPROM）

51 Single chip learning —— Timer / Counter

51 Single chip microcomputer ——A/D transformation

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Preface

A/D The conversion module is a high-function module of MCU , Used for digital conversion of analog signals , This article takes STC8H8K64U For example, SCM （ Principle and others 51 There is no difference between single-chip computers ）, The single chip microcomputer A/D The module is 16 passageway 12 position . This article mainly excerpts STC Ding Xiangrong 《 Single chip microcomputer principle and Interface Technology 》.

A/D Structure of conversion module

STC8H This single chip computer A/D The input channel of the conversion module has 16 individual （ More channels , At the same time, the more analog quantity can be received ）, Respectively ADC0——ADC15, among ADC15 Used to test internal 1.19V Reference voltage , Working hours , Each input channel works in the high resistance state .

About digits ：12 Bits are used for precision , The more digits , The higher the accuracy , With 5v Voltage as an example , When one , It can only be divided into two ,2.5v Above is 1,2.5v The following is a 0, When two , Can be divided into 4 Share , That is to say 1.25v,2.5v,3.75v,5v Demarcation , Improved accuracy , And so on .

The single chip microcomputer A/D The conversion module consists of Multiple selector switch 、 The comparator 、 Compare registers one by one 、12 Bit digital to analog converter (D/A Conversion module )、A/D Conversion result register 、A/D Conversion module control register and A/D Conversion module configuration register constitute .

A/D Classification of conversion modules

According to the principle of transformation ： Successive approximation type 、 Double integral type 、 parallel / Serial comparison type 、 Voltage frequency conversion type, etc
Classification by conversion speed ： Superhigh speed <=1ns、 High speed <=20us、 Medium speed <=1ms、 low speed <=1s.
Classification by converted digits ：8、12、14、16.

At present, there are mainly Successive comparison converter （ Most commonly used ） and Double integral converter , So next, we will mainly talk about the successive comparison converter .

Successive comparison converter

The successive comparison type analog-to-digital converter is based on the logic of successive comparison , From the top （MSB） Start , The voltage of each input is simulated and internal D/A The converter outputs are compared , After many comparisons , Make the converted digital quantity approach the corresponding value of the input analog quantity , until A/D End of conversion .

The following figure is the schematic diagram of the successive comparison converter

A/D Reference voltage source of conversion module

The A/D The power supply of the conversion module is the same as that of the MCU , but A/D The module has an independent reference voltage source input .
When the measurement accuracy is not high , You can directly use the working voltage of the MCU , Use accurate reference voltage for high accuracy .

A/D Control of conversion module

A/D The conversion module mainly consists of ADC_CONTR、ADCCFG、ADC_RES、ADC_RESL and A/D Conversion module timing control register ADCTIM And control A/D Control and management of the interrupt related control registers of the conversion .

Because there are too many register flag bits , Here, only the more important individual registers are listed .

/**************** measurement AD value *************************/
uint	Get_ADC12bitResult(uchar channel)	//channel = 0~15
{
    
	ADC_RES = 0;	// Clear the conversion result register 0
	ADC_RESL = 0;

	ADC_CONTR = (ADC_CONTR & 0xe0) | 0x40 | channel; 	// start-up ADC
	_nop_();  	_nop_(); 	_nop_();	_nop_();// You need to wait for the circuit to stabilize at the beginning for the first time 

	while((ADC_CONTR & 0x20) == 0)	;	// wait for ADC complete 
	ADC_CONTR &= ~0x20;		// eliminate ADC End mark 
	return	(((uint)ADC_RES << 8) | ADC_RESL );
}

The registers that appear in this code are ADC_CONTR,ADC_RES,ADC_RESL.
among ADC_RES and ADC_RESL Together as a storage result register ,RES It's high 8 position ,RESL It's low 8 position .

ADC_CONTER Everyone is shown in the figure below ：

B7 yes A/D Power control bit of the module ,1 open ,0 close .
B6 yes A/D Switch start control bit ,1 Start the conversion , Automatically clear after conversion 0, by 0 It has no effect on （ When A/D After the conversion module starts , Even if you write 0 Can't stop ）
B5 Is the completion flag bit , After conversion 1, Manual cleaning is required 0.

The main ： start-up A/D Make sure that A/D The conversion module is powered on ,A/D Close after conversion A/D Converting module power can reduce energy consumption . Appropriate time delay is required when opening for the first time , Wait for the internal circuit to stabilize before starting A/D transformation .

A/D Conversion module conversion result selection

A/D After the conversion of the conversion module , The results are saved to ADC_RES and ADC_RESL in （ the reason being that 12 position , A register 8 position , So you need to put together two registers to store the result ）, But there are two storage formats , from ADCFG Medium RESFMT control .
RESFMT=0 when , The result is left aligned , The empty space on the right is automatically 0.
RESFMT=1 when , Align results right , The left space is automatically 0.