利用MAX7651实现ADC转换

摘要：本文提供了利用MAX7651评估板实现模/数转换的源代码和函数。它是MAX7651评估板提供的3个应用例程之一，能够进行写、编译和下载等简单的编程功能。

另请参考：

• 为MAX7651评估板配置Keil µVision IDE参数
• 利用MAX7651评估板的串行下载器将程序装载到闪存内

本文利用8051兼容的微控制器MAX7651实现模/数转换，并给出了程序代码。其中包括一些关键的写操作、编译和程序下载步骤，支持MAX7651EVKIT进行模数转换，并可读取转换结果。程序使用了MAX7651内部的12位集成ADC。

本例源代码利用Keil DK-51 IDE创建，需要从www.keil.com下载并安装Keil DK-51演示软件，以简化例程使用。Keil程序包含MAX7651基本的库文件，接下来，应该下载µVision2®项目文件和源代码：AN3083_uVision2_Project_Files.zip。

本例使用MAX7651EVKIT，当然，必要时也可以使用Keil µVision® IDE仿真器演示这些源代码，无需使用MAX7651EVKIT。软件安装请参考MAX7651EVKIT的快速入门使用手册，其中提供了关于Keil软件安装的详细信息。

注意：这里提供的例程是Maxim给出的公开源代码，可以免费复制，并根据具体应用进行修改。

C程序说明

为了有效使用该例程，需要熟悉C语言编程。程序包括三部分：“ADC Test Function”主程序(main())和两个调用子函数：convert_all_channels()和convert_channel()。主程序首先对MAX7651的寄存器进行初始化，并提供一些标准的库函数。然后配置MAX7651，与PC的标准串口进行通信。

主程序main()调用两个子函数，函数convert_all_channels()顺序对8个ADC输入通道进行转换；函数convert_channel()能够使用户自行选择转换通道并返回ADC转换结果。

为了启动一次转换，只需简单调用函数convert_all_channels()或convert_channel()。函数通过将所要求的转换通道写入MAX7651的ADCON寄存器执行转换操作，然后通过循环检测ADCON寄存器终止转换。转换结束后，将转换结果返回到调用函数内。主程序main()通过调用stdio.h内的printf()函数，通过串口将结果显示在显示器上。

/*-------------------------------------------------------------------------
ADC Test Function (main.c)
Copyright: Maxim Integrated Products
Target: MAX7651
Date: Feb 26, 2004
Author: Maxim Integrated Products

Description: This program will convert 8 channels using the MAX7651
and send the results to Serial Port 0
--------------------------------------------------------------------------*/
#include 					//include MAX7651 register definitions
#include 					//Standard I/O, Print() function.

#define NUMBER_OF_CHANNELS 8			//Convert 8 channels

void convert_all_channels(int* buffer);			//Function declaration
int convert_channel(int adc_ch);  			//Function declaration

void main(void)					//Begin Main()
{
int Adc_Results[NUMBER_OF_CHANNELS];	//Array to store conversion result
int i;					//for loop counter

	convert_all_channels(Adc_Results);		//Convert all channels, Store results in Adc_Results

	#ifndef MONITOR51			//Setup Serial Port if not using Keil Monitor
SCON  = 0x50;				//9600 Baud, 8N1, Xon, Xoff
	TMOD |= 0x20;
	TH1   = 0xFA;
	TR1   = 1;
	TI    = 1;
	PCON |= 0x80;
	#endif

	for (i=0; i < NUMBER_OF_CHANNELS; i++)	//Display contents to Adc_Results
	{
	printf ("CH %d:%x ",i,Adc_Results[i]);	//print the hex
	}

	printf("
 channel 0 %x", convert_channel(0));		//Convert a single channel and display
	printf("
 channel 1 %x", convert_channel(1));
	printf("
 channel 2 %x", convert_channel(2));
	printf("
 channel 3 %x", convert_channel(3));
   	printf("
 channel 4 %x", convert_channel(4));
	printf("
 channel 5 %x", convert_channel(5));
	printf("
 channel 6 %x", convert_channel(6));
	printf("
 channel 7 %x", convert_channel(7));

	while(1);					//End Program, Start infinite loop since there is no OS
}
 /*-------------------------------------------------------------------------
Function: convert_all_channels
Copyright: Maxim Integrated Products
Target: MAX7651
Date: Feb 26, 2004
Author: Maxim Integrated Products

Usage: The function will return 8 conversion results to an array.

Parameters: p_buffer, pointer to an 8 location array stores the conversion results

Return: Values are returned to the calling function using the function parameters

/*-------------------------------------------------------------------------
Setup ADC in the MAX7651
--------------------------------------------------------------------------*/

sfr ADCON  = 0xC5;				//Define address of  MAX7651 ADCON
sfr ADDAT1 = 0xC3; 				//Define address of  MAX7651 ADDAT1 (8MSBs)
sfr ADDAT0 = 0xC2;				//Define address of MAX7651 ADDAT0 (4LSBs)

void convert_all_channels(int* buffer);
--------------------------------------------------------------------------*/

#define NUMBER_OF_CHANNELS 8

void convert_all_channels(int* p_buffer)            			//pointer Buffer to return
{
	int adc_ch;
	int conv_val;

/*-----------------------------------------------
Convert all ADC channels
-----------------------------------------------*/

	for (adc_ch = 0; adc_ch < NUMBER_OF_CHANNELS; adc_ch++)      //for ADC channels 1 to 7
{

/*-----------------------------------------------
Start a conversion and wait for it to complete.
-----------------------------------------------*/

	ADCON = adc_ch;					//Start conversion and select channel
while ((ADCON & 0x80) == 0);			//wait for conversion to complete
conv_val = (ADDAT0 >> 4) |  (ADDAT1 << 4);		//Format the data in 12 bit format

	  *(p_buffer+adc_ch) = conv_val;		//Write result back to calling function
}					//End For
}						//End function convert_all_channels()

 /*-------------------------------------------------------------------------
Function: convert_channel
Copyright: Maxim Integrated Products
Target: MAX7651
Date: Feb 26, 2004
Author: Maxim Integrated Products

Usage: The function will convert and return the result of a Channel.
Parameters: adc_ch, Select ADC channel to be converted.

Channels 0-7 = single ended channel 0-7.
	Channels 8-11 = differential channel pairs {CH0,1}, {CH2,3}, {CH4,5}, {CH6,7}
	Channel 12 = differential reference measurement {REF+,REF-}

Return: Function returns Integer with the conversion result

Function Declaration: int convert_channel(int adc_ch);
--------------------------------------------------------------------------*/

/*-------------------------------------------------------------------------
Setup ADC in the MAX7651
--------------------------------------------------------------------------*/

sfr ADCON  = 0xC5;				//Define address of  MAX7651 ADCON
sfr ADDAT1 = 0xC3; 				//Define address of  MAX7651 ADDAT1 (8MSBs)
sfr ADDAT0 = 0xC2;				//Define address of MAX7651 ADDAT0 (4LSBs)

int convert_channel(int adc_ch)
{
	int conv_val;

if (ADCON <0 || ADCON >12){		//Check for valid channel
return (-2048);				//Using -FS for the error code
}

	ADCON = adc_ch;				//Select channel and Start conversion
while ((ADCON & 0x80) == 0);		//Wait for the conversion to complete

	conv_val = (ADDAT0 >> 4) |  (ADDAT1 << 4);		//Format the data in 12 bit format

	return (conv_val);					//Return result back to calling function
}							//End function convert_chan