MCP4922.cpp

00001 #include "MCP4922.h"
00002 
00003 MCP4922::MCP4922(int SPIchannelNum, PinName _CS, PinName _LDAC) : DAC_SPI(SPIchannelNum, _CS, _LDAC){    
00004     messageBits(16);  //messages use a 16 bit word
00005     frequency(20000000);  //set default frequency to 20MHz    
00006     autoUpdate=1;
00007     VrefA=5000; //assume a 5V reference voltage for use with write_mv().  This value can be configured;
00008     VrefB=5000;
00009     gain =1; //set gain to x1 to match Vref.
00010     buffered =0; //leave output buffer off
00011 }
00012 
00013 void MCP4922::select(char DACnum){
00014     DACselect=DACnum & 0x01; //choose between DAC A and DAC B
00015 }
00016 
00017 void MCP4922::write_mV(int mV){
00018     int Vref;
00019     if (DACselect==1){
00020         Vref=VrefB;
00021     }
00022     else{
00023         Vref=VrefA;
00024     }
00025     int DACvalue= mV* 4096/Vref *(gain+1); //scale voltage to a DAC value.
00026     write(DACvalue);
00027 }
00028 
00029 void MCP4922::write(int value){
00030     //valid input values are 0 - 4095.   4096 should scale to Vref.   
00031     //All serial commands are 16 bit words.   The highest 4 bits are control bits, while the last 12 are the data bits for the 12-bit DAC MCP4822.
00032     //bit 15: select which DAC to use.
00033     //bit 14: 0=unbuffered , 1= buffered
00034     //bit 13: 0= gain x2, 1= gain x1
00035     //bit 12: 0= DAC active, 1= shut down DAC
00036     //bit 11-0: Data bits for the DAC.
00037     
00038     if (value > 0xFFF){
00039         value = 0xFFF;  //any out of range values will be truncated to our max value
00040     }    
00041     value=value & 0xFFF; //limit our value to 12 bits.
00042     
00043     //SCK=0;  //set the clock low.   Data is read on the rising edge of the clock.  
00044     LDAC=1;
00045     CS=0; //enable the chip to recieve data
00046     //bit 13: 0= gain x2, 1= gain x1
00047     char gain2 = 0;
00048     if( gain == 0 )
00049         gain2 = 1;
00050     char controlbits=(DACselect<<3) + (buffered<<2) + (gain2<<1) +1;
00051     
00052     int message= (controlbits<<12)+value;
00053     (*DACspi).write(message);
00054     CS=1;  //signal end of message.   The data will be loaded into the internal registers.
00055     if(autoUpdate){ LDAC=0;} //trigger the update of the output voltage.
00056       
00057 }
00058 void MCP4922::update(){
00059     //triggers the DAC to update output on command.   Useful if we wish to synchronize the DAC output value with another event.
00060     LDAC=0;
00061 }
00062 
00063 //Set the multiplying factor for the output.  Valid inputs are gains of 1 and 2.*/
00064 void MCP4922::setGain(int gain_value){
00065     if (gain_value>1){ 
00066         gain =1; //Set gain to x2
00067     }
00068     else{
00069         gain=0; //Set gain to x1.   Limits range to 2.098mV
00070     }
00071 }