could I just use 6 bit A/D in CCS or with PIC18F458?

SOS_help · Guest

for some reason, I don't have to use 8 or 10 bit A/D , I just need 6 bit. how I can do it in CCS with PIC18F458? or I have to change pic chip to 6 bit pic? thanks

Ttelmah · Guest

rnielsen · Joined: 23 Sep 2003 Posts: 852 Location: Utah

I might do it like this:

val = read_adc() & 0x3F;

This will strip the two highest bits from the value. I believe the >> would change the value by actually moving the bits to a different location in the word.

Ronald

languer · Joined: 09 Jan 2004 Posts: 144 Location: USA

Using Ttelmah's approach, you get the 6-MSBs (which is what you want),

Sherpa Doug · Guest

MSB or LSB depends on how big your input signal is. If 256 is 5V then the 6 MSBs will range from zero to 5V. The 6 LSBs range from zero to 1.25V. If your signal is small you don't have to amplify it as much using the LSBs. However the LSBs may give you a little more noise.

Sherpa Doug

Ttelmah · Guest

Guest · Posted: Sat Jul 31, 2004 6:35 am

Generally when I work the ADC I try to make sure the MSB in the data is the MSB of the ADC. That is I assume the data is value between 0.0 1.0 where zero maps 0x00 and 1.0 mapps to 0xFF.

For example if I was doing a 6 bit ADC then I would

value=ADCresult & 0xFC; //6 bit result.

Now if I wanted to read the voltage output as 0.0-5.0 volts with one decimal place. You could do this by multiplying the value by 50.

int16 volts;

volts=value*50;
//volts now has the value of 0-0x31CE

volts=volts>>8; //get the upper word (note faster methods than shift)

//at this point volts is 0-0x31 or 0-49

Now what happens is the volts will be a value between 0-50 which maps to the voltage 0.0-5.0...

Doing the ADC this way is nice in that 6 or 8 bit ADC it does not matter. Even if you have 10 bit ADC then the only thing is the variables need to be 16 bit and the ADC needs to be left justified.

Trampas.

SOS_help · Guest

I can't try the methods you guys metioned here at home, but I am wondering if I do so, 5V input still matches to 64(6 bit)?or just reduces the input voltage range?
thanks again,