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What is faster? ADC() conversion and multiplication + divis.

 
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Christophe



Joined: 10 May 2005
Posts: 323
Location: Belgium

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What is faster? ADC() conversion and multiplication + divis.
PostPosted: Tue May 23, 2006 2:59 am     Reply with quote
Hi,

here is my routine to measure analog Battery Voltage:

Code:
//***********************Meten Batterijspanning*******************************//
//Vbat = (150+120)/120*waarde*3/1023
//digitale waarde = 10 bits
void Meet_Vbat()
{
   setup_adc_ports(AN0_AN1_AN3);
   setup_adc(ADC_CLOCK_DIV_32);
   set_adc_channel( 0 );
   delay_us(20);
   Vbat = read_adc() * (270.0 / 120.0) * (3.0 / 1023.0);
   setup_adc(ADC_OFF);
   setup_adc_ports(NO_ANALOGS);
}


Now my Q is about the line:

Code:
Vbat = read_adc() * (270.0 / 120.0) * (3.0 / 1023.0);


wouldn't it be faster to do the arithmetic at first and stating this:

Code:
Vbat = read_adc() * 0.006598240469;
Ttelmah
Guest
PostPosted: Tue May 23, 2006 4:01 am     Reply with quote
If you have 'constants' like this, the compiler works them out at compile time, _provided_ the arrangement is such that this part of the equation can obviously be solved first.
So:

Vbat = read_adc() * ((270.0 / 120.0) * (3.0 / 1023.0));

(notice the extra pair of brackets), will result in exactly the same code as using your 'presolved' version.:-)

As written, the compiler is not quite smart enough to realise that the whole block can be solved.
As a seperate comment, consider using integers instead. If you use integer 1/100th volts, then you can use integer arithmetic. Much faster and smaller, and also quicker for tests/printouts too. The compiler does have the ability to print an integer as a 'scaled' value. Might be worth considering.
As a final comment, the value at the end of the equation, should be 1024, not 1023. A ten bit ADC, gives 1024 'steps', with the full scale voltage (3v), never actually being reached. If it was, it'd be a count of 1024, not 1023, with the transition exactly 'at' the full scale voltage.

Best Wishes
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