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How to calculate the analog value from read_Adc()

 
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Johnsecada
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How to calculate the analog value from read_Adc()
PostPosted: Mon May 19, 2008 4:35 pm     Reply with quote
hello

I has 8-bit adc . I read the digital value (int8) from read_adc() function. I want to

I found this statement analog_value = (float)(read_adc()*5/256)
my reference voltage is 5V.
How it works ?
read_adc()returns 8-bits binary number like this 00000001 . When we do *5 the binary number should be automatically convert to int.

Could someone tell me what actually happen here?

10x all
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Mon May 19, 2008 5:03 pm     Reply with quote
Try this instead:
Code:
analog_value = (float)(my_read_adc()*5L)/256;
Johnsecada
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I want to undestand how it works
PostPosted: Tue May 20, 2008 1:03 am     Reply with quote
Hello

i want to understand how it works ... Could you give me explanation.
What is 5L ?
RLScott



Joined: 10 Jul 2007
Posts: 465

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Re: I want to undestand how it works
PostPosted: Tue May 20, 2008 5:30 am     Reply with quote
Johnsecada wrote:
Hello

i want to understand how it works ... Could you give me explanation.
What is 5L ?

The default size of type int is 8 bits. If you have read_adc()*5/256, then all integer arithmetic will be conducted with 8-bit values. But read_adc() * 5 can overflow the 8-bit limit of 255. 5L means 5 "Long", so that 5 is treated as an int16. This forces read_adc() also to be promoted to int16 before the "*" operation, so there will be no overflow. Another way to express it is with a cast:

read_adc() * (int16)5

Robert Scott
Real-Time Specialties
Johnsecada
Guest
Great
PostPosted: Tue May 20, 2008 6:16 am     Reply with quote
10x very much Smile
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