ADCH 8bit --> 2 x 4 bit conversion

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mtlost
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Posted by mtlost: Tue. Apr 20, 2010 - 08:07 PM
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Hey AVRFreaks

When you read the ADC0 on an Atmega8, you get an 8bit value in the ADCH. I hope I'm correct on that.

I want to display the ADC reading on 2 x 7 segment displays that is connected to one 74HCT47 7 segment driver and I control the displays with transistors. The ADC data will be shifted into a shift register, one display switched on, switched of again then the next display's data will be shifted into the shift register.

Now the question, how do I split the data in ADCH that is 8 bits so that I can send the first 4 bits to the shift use those bits and then shift in the next 4 bits and use them then?

Regards,

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Tools, Compilers and General Programming, ATmega8
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Koshchi
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Posted by Koshchi: Tue. Apr 20, 2010 - 08:52 PM
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Quote:
When you read the ADC0 on an Atmega8, you get an 8bit value in the ADCH.
Well, you can get an 8 bit value in ADCH. You will have to tell the ADC to do that, though.
Quote:
Now the question, how do I split the data in ADCH that is 8 bits so that I can send the first 4 bits to the shift use those bits and then shift in the next 4 bits and use them then?
You don't. An 8 bit value has the range of 0 to 255, so you need 3 digits to display the value. Unless, of course, if you want hexadecimal output (and your 7 segment drivers can handle it), then you just output the 4 bits you want for each display.

Regards,
Steve A.

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Bronze MarkJohanEkdahl
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Posted by JohanEkdahl: Tue. Apr 20, 2010 - 08:56 PM
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Quote:

Quote:
Now the question, how do I split the data in ADCH that is 8 bits so that I can send the first 4 bits to the shift use those bits and then shift in the next 4 bits and use them then?
You don't. An 8 bit value has the range of 0 to 255, so you need 3 digits to display the value.

Unless the value is displayed in hex..

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mtlost
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Posted by mtlost: Wed. Apr 21, 2010 - 05:28 AM
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Koshchi wrote:
You don't. An 8 bit value has the range of 0 to 255, so you need 3 digits to display the value.

I can add the third display but I doubt if the measurement would go up do 80 so a third display will propebly never be used. Come to think of it, I'll add the third display for futue expansion.

The shift register contains 8 values, but only 4 lines are hardwired and the other 4 pins is NC.

WIll it work if I shift in the 8 bits, switch the first display (only 4 bits will be utilised as only 4 is wired) then I clock the shift 4 more times to shift the data 4 bits then switch the next display?

Regards,

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Gold Markclawson
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Posted by clawson: Wed. Apr 21, 2010 - 08:55 AM
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Quote:

I can add the third display but I doubt if the measurement would go up do 80 so a third display will propebly never be used.

Which suggests you ADC voltage never exceeds 80/256 * Ref? If Ref=5V then are your readings all below 1.56V. Or are you saying you are performing some conversion on the ADCH value after you've read it to limit its range to 0..80?

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mtlost
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Posted by mtlost: Wed. Apr 21, 2010 - 10:25 AM
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There is no conversion ans the max voltage read will be about 1.2V.

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sternst
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Posted by sternst: Wed. Apr 21, 2010 - 10:57 AM
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Quote:
I can add the third display but I doubt if the measurement would go up do 80 so a third display will propebly never be used.
This implies that you want to display a decimal value. Then a simple split into 2 x 4 bit won't help you. You need to split it like this:

uint8_t digit0 = adc_value % 10;
uint8_t digit1 = adc_value / 10;

Stefan Ernst

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mtlost
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Posted by mtlost: Wed. Apr 21, 2010 - 06:05 PM
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O yes, that rings a bell, I completely forgot about that.

Thanx, I'll give it a try.

It's been way too long since I last did some programming......it's a shame that it took me so long to start again.

Thanx sternst

Edit: Thanx, it worked, now just to play around with the values a bit.

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