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How to convert a 24 bit, two’s complement value into a signed integer?

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I want to read an AD converter using the Raspberry Pi SPI interface controlled by Matlab. The AD converter outputs 24 bit data in the two's complement format, MSD first. The Matlab writeRead command returns the data as a row vector of data type char
How can I quickly translate the data into an integer of data type int32 ?

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Balamurugan S
Balamurugan S on 19 Apr 2018
Dear Walter Roberson, Thank you for your reply. I am expecting -3 as answer, it is working fine for me if p=16 bit two's complement number ( using w=typecast(uint16(bi2de(w)),'int16')) but when p=64 bit two's complement number, it is not giving correct answer as -3. And the function "swapbytes(typecast(uint8(bin2dec(char(reshape(P,8,[]).' + '0'))),'int64'))" not giving correct answer.
Walter Roberson
Walter Roberson on 19 Apr 2018
>> P=[1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1]
P =
Columns 1 through 23
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Columns 24 through 46
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Columns 47 through 64
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1
>> swapbytes(typecast(uint8(bin2dec(char(reshape(P,8,[]).' + '0'))),'int64'))
ans =
int64
-3
Looks like -3 to me.

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Accepted Answer

James Tursa
James Tursa on 10 Jun 2014
Edited: James Tursa on 10 Jun 2014
s = 2's complement 24-bit string to convert
b = [s([1 1 1 1 1 1 1 1]) s]; % sign extension
k = typecast(uint32(bin2dec(b)),'int32'); % equivalent decimal number

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Evangelos
Evangelos on 17 Oct 2014
This is also very helpful to me. However I have one question. When you say 24-bit string you mean something like 0bxxxxxxxxxxxxxxxxxxxxxxxx or a 24-bit hex string like 0xaaaaaa ?
Guillaume
Guillaume on 17 Oct 2014
This is is obviously a 24-bit binary string. Hence the extension by 8 bits to 32 bits.

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More Answers (1)

Murat Belge
Murat Belge on 10 Jun 2014
There is an example MATLAB class in Raspberry Pi support package that does something similar for MCP300x ADC's. Here is the readVoltage() method for this class:
function voltage = readVoltage(obj, adcChannel)
validateattributes(adcChannel, {'numeric'}, ...
{'scalar', '>=', 0, '<=', obj.NumAdcChannels-1}, '', 'adcChannel');
adc = obj.getAdcChannelSelect(adcChannel);
data = uint16(obj.spiObj.writeRead([1, adc, 0]));
highbits = bitand(data(2), obj.Lsb2);
voltage = double(bitor(bitshift(highbits, 8), data(3)));
voltage = (obj.VoltageReference/1024) * voltage;
end
where obj.Lsb2 is defined as bin2dec('00000011'). The value read from ADC is 10-bits stashed into two 8-bit values. The upper two-bits is in byte 2 and the 8 least significant bits are in byte 3.
You can take a look at the entire class definition here:
<Support package installe dir>\raspi\+raspi\+internal\mcp300x.m

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