Packing 12-bit date into int

Think a struct with union should do.

That's probably not the best way to approach the problem, since there is no 12-bit data type in C.

One way would be to divide the circular buffer into 96-bit blocks (3*32 bit and 8*12 bit). A function that reads/writes to the buffer then first finds the block that contains the requested data. It then extracts the data using a scheme similar to:

data 0 - bits 0:11 of word0
data 1 - bits 12:23 of word0
data 2 - bits 24:31 of word0 OR bits 0:3 of word1
data 3 - bits 4:15 of word1
data 4 - bits 16:27 of word1
data 5 - bits 28:31 of word1 OR bits 0:7 of word2
data 6 - bits 8:19 of word2
data 7 - bits 20:31 of word2

This can be implement with a little bit of bit-masking and bit-shifting.

Think a struct with union should do.

That's probably not the best way to approach the problem, since there is no 12-bit data type in C.

One way would be to divide the circular buffer into 96-bit blocks (3*32 bit and 8*12 bit). A function that reads/writes to the buffer then first finds the block that contains the requested data. It then extracts the data using a scheme similar to:

data 0 - bits 0:11 of word0
data 1 - bits 12:23 of word0
data 2 - bits 24:31 of word0 OR bits 0:3 of word1
data 3 - bits 4:15 of word1
data 4 - bits 16:27 of word1
data 5 - bits 28:31 of word1 OR bits 0:7 of word2
data 6 - bits 8:19 of word2
data 7 - bits 20:31 of word2

This can be implement with a little bit of bit-masking and bit-shifting.

It might be easier to use a 6k array of bytes. Then there are only two pack alternatives for a 12-bit value. Even ADC values starts with a byte and and requires 4 bit from next byte. Odd ADC values should take 4 bits and then the next byte.

It is trivial to convert from ADC value n into the start offset in the byte array.

It might be easier to use a 6k array of bytes.

Depending on the exact ARM architecture version used, using bytes may be fairly wasteful performance-wise. Memory accesses are costly on ARM, and a byte access takes as many (or even more) cycles as a word access.

Correct, but most users don't use a sampling speed high enough that it matters, and the simplified handling of being able to read/write values with a single odd/even if statement may offset the memory bandwidth issue.

It's the usual KISS contra maximum optimization.

Well, the OP now has a good set of options to choose from.

Only he can evaluate their relative merits according to his specific requirements, constraints, etc - unknown to the rest of us...