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Lets say you have Q2.2 notation. You have two numbers:
01.11=1.75
x 10.00=2.00
The product should be 3.50
If I do the binary multiplication in my calculator I get a Q4.4 result:
00111000
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So far, soo good.
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Now I perform the rounding by adding P(3) to P(7:4) I get:
0011+0001=0100
If I'm interpreting this correctly, it appears that my 3.5 rounded to a 4. But, there isn't any fractional part of the number. The rounding forced the result to Q4.0. Instead of getting 11.10=3.5, I got 0100=4.
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Your mistake is not in the rounding part, it's in the cutting part.
You had a Q4.4 value and trimmed it's 4 LSBs. Thus, you got a Q4.0 value. Q4.0 can only represent 3 or 4, not 3.5.
The rounding is actually correct for a conversion to Q4.0: it rounded 3.5 to 4.
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