For loops in stateflow

I'm not understanding how for loops are expanded in state flow. I the chart below I'm attempting to break a 32-bit value into 8, 4-bit chunks and store them into an array. I'm expecting bits 32-29 to go in buf(1), bits 28-25 in buf(2), bits 24-21 in buf(3) and so on. However, it seems that bits 1-4 get stuffed into each element of buf. Why is this not working properly and what is the correct way to implement something like this?

Answer 1

[Samar]

Hi John,

In your chart, the shift distance is computed using unsigned fixed‑point (fi) arithmetic (because i is an unsigned fi). That can cause the negative shift count you intend (right shift) to be type-promoted/wrapped instead of staying a proper negative integer, so the bitshift effectively behaves like “no meaningful shift,” and then bitand(..., 0xF) keeps returning the lowest nibble for every element. This is consistent with Stateflow fixed‑point promotion behavior and fixed‑point operation rules stated in the MathWorks Documentation links given here: https://www.mathworks.com/help/stateflow/ug/how-fixed-point-data-works-in-stateflow-charts.html

The method explained below can help:

Make the shift amount a built‑in signed integer (e.g., int32), or use built‑in integer loop indices so the shift count is not a fi.

value = uint32(hex2dec('12345678'));
for i = 0:7
    sh = int32(28 - 4*i);
    buf(i+1) = bitand(bitshift(value, -sh), uint32(15));
end

Negative shifts mean right shift, and this works reliably when the shift count is a proper signed built‑in numeric type.

You can refer the following MathWorks Documentation/MATLAB Central links for more understanding:

• MATLAB bitshift doc (negative k shifts right): https://www.mathworks.com/help/matlab/ref/bitshift.html
• Fixed‑Point fi bitshift doc (shift count k must be a built‑in numeric scalar): https://www.mathworks.com/help/fixedpoint/ref/embedded.fi.bitshift.html
• MATLAB Central thread on type/signedness affecting bitshift: https://www.mathworks.com/matlabcentral/answers/852420-bitshift-cannot-work-on-negative-numbers

Hope this helps!

Regards,

Samar

[John]

Thanks, that cleared some things up for me. It seemed like my main problem was that the index variable, i, was unsigned.