MC56F802x 56800E core - from ADC value to real Frac16

[sandrom]

Hello,

I'm using MC56F8023 DSC and I would like to better understand relatioship between ADC value read from A/D converter and the Fractional arithmetic. Consider this example (based on AN1911).

We have the A/D converter (12 bit) that reads the DC Bus from a classic voltage divider. The fractional values used in the algorithms are scaled on 309.0 V, so for example 60V become ((60.0 / 309.0) * 32767) = 6362.

My question is: how do I convert the 4095-based value read from ADC to its corresponding Fractional value?

I tried this:

Frac16 value;

value = FRAC16(adcValue / 4096.0);

But I think this is not the right way (I get almost the value I expect, but not the precise one), am I right?

Thanks in advance

Sandro

[xiangjun_rong]

Hi, Sandro,

This is the ADC result register format:

The maximum positive sample value is 0x7FF0 when the analog voltage is 3.3V in single-ended mode, the minimum negative sample value is 0x8000 when the analog voltage is -3.3V in differential mode or the OFFSET register value is 0x4000 in single-ended mode.

In conclusion, after you read the ADC result register to a variable, the variable is in FRAC16 format itself, you do not need any operation.

Hope it can help you

BR

Xiangjun Rong

[sandrom]

Hello Xiangjun,

thank you for the answer, this is cool! I didn't figure out that the 3-bit offset would adapt the register with the Fractional arithmetic. My mistake was that I use macro from PE to get the ADC value, and this macro give the value without the 3-bit offset, correctly, but not for my need.

I try now to use the value with the offset, and if I still have problem I will let you know.

OT: do you think that this register-format was made apposite for Frac purpose?

Thank you very much Xiangjun,

BR

Sandro

[xiangjun_rong]

Hi, Sandro,

Yes,  the ADC result register format is designed to be  apposite for Frac purpose.

Hope it can help you

BR

Xiangjun rong