MSP430, sigma-delta AD

Jun 27, 2006 4 Replies
B

Dear All,



Running on TI MSP430, a sigma-delta AD design, read a signal from UART RX, do SD AD, and then output to UART TX.



Setup: UART is 9620 baudrate.



1 bit Sigma-delta AD. Switched-capacitor input. load capacitor 14pF ( need to change? )
1.2V reference voltage ( provided by internal )

Here is partial code: ///////////////////////////////////// ... unsigned int RXTXData,results; ....



while (1) { RX_Ready(); // UART ready to RX one Byte _BIS_SR(CPUOFF + GIE); // Enter LPM0 Until character RXed



SD16CCTL0 |= SD16SC; // Set bit to start conversion while ((SD16CCTL0 & SD16IFG)==0); // Poll interrupt flag results = SD16MEM0; // Save CH0 results (clears IFG) _NOP();



if (results >32767) { results=results-32767; RXTXData=results/26.214; } if (results == 32767) { RXTXData=0; } if (results


Vote
Twitter Facebook LinkedIn
B

"Boki" ???????: snipped-for-privacy@j72g2000cwa.googlegroups.com...

Doing shift ? due to unsigned ?

Vote
Twitter Facebook LinkedIn
J

I very much doubt it -- it just looks like the code after _NOP() just performs scaling.

Because apparently the input device connected to your A/D produces voltages that are supposed to be interpreted as 1250 down to 0 (1250=32768/26.214) as the input voltage rises from 0 to half the A/D's reference voltage, and then 0 to 1250 as it ranges from half the reference up to the reference. This assumes the ADC produces codes from 0-65535 as the input voltages rises from 0 to the reference voltage; this is not at all true on some ADCs, so should be checked in the part's data sheet.

The test for "results == 32767" appears unnecessary if you change the first "if" statement to include "if (results >= 32767) ..."

What is the A/D converter input connected to?

---Joel Kolstad

Vote
Twitter Facebook LinkedIn
I

The sequence of ifs looks very odd to me. I would understand it better if the last two were else ifs.

Assuming that they means else ifs, which is a big assumption, this seems to be code that gets absolute value of deviation of SD16MEM0 from 32768, normalized to whatever by dividing by 26.214.

i

Vote
Twitter Facebook LinkedIn
B

Joel Kolstad =E5=AF=AB=E9=81=93=EF=BC=9A

forms

es

4) as

hen 0

rom 0

be

first

Yes, you are right, "results =3D=3D 32767" is unnecessary.

It is a pulsation application, what I strange is the output will become a 'V' form...

0~32767: when RX higher, the output TX value becom lower. 32768~65535: when RX higher, the output becom higher.

Still can't understand why do this..

Best regards, Boki.

Vote
Twitter Facebook LinkedIn

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required