Frequency of ACLK on lauch pad MSP430FR6989 is not as expected

Klaus Schlitt

I have selected ACLK as clock input for Timer A (TASSEL=1).The two clock dividers of Timer A are configured to divide ACLK by 64.

With this configuration I would expect Timer A is clocked by 32768 Hz / 64 = 512 Hz.

To prove this I start the timer A and wait 60 s.
After this wait time, the TAxR is always about 35210.

This means the clock of Timer A is not 512 Hz but 35210 / 60 s = 587 Hz.
So ACLK is 587 HZ * 64 = 37568 Hz (instead of 32768)

What can be the reason for this?

over 9 years ago

0 Venu S over 9 years ago

TI__Intellectual 960 points

Did you check the content of the TAR register, when you started your timekeeping?
ie, Was the TACLR bit set, exactly when you started with the 60s wait time? (after configuring the timer clock divider and mode)

0 Klaus Schlitt over 9 years ago in reply to Venu S

Prodigy 170 points

After the 60 s is over, the TACLR is set in TAxCCTLn register.
So after 60 s the counter is cleared and the measurement starts again.

0 Ilmars over 9 years ago

Guru 46710 points

Instead of building frequency meter I would configure ACLK pin for output clock function and measure frequency on it directly using scope or freq.counter.

0 Klaus Schlitt over 9 years ago in reply to Ilmars

Prodigy 170 points

Now I have configured P4.1 with ACLK (SELA=0) and P4.0 with MCLK (DCORSEL=0, DIVM=3) to measure it with a scope.
Result:
ACLK: 37736 Hz (15 % too high)
MCLK: 1 MHz (as expected)

Why ACLK is not exactly 32768 Hz?

0 old_cow_yellow over 9 years ago in reply to Klaus Schlitt

Guru 58965 points

You probably did not program LFXT correctly and ACLK defaulted to LFMODCLK (which is ~37.5KHz).
See sample code: msp430fr69xx_cs_03.c of slac668.

0 Klaus Schlitt over 9 years ago in reply to old_cow_yellow

Prodigy 170 points

Yes, you are right! This was the missing part.
Now the ACLK is exactly 32768 Hz.

Thanks a lot for your help!

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Frequency of ACLK on lauch pad MSP430FR6989 is not as expected