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MSP430G2755: Timer A1 / DCOCLK / SMCLK variation with temperature

Roy Chisem
Prodigy 240 points
Part Number: MSP430G2755

I believe I am seeing a variation of a timer period of 20% in the temperature range -40 deg C to 80 deg C.
I have an ISR that raises an interrupt every 50ms on timer A1, the timer source is SMCLK which in turn is
sourced from DCOCLK which is set at 16Mhz using the constants:

CALDCO_16MHZ
CALBC1_16MHZ

defined in msp430g2755

Is this possible, I believe the data sheet indicates a variation of only + / -6%.

  • 0
    TI__Expert 6635 points

    Hey Roy,

    I am seeing the same +/- 6% spec in the datasheet. I have a few quesitons:

    1. What supply voltage are you using?

    2. What is your method of measuring the timer ISR period?

    Thanks!

    -Mitch

  • 0 in reply to Mitch Ridgeway
    Prodigy 240 points

    Hi Mitch

    Thanks for the response, the supply is 3.3V, the measurement is taken from a discrete signal on P1.6 which forms a square wave changed from high to low / low to high every 50ms monitored on an oscilloscope.  The temperature variation is in an atmospheric chamber which accurately controls / reports temperature.

    At 30 C the period for the change in the P1.6 signal is 50ms (correct)

    At -40 C the period for the change in the P1.6 signal has been recorded as 61ms

    At 80 C the period for the change in the P1.6 signal has been recorded as 40ms

    We are about to repeat this with a second piece of the same hardware in an attempt to verify that what we see is consistent.

  • 0 in reply to Roy Chisem
    Prodigy 240 points

    Hi Mitch

    Thanks for help, I have now discovered the problem, I am not sourcing the timer that generates the interrupt from SMCLK as I bilieved but from ACLK & and internal VLO - data sheets indicates a 0.5% drift per degree C between -40C to 80C which is approxiamtly what we are seeing. 

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