MSP430G2553: MSP430G2553 Clock Speed

Abu Bakar

Part Number: MSP430G2553

Hi all:

I am having trouble figuring out this problem. Maybe someone here can give me some ideas. I am using the TI MSP430 Launchpad.

I have fixed the MSP430G2553 clock speed at 8 MHz in the code, and according to the datasheet, the clock speed should increase with increasing voltage. However, when I measure it using DCO calibration technique, I get a decreasing clock speed with increasing voltage. The variation is not too much; it varies between 7.92 to 8.16MHz. But shouldn't it be fixed at 8MHz, or maybe increase? I see nothing in the datasheet that explains this anomaly.

Any ideas?

over 8 years ago

0 Clemens Ladisch over 8 years ago

Guru 299840 points

Where does the datasheet say that higher voltage results in higher DCO frequency?

0 Abu Bakar over 8 years ago in reply to Clemens Ladisch

Prodigy 70 points

0 Clemens Ladisch over 8 years ago in reply to Abu Bakar

Guru 299840 points

This figure shows allowed frequencies. For example, when V_CC is 2.7 V, then you must not use any frequency higher than 12 MHz.

0 Abu Bakar over 8 years ago in reply to Clemens Ladisch

Prodigy 70 points

Thanks Clemens.
Can you please answer the other half of my question i.e., why is the clock speed decreasing with increase in voltage? I am using following code to set the DCO frequency to 8MHz

BCSCTL1 = CALBC1_8MHZ;
DCOCTL = CALDCO_8MHZ;

I am getting these frequency values

Voltage(V) Clock Speed(MHz)
2.3 8.2
2.5 8.1
3.0 8.0
3.5 7.9

0 Abu Bakar over 8 years ago in reply to Abu Bakar

Prodigy 70 points

Voltage(V) --------Clock Speed(MHz)
2.3------------------ 8.2
2.5------------------ 8.1
3.0------------------ 8.0
3.5------------------ 7.9

0 Dubnet over 8 years ago in reply to Abu Bakar

Intellectual 965 points

If I read the information on page 30 of the MSP430G2553 data sheet correctly, that actually falls into the specification for power supply rejection error of the DCO at +/-3% from 2.2 to 3.6V

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MSP430G2553: MSP430G2553 Clock Speed