ADC calibration

[ianbenton]

On a slow-rising power supply, the LPC1517 gets stuck in ADC calibration. Any ideas why?

[ianbenton]

Or could it be that I switched to system oscillator without waiting for it to start up, and then immediately started the ADC calibration (which would probably have started before the system oscillator was stable)?

[jeremyzhou]

Hi Ian Benton,

Thank you for your interest in NXP Semiconductor products and
Actually, I don't the exact reason which causes this phenomenon, as the ADC calibration process is unknown, however, in general, it needs many conditions must be stable during calibration, for example, ADC's clock source and frequency, power configuration, voltage reference selection, sample time, etc.
Hope it helps.
Have a great day,
TIC

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[ianbenton]

Does NXP make this device? If so, then SOMEBODY there knows how the ADC calibration process works, and what factors would prevent it from terminating.

[jeremyzhou]

Hi Ian Benton,,

Thanks for your reply.
As mentioned above, it needs many conditions must be stable during calibration, for example, ADC's clock source and frequency, power configuration, voltage reference selection, sample time, etc. Otherwise, it may interferes the ADC calibration or sample process.
I was wondering if you can introduce the testing procedure which can definitely replicate the phenomenon,it can let me dig much deeper in this issue.

Have a great day,
TIC

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[ianbenton]

I think I may be closer to understanding what has happened. The device is powered from a 48V supply via a linear regulator, which has a dropout of <1V. There is about 70,000uF of capacitance on the supply, which takes a very long time to discharge, especially below 5V. It appears as though the voltage dropped low enough to cause a power-on reset next time the power supply was started, but the ADC seems to be in a "brownout" situation. So, I could possibly add a microprocessor reset IC to the circuit, such as an LM809, or make a regulator with a dropout of about 12V so that the supply is guaranteed to reach zero. 

But, before I start changing pcbs,  would it be possible to fix it in software? Would asserting the ADC bit in PRESETCTRL0 before sending the ADC into calibrate fix the problem?

[jeremyzhou]

Hi Ian Benton,,

Thanks for your reply.
1） Would assert the ADC bit in PRESETCTRL0 before sending the ADC into calibrating fix the problem?
-- No, I'm afraid not, if the stability of power voltage is not guaranteed.

Have a great day,
TIC

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[ianbenton]

Apologies for being unclear, but the supply HAS now returned to stability, and the program restarts, but the ADC appears to be in an undefined state and won't calibrate. Would resetting it with PRESETCTRL fix it?

[jeremyzhou]

Hi Ian Benton,

Thanks for your clarification.
Yes, I agree with you, it'd better reset the ACD prior to reinitializing the ADC, you can give a try.:smileylaugh:

Have a great day,
TIC

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