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MPR121 - Electrode Filtered Data Register UPDATE Rate Too Slow?

Question asked by Mark Kooy on Jun 2, 2016

Hello,

 

I am using an MPR121 and polling the Electrode Filtered Data Register (0x04~0x1D) for capacitance values.  This is working OK, with good sensitivity and low noise.  I am receiving data items in sub-1ms intervals without problem.  My problem is that I only get UPDATED data every 7.5 milliseconds (roughly).  That is, I get a certain data value, and that data value repeats a dozen or more times (at my polling read-rate) before it changes - the changes occur only every 7.5milliseconds.

 

The datasheet, section 5.3, says that this register (both bytes) are updated every {ESI * FSI}, which leads me to expect that I should get updated data every 4ms

 

Presently, I have the Filter/Global CDT Configuration Register (0x5D) set such that:

CDT = 1 --> 0.5 microsecond

SFI = 0 --> samples is 4

ESI = 0 --> period is 1 millisecond

 

(and if it matters, this is the only electrode enabled, and the Global config register settings certainly do affect the output as expected, so it seems clear that everything is set up properly).

 

Is there some way I can increase the UPDATE rate to match the specification in the datasheet?  Or is this an error in the datasheet?  The datasheet I'm looking at is:

 

Document Number: MPR121

Rev. 4, 02/2013

 

Thanks for any response.

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