LPC55: Is BIAS_VREF_1V really 1.0V?

[jayk_edmi]

I'm trying to measure the internal 1V bandgap reference, BIAS_VREF_1V, with the ADC16, and it's coming out as ~0.91V.

I can't find any specifications for the reference in v2.7 of the User Manual, nor v2.5 of the Datasheet.

I've found elsewhere on the forums that "BIAS_VREF_1V is 1.5% accuracy over working voltage and temperature range", so 0.91V seems a bit far off if it's really supposed to be 1.00V.

Is it actually supposed to be 1.00V, or is it actually a 0.90V band-gap reference? What is the nominal voltage that the 1.5% accuracy is centred on?

[jayk_edmi · Accepted solution]

OK, if anyone else is having this problem: the User Manual marks most of the AUX_BIAS register as "reserved, write as zero". This is incorrect; if you write these fields as zero it resets the calibration / trim fields!

If I write the current value + VENABLE, rather than just writing = VENABLE, I now get a 1.0V reading.

[xiangjun_rong]

Hi,

I attach the training doc.

Hope it can help you

BR

Xiangjun Rong

[xiangjun_rong]

Hi，

I suppose that you use LPC553x family.

The Vref_out pin of  LPC553x must be connected to a 220nF cap.

The VREF_OUT pin can output 1.0 to 2.1V DC voltage.

You can write the trim register to adjust the output voltage.

each step in the TRIM2V1 bits is 100mV. For example, the TRIM2V1 is 5, the original voltage is 1V, so the voltage will be 1.0V to 5*100mV=1.5V.

The VREFTRIM is fine turning. Assume it is 5, the voltage will be 5*0.5*1.5mV=3.5mV.

so the voltage will be 1.503V.

Hope it can help you

BR

XiangJun Rong

[jayk_edmi]

Sorry, I forgot to specify the exact chip - LPC55S69.

And it's this channel on the ADC16

[xiangjun_rong]

Hi,

Okay, I see you use LPC5569.

But the Vref module is the same for both LPC553x and LPC556x, all the code and training doc is valid.

Hope it can help you

BR

XiangJun Rong

[jayk_edmi]

There's no VREF_OUT pin on the LPC55S69, there's no VREF module (it's called AUX_BIAS, if anything?), and there's no TRIM2V1 or VREFTRIM registers in the SDK anywhere I can find.

Can you point me to the relevant training material, please?

The LPC55S69_cm33_core0.svd does give the AUX_BIAS register ITRIM, PTATITRIM, VREF1VTRIM, VREF1VCURVETRIM, ITRIMCTRL0 and ITRIMCTRL1 subfields, but they definitely don't match the UTRIM register you posted. (AUX_BIAS.VREF1VENABLE would be in the middle of UTRIM.TRIM2V1, if they were the same.)