K64 (Teensy) Voltage Reference Factory Trim Ambiguity

[TomE]

I'm using a K64FX512 on a Teensy 3.5 board.

We were using the Bandgap as a software reference voltage for the ADC, but that looked to about 0.8% high. Which is expected as it is only specified as being +-3%.

I've switched to the Voltage Reference, and that is far better. But the Reference Manual is confusing on its use. The manual is "K64 Sub-Family Reference Manual, Rev. 4, Oct 2019".

Section "38.1.2 Features" says:

* Programmable trim register with 0.5 mV steps, automatically loaded with factory
  trimmed value upon reset.

But the register definition is as follows:

• It says everything except CHOPEN is "Undefined at reset", which usually means "random" and not "Factory Set".
• It says Bit 7 is "loaded with a factory trim value".
• It doesn't say the bottom 6 bits are loaded with the factory trim.
• The "Reserved" bit might be read-only or it might be read-write. Its value probably should be preserved on any writes to this register, but that isn't clear.

Checking the Teensy driver sources for clarification doesn't help. It overrides all of the trim bits with 0x20 as the "middle value". AND it overwrites the "Reserved" bit with zero, which is probably wrong.

Could the proper use of this register be clarified, please? Is it better in any of tha other Kinetis manuals that use the same module?

Tom

[diego_charles · Accepted solution]

Hi @TomE 

I hope you are doing well during this times !

Although the register is shown as undetermined during reset, after reset the register is loaded with a VREF factory trimmed value for a nominal VDDA and a 25°C temperature.

The VREF voltage may be trimmed manually by the user depending on ambient temperature variations for the end application. Since the external VREF pin voltage is dependent on this factor.

Unfortunately, there are no curves of temperature vs VREF that I may provide. You may trim VREF using an multimeter and its external pin.

My apologies for the late reply!

Please, let me know if this clarifies your inquiry.

Regards,

Diego

[TomE]

Thank you for confirming its operation. From my measurements with a good voltmeter it seemed to be generating a very accurate reference.

But Section 38.2.1 is misleading to the point of being wrong. It needs to be updated to say that all those bits are set from the factory values. Could you please advise the Documentation people that it needs a revision or an Errata. This probably applies to other devices that have reference manuals with the same chapters.

Tom