Kinetis MKM34Z256VLX7: Vref trim values

[matteoperego]

Hi,
I am using development board TWR-KM34Z75M and trying to trim the vref values (VREF_H and VREF_L).
On page 654 of the reference manual there's the description of the VREF Trim Register (VREF_VREFH_TRM), and the manual says it is possible to trim the vref value with 0.5mV step.
The minimum value is supposed to be 000000 and the maximum 111111 (x3F), so this means 64 steps of regulation and a total range of 32mV.
Tha manual says to refer to the data sheet for min and max voltage reference values, and on page 32 of the data sheet the possible values of VREFH are:
min. = 1.178V
max. = 1.202V
this means a range = (max - min) = 24mV with 48 steps of regulation (based on the 0.5mV step).
How is it possible?

Can you please explain this mismatch?

Best regards

[xiangjun_rong]

Hi, Matteo,

Regarding your question, I suppose you misunderstand the spec

VREFH Voltage reference output — user trim 1.178 — 1.202 V

The spec means that the Vrefh pin voltage may vary from 1.178V to 2.202V because of temperature variation after user set the trim register to a fixed value. The temperature range (ambient) is –40 to 105 °C for the MKM34Z256VLX7

Because the Vrefh pin voltage varies with the temperature, user can measure the ambient temperature and change the trim register value based on the measure temperature, but we do not provide the curve for voltage vs temperature.

There are 64 steps for VREF_VREFH_TRM, each step means 0.5mV voltage.

Hope it can help you

BR

XiangJun rong

[matteoperego]

Hello Xiangjun Rong,
thank you for your answer,
I didn't notice table 32 was about full-range operating behaviour.

So now let's work at 25°:
what is the value of VREFH corresponding to 000000 in VREF Trim Register?
what is the value of VREFH corresponding to 111111 in VREF Trim Register?

In other words, when I write 000000 in the trim register, what is the voltage that I should have on VREFH?
The manual says to refer to the data sheet for min and max voltage reference values, but I can't find this information.

Best regards.

[xiangjun_rong]

Hi, Matteo,

User trim is a register to fine tune the VREF. If the VREF generated is, say, 1.195V, you can use the trim to take it to 1.2V by setting trim value to 10 (the resolution of trim is +/-0.5mV). Let me give you an extreme example, if the current VREFH pin voltage is 1.178V with TRIM value 2, after you change the trim to zero, the VREFH pin will be 1.178V-0.5mV*2=1.177V. Note no matter what the current Vrefh pin voltage is, decreasing the trim value  for example x, the Vrefh pin voltage will decrease by 0.5mV*x.

Please note that the trim will not change its value according to temperature, so if you have to set a different value of trim according to chip temperature, then you have to measure temperature and manually change the trim register.

Hope it can help you

BR

Xiangjun Rong