STM32F105VCT micro GPIO input thresholds

SandeepGCT · ‎2024-09-11

We are using STM32F105VCT micro in our product from many years.

In the recent hardwares, we have seen that input logic level is changing only with 200mv change.

When the input voltage to any port pin is crossing 1.5 we are seeing as logic level 1, when input voltage reduces more than 1.3 volts we see it as logic level 0, only 200mv gap. This is causing smaller noise on the input pin causing edge detect triggers in the firmware.

With our old batch of micros we see input logic level 1 at 1.7v and logic level 0 at 1.3v. So gap is 400mv

As per the data sheet we should see gap 600mv approx between two logic levels

Our VDD is 3.3 V (CMOS)

As per this VIL max threshold should be 1.136V

VH min threshold should be 1.833V

There is a gap of 697mv theoretically.

Can you please let us know what is the reason behind this, why in both cases it is not matching the data sheet?

waclawek.jan · ‎2024-09-12

Because the semiconductor manufacturing processes are not perfect.

The threshold voltages of the MOSFET transistors used depend heavily on various technological parameters, e.g. the exact amount of N- and P-type impurities introduced during the diffusion steps. These are not exactly the same at each silicon wafer, and they even vary across the area of the wafer. The manufacturer has characterized, how much the thresholds vary using given manufacturing equipment, materials and steps, and that's where the values in the datasheets come from.

JW

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waclawek.jan · ‎2024-09-12

The general problem with datasheet parameters is, that they usually don't tell, whether it's a requirement on the user or a guarantee by the manufacturer. In this case, the VIL/VIH values are requirements on the user, i.e. VIL=max.1.136V means, that "if you want the pin to detect low, you must not put more than 1.136V on it".

What you are looking for is the hysteresis value:

JW

SandeepGCT · ‎2024-09-12

Thanks for the reply.

But why different micros have different VIL and VIH practical values...

waclawek.jan · ‎2024-09-12

Because the semiconductor manufacturing processes are not perfect.

The threshold voltages of the MOSFET transistors used depend heavily on various technological parameters, e.g. the exact amount of N- and P-type impurities introduced during the diffusion steps. These are not exactly the same at each silicon wafer, and they even vary across the area of the wafer. The manufacturer has characterized, how much the thresholds vary using given manufacturing equipment, materials and steps, and that's where the values in the datasheets come from.

JW

SandeepGCT · ‎2024-09-12

Thanks for the quick reply