Problems increasing VCO frequency above datasheet recommendations

Question

Firstly, yes I contacted ST for answers. Until now, no answer.

I'm using STM32F429 MCU and embedded USB FS PHY. To keep the 48 MHz USB PHY clock the maximum PLL clock that I can achieve is 168 MHz, unless the VCO clock is increased to 720 MHz. Then, I can achieve the PLL maximum frequency of 180 MHz and also provide the 48 MHz clock.

The datasheet says that the VCO clock should be on an interval of 100 to 432 MHz. Nevertheless, the same datasheet says that those conditions (100~432MHz) are valid for a source voltage of 1.7 to 3.3V.

Once my application always works at 3.3V, I've made some tests with VCO Clock at 720 MHz and everything worked!

Now, my questions are:

1. Is it safe?
2. Which tests should I run to verify the safety of this approach?

EDIT 1

No, it's not a hobby project. In fact, I'm trying to make some low-level performance improvements once the opcode has grown bigger and bigger since the first market release. Today it has 1.8 MB.

Anyone who has worked on a long-term project that has hundreds of thousands of lines of code knows that there are a lot of 'truths' or myths that are not entirely true. So, it's a good thing to question those from time to time.

By design, if a VCO can operate within a range of frequencies with a power source of X watts, it surely can operate in a bigger range with a power source of X + Y watts. Of course, components' saturations, and resonations must not be reached. How much? I do not know. Hopefully, ST experts know.

If ST would put into the datasheet all the information gathered during MCU design and rehearsal it would be unreadable. But, only because some information is not in the datasheet it not means that the information does not exist or it could not be true.

We are not mad to release something that still unclear, without all necessary tests.

Finally, I know that I'm going on an unknown path, that is the reason that I've come here. To hear some wise words from someone that has done something similar.

Answer 1

Well, 720 MHz is 66% over the 432 MHz limit so highly unlikely it will work.

As you have correctly calculated, you can achieve 168 MHz as the highest core clock when 48 MHz USB clock is required. That configuration is guaranteed to work.

If you really need to run at 180 MHz instead of 168 MHz, and intend to overclock, then you have already selected the wrong MCU and patching that problem with something that is not guaranteed to work.

1. Safe depends on what your MCU does and what happens if it does not work when you exceed specifications.

What you might just want to know if it will it work or not, and since you are exceeding specifications, it is of course not guaranteed to work. That is why the manufacturer gives out the specifications. The manufacturer will also likely say overclocking is unsupported by them and you are on your own if you try it and complain that it does not work.

2. The point is not which tests you can run on your microcontroller that runs on your desk at 25 °C and at 3.300V. The point is will all microcontrollers from different batches, with different manufacturing tolerances, from other manufacturing plants, with other temperatures and supplied with other supply voltage will work at all.

Answer 2

Is it safe?

No, it is not. If it is a hobby project run on your desk - you can experiment. But if you want to put it into the drone - never do it. In commercial projects - never do it. One of my friends did something very similar using AVR ATMEGA uC many years ago. He needed to implement 1-wire overdrive driver. AtMega was too slow and he had overclocked it. The company started to sell devices and eventually was overflowed by customer complaints.

Which tests should I run to verify the safety of this approach?

Such a test does not exist.