STM32N6 minimal hardware design

[asdfasdf]

Hi there,

I'm designing a custom PCB using the STM32N657Z0 MCU. The design is extremely space-constrained, so I try to leave out everything I can. Since this deviates quite a bit from the reference design (well, the N6 Discovery kit, that is) I'd love to get some clarifications from someone from ST experienced with the N6 hardware side of things:

1. I'm not using USB at all. What should I do with the USB power pins VDD33USB and VDDA18USB, can I just connect them to ground? Or leave unconnected? Are the tuning resistors OTG1_TXRTUNE and OTG2_TXRTUNE necessary when I don't use USB, or can I just leave these pins unconnected? I assume all other USB pins (data etc) can just be left floating.

2. I do not use any external crystals or oscillators, can I just leave the OSC_OUT, OSC_IN, OSC32_OUT and OSC32_IN pins unconnected?

3. I use an external regulator to supply the VDDCORE domain (instead of the internal SMPS). Am I understanding AN5967 and AN6000 correctly that in this case the pins for the internal SMPS (VDDSMPS, VDDA18PMU, VFBSMPS, VLXSMPS) must all be connected to ground?

4. Since the external switching regulator supplying VDDCORE (see last question) is pretty delicate I try to copy the design from the N6 Discovery kit as much as possible. However, in that design the input and output capacitance for the switching regulator (U2 - TPS62088YFP) is more than four times as large as what Texas Instruments recommends for this IC (4u7F input capacitor, 2x10uF or 1x22uF output capacitance). I'd love to leave out a few of these large capacitors but before I do it would be great to understand why the designers of the Discovery kit did it this way, maybe they found during testing that TI's recommendation was not enough? Is it safe to leave out, say, halve of these capacitors?

Thanks a lot for all your help!

Kind regards, Michael 

[Andrew Neil]

The Nucleo board would be more of a "minimal" design than a Discovery.

Have you seen Application Note AN5967, Getting started with hardware development for STM32N6 MCUs:

https://www.st.com/resource/en/application_note/an5967-getting-started-with-hardware-development-for-stm32n6-mcus-stmicroelectronics.pdf

via: https://www.st.com/en/microcontrollers-microprocessors/stm32n657z0.html#documentation

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@asdfasdf wrote:

2. I do not use any external crystals or oscillators, can I just leave the OSC_OUT, OSC_IN, OSC32_OUT and OSC32_IN pins unconnected? 

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Yes.

[asdfasdf]

Thanks @Andrew Neil , agreed, the Nucleo is more minimal but in this case the parts relevant to my questions are identical to the Discovery kit. I'm aware of AN5967 (and AN6000), however, it does not provide answers to my questions (for instance, there is information about the USB power supplies, but nothing about what to do if USB is unused).

[RomainR. · Accepted solution]

Hello @asdfasdf 

I do not recommend to keep floating peripherals power supply or GPIOs.
- Connect VDD33USB and VDD33USBCC to 3.3V supply, this will not impact your power consumption.
- Group VDDA18USB with VDD18CSI to 1.8V supply.
- USB PHY should be deactivated and related USB GPIOs (DP and DM) should not be used.
- I also recommend to keep OTG1_TXRTUNE and OTG2_TXRTUNE with some pull-down resistors.

For all GPIO's (including OSC_OUT/OSC_IN...) refer to AN5967 section 12.7 Unused I/Os and features.

When using external DCDC for VDDCore (external supply (bybass), you can apply the design in AN5967
Figure 4. System supply configurations.

Regarding the values of the decoupling capacitors of U2 on the STM32N6570-DK board, it is purely internal.
There has been a redesign since the first versions of the boards which did not have the same external VDDCore
regulator. In your case, use the values recommended by TI.

Let me know if it answers to your questions?

BR

Romain,

[asdfasdf]

Thanks a lot @RomainR. !

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When using external DCDC for VDDCore (external supply (bybass), you can apply the design in AN5967
Figure 4. System supply configurations.

Figure 4 you mention seems to indicate that VLXSMPS should be left unconnected in this case. However, Figure 1 in AN6000 (How to build the discrete power supply for STM32N6 MCUs) connects it to ground. Can I assume that both will work in this case?

Note also that the aforementioned Figure 1 in AN6000 also connects the USB supplies to ground, contrary to your suggestion?

Again, I appreciate your support!

Best, Michael

[RomainR.]

The two cases of VLXSMPS will work.

Regarding the AN6000 and the USB supply, I will check internally as there is probably a small document adjustment to be made to avoid doubts and clarify the point.

If your firmware application will never mistakenly enable the USB-PHY and you can guarantee it for the entire life of your future product and any derivative products that may arise from yours, then connect the USB supply to VSS.

Otherwise, if you cannot guarantee anything, follow my recommendation.

BR

Romain,