STM32MP2 Layout requirements, Starting point

[kbch]

Hello

I'm considering the requirements and learning process for designing a board with the STM32MP2.

I've never done DDR4 routing before—what's the best starting point?

Also, is it feasible to design such a board using a tool like KiCad, or is a tool like Altium essential?

Thanks!

[jaehoon · Accepted solution]

I made MP157 custom board using KiCAD. no problem to design. but some PCB manufacturer can not make high precision PCBs. so, you need to check PCB manufacturer and select BGA pitch.

https://www.st.com/en/microcontrollers-microprocessors/stm32mp2-series/documentation.html

https://www.st.com/resource/en/application_note/an5723-guidelines-for-ddr-configuration-on-stm32mp2-mpus-stmicroelectronics.pdf

https://wiki.stmicroelectronics.cn/stm32mpu/wiki/STM32_MPU_resources

press ctrl + F keys then, search `example`. then you can see DDR routing examples.

it contains memory length matching excel sheets.

[kbch]

Thanks for this valuable answer, good to hear that this idea is not completely out of scope! =)

By the way, is your KiCAD project available publicly?

[jaehoon]

I didn't upload my project to anywhere yet. I'm second time to design these MPU board, so, no good layer-plan did. just ground planes are setup between signal layers. just I target to make my board working. then, I'll study more details, and modify my designs.

my board's target size is small so (65mm x 38mm including LTDC routings), no 3W rule could be applied, so I just adjusted to 2W. And no additional fees for via in pads at JLCPCB I used, so I just drawn like above. JLCPCB does not support micro vias including blinded via, etc, so I just placed through hole vias. (due to make it cheap as possible)

In DDR3, ST's application note says that, no termination resistors are needed for `point to point` topology, so its are marked as `no solder, no place`. and bit swaps in byte groups are allowed.

And I checked the 24MHz crystals and 12MHz crystals, so 24MHz is more stable than 12MHz in DFU mode.

due to no crystals available I could buy in my near shops except 18pF for load capacitance,  I choose it as alternative. 18pF = 2 / C + S, supposed S = 3.

And online altium viewer is available to check ST's official example gerber files.

P.S. it's important to check BGA pitch availability of PCB manufacturer. for example, JLCPCB's capability couldn't cover MP157DAC1. at first, I bought MP157DAC1, it could'nt be used with JLCPCB. so I just gave up, and chnaged to MP157DAB1.

[kbch]

Thanks for sharing all these details! The insights are really helpful.

[debugging]

Thanks for the tip about JLCPCB. Its surprising they cannot handle 0.5mm pitch BGA, or was it because hole in via for 0.5mm ? 18pF caps seem to need 10pf int. load xtals which are hard to find. What about using a 3.3V 24 Mhz OSC instead of an XTAL ?  I plan to use an 12.5pf XTAL with 22pf caps.  Making boards with 6~8 layers (ENIG) with correct stack-up for impedance so far seems a costly affair, specially for low quantities. (5-10pcs)