M3 replacement for STM32F446RCT6 (pin-to-pin compatible) - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: August 08, 2023, 11:51:46 am
content: Hi,I am new to ST micros (always used Microchip's) and I am looking for an M3 replacement to the STM32F446RCT6 (which is an M4) that is a drop-in replacement and is pin-to-pin compatible,where all the peripherals are also mappable exactly the same.The main reason is cost savings for production. For this application the micro only does vary basic and low-speed IO control, PWM and comms. So no need for DSP / floating point unit.After going through Digikey's various filters and refining the search as much as possible I still get a 54 micros option that are M3 with that same footprint. It would take ages to go through all of them to check pin by pin and pin functionality.Does anybody know what a low-cost M3 equivalent? Or another low cost ST Micro Cortex family micro?Or is there perhaps an ST tool that can help with that?Thank you

author: ataradov
author_rank: Super Contributor
timestamp: August 08, 2023, 03:39:32 pm
content: Why would you want M3? There are not a lot of M3 devices, since everyone just moved to M4 after it came out. The timing was such that significant update to the product line was due anyway, so everyone jumped on that opportunity.

author: ricko_uk
author_rank: Super Contributor
timestamp: August 08, 2023, 08:48:25 pm
content: Thank you@ataradovfor cost reasons (in production). The application is extremely "simple" and does not need do any calculations or anything other than communication between multiple devices using various peripherals and very basic on/off and a few PWM. So no need for DSP / floating point unit which, form my understanding, is the main difference between the M3 and M4 and the reason for being more expensive?So are you saying that the M4 is now the new "standard" replacement and the the M3 being phased out?What other low-cost alternative would you use/suggest as low-cost drop-in replacement?Thank you

author: ataradov
author_rank: Super Contributor
timestamp: August 08, 2023, 09:16:11 pm
content: M3 has been phased out for more than a decade for all practical purposes. There are old devices that are still made, but I have not seen any new device using that core in many years.Device price has very little to do with the core or additional functions, it is pure marketing. Some older design M3-based devices may be cheaper, but it is mostly because they are old and are there to support existing products.M4 can be implemented without an FPU, although in practice there are not a lot of devices that do it, because again, it does not affect the cost that much.There are no drop-in replacements, if you want the power of M4, you have to use M4. You can look at cheaper vendors, like GigaDevice. Or even RISC-V MCUs, but non of that is drop-in.

author: ricko_uk
author_rank: Super Contributor
timestamp: August 09, 2023, 12:15:07 am
content: Thank you@ataradov

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 09, 2023, 02:12:22 am
content: Do you expect making such generic question to work?We can always take the magic glass ball and guess your RAM/flash/speed/peripheral requirementsAnyways the only mcus cheap enough are the STM32F302RB or the STM32F103R8/RC, almost half the cost of the 446, same pinout except Vcap which won't affect anything, but way lower memory and speed, no idea about the peripherals.Otherwise other options are just <$1 less.You could also use a lower tier F4 series.53 MCUs isn't that much, remember 2/3 of them are the same thing with different RAM/Flash sizes, so it's pretty simple.

author: ArdWar
author_rank: Frequent Contributor
timestamp: August 09, 2023, 07:59:15 am
content: If you don't expect to do heavy calculation you can even go all the way down to STM32F030, a cortex-M0 parts, while still maintaining *physical and electrical* pin compatibility. Peripheral pinout is a bit messy however, look first if your selection of usage maps nicely.Quote from: ricko_uk on August 08, 2023, 11:51:46 amA rather unlikely source for reference is to look at certain Nucleo board and see which parts share the same particular board revision. It does not guarantee full functional compatibility, but it'll at least guarantee things won't blow up and major peripherals to work okay.

author: ricko_uk
author_rank: Super Contributor
timestamp: August 10, 2023, 01:20:22 am
content: Thank you both.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 10, 2023, 06:58:48 am
content: If it has to be the same both in pinout and functionality, then you do not give yourself much room for selecting another one.It's quite common to respin a PCB to optimize for production costs.Just recently I also did a short comparison between some different STM32 models. The reason I did this is that I want to make my own develop / breakout board and I was curious how many different chips would fit on it, and curiousity to general pin compatibility between STM32 models.When you have STM Cube open, you can easily sort on package size, peripherals, Flash, RAM, etc.For the pinout, I found out that just loading the chips in KiCad was the quickest (any other PCB program with libraries for those chips should work too)If you want the pins in a table in a spreadsheet format, that is also easy to do from KiCad:1. Select the symbol on the KiCad schematic.2. Press Ctrl + E to load it in the schematic editor.3. Symbol Editor / Edit / Pin table4. Sort on pin number.5. Select all the fields in the "name" column.6. Copy the column, and paste it in a spreadsheet.7. Close the Symbol Editor without saving, and then repeat with some other IC.In the screenshot below, I mirrored the STM32F303R6Tx and you see immediately it has one less IO pin. The 446RCT has Pin 30 as a VCAP_1 as some other function.In KiCad you can also narrow the selection with multiple (partial) search terms.For example: stm32f3 and tqfp64", and this narrows it down to about 30 models in KiCad.Also, scrolling through the list immediately shows the pinout preview in the right which also helps with pinout comparison.Also, when differences are very small (such as with the VCAP_1 pin) you may be able to just leave such a pin open in your design, or plan a few 0 ohm resistors into the design for variants. But I'm not sure if planning that far ahead is useful.

author: ricko_uk
author_rank: Super Contributor
timestamp: August 14, 2023, 03:17:30 pm
content: Thank you@Doctorandus_P