Cortex-M4 mini-evaluation board

05 February 2021
Although I had previously made a host board for the ST Microelectronics STM32L4R5 back in 2018 all I ever got round to doing with it was get together a working linker script with bootstrap code and do some basic GPIO. At the time it got left behind as I put gave priority to getting up to speed with the NXP LPC11(U)xx and later the Kinetis KE0x Cortex-M0 microcontrollers, and then was overtaken by events. Aside from working out how to to get RS232 send & receive working, which in practice would mostly be about working out the idiosyncrasies of the UART's BAUD rate generator, there was little else I could have done with it.

STM32L4R5 mini-eval board

I recently found the time to design and build a more featureful “mini-evaluation” board and the result is shown in the image above. The main additions are fanning out the pins for I²C and USB because getting those two things working would be the first thing I would want to do with this Cortex-M4 chip. With fabrication costs also being less sensitive to board size I decided to add in some extra bits for convenience such as a power jack.

Circuit design

Because the previous host board was designed using KiCad 4 which I have long since stopped using in favour of KiCad 5, this revised board is a complete redesign although some superficial similarities remain. The circuit design follows the power supply recommendations within technical note AN4555, and fans out dedicated connections for programming, I₂C, RS232, and USB. In trying to keep it general-purpose some pins power on-board indicator LEDs and there are two connectors intended for GPIO — one 8-way 2.54mm pin socket and one 16-way surface-mount MicroMatch connector.

PCB layout

Fabrication & building

Like many of my more recent PCB designs the KiCad files have been open-sourced and are available from my Bitbucket repository. Due to horrendous intercontinental shipping delays I opted to use Aisler rather than JLCPCB as last time I used the latter the boards got stuck in Belgium and due to travel restrictions was unable to receive them. Although I did order in a solder stencil I ended up hand-soldering the circuit boards instead of reflow soldering them — the trick seems to be to liberal with the flux and conservative with the solder.

List of components

Most components are from recently-ordered stock but since things got a bit mixed up in relocating all my electronics stuff some of the order codes below might be wrong. Based on my experience with the first-generation breakout board a lot of the capacitors could probably be omitted, and in the case of the ferrite bead FB1 there is no recommended value beyond mentioning that it could be replaced with a short. One thing to watch out for is tantalum capacitors being polarised — I only realised this when it came to soldering the circuit, and had I known in advance I would have used ceramic capacitors instead.

Pad	Component	Manufacturer	Part number
`U1`	ARM Cortex-M4	ST Microelectronics	`STM32L4R5VITx`
`U2`	3.3-volt power regulator	Texas Instruments	`UA78M33CDCYR`
`X1`	16MHz Oscillator	Multicomp	`MCSJK-6NC2-16.00-25-B`
`D1,D2,D3,D4`	LED	Kingbright	`L-113IDT`
`R1,R2,R3,R4`	10kΩ resistor	Multicomp	`MCWR08X1002FTL`
`R5,R6,R7,R8`	1kΩ resistor	Multicomp	`MCWR08X1001FTL`
`C1,C4,C5,C9`	0.1μF (100nF) capacitor	Samsung	`CL21B104KACNNNC`
`C10,C11,C12,C13`	0.1μF (100nF) capacitor	Samsung	`CL21B104KACNNNC`
`C2,C3,C6`	1μF tantalum capacitor	Vishay	`TMCP1D105KTRF`
`C7`	0.01μF (10nF) capacitor	Samsung	`CL21B103KCANNNC`
`C8`	4.7μF capacitor	Vishay	`VJ0805G475KXXTW1BC`
`FB1`	Ferrite bead	TDK	`MPZ2012S601AT`
`J1`	16-way MicroMatch socket	TE Connectivity	`1-2178711-6`
`J2,J3,JP1`	3-way pin header	n/a
`J4`	8-way pin header	n/a
`J5`	Barrel jack	Cliff Electronic	`FC681465P`
`J6`	USB Mini-AB	Multicomp	`MC32598`
`J7`	20-way IDC socket	Multicomp	`MC-254-20-00-ST-DIP`
`SW1`	Push button	n/a

I have forgotten where I obtained the push button switches but suspect it is from a batch I ordered off E-Bay quite a while back; I am also unsure what order number was used for the pin headers I cut down to size. The price of Cortex-M4 chips has gone up a lot in the last year or two and it alone accounts for the majority of the cost of components.

Remarks

At time of writing I have only done a smoke test of checking whether a programmer can communicate with the chip, as writing firmware to do things like serial communications is a long-term project. Figuring out RS232 and I²C on the NXP microcontrollers took several weeks whereas USB was on-off for about a year. For the time being the Cortex-M0 chips I have experience with are more than adequate for my needs and I only got the Cortex-M4 for when I need to write more demanding firmwares, but that time will come and I want to be prepared.