While I’ve been working on my USB-to-ADB converter with a PIC32, I’ve received a steady barrage of feedback saying I ought to consider an ARM Cortex-M microcontroller instead. There are several reasons I haven’t done that, but one of the biggest reasons is that there are simply too many ARM options for me to keep them all straight! Everyone knows what an AVR is, or a PIC, but my mind is a gray blur when it comes to the various LPC, MSP, and XYZ microcontroller offerings based on ARM cores. While having so many ARM options is good from a price competition standpoint, I have to believe this fracturing of the ARM Cortex-M segment hurts it in its competition with AVRs and PICs. If there were only a single big player making and promoting ARM Cortex-M microcontrollers, I suspect they would have made bigger inroads against Atmel and Microchip.
To help clear the mental fog, I decided to make a list of all the ARM Cortex-M microcontroller families I could name off the top of my head, plus any that have been mentioned here recently by commenters. This isn’t meant to be an exhaustive list, but just the options that are mostly likely to be of interest to hobbyists. I did 10 minutes of research into each one, to find some basic info on distinguishing features and pricing. Maybe this will help you, as it helped me. Pricing is Digikey’s lowest listed price for a device with at least 128K of program Flash memory, in a hobbyist-friendly package (no BGA or QFN), and in single unit quantities.
SAMD (Atmel) – ATSAMD20E17A-AU $3.44, ATSAMD21E17A-AU $3.65. ARM Cortex-M0+. Speeds to 48 MHz. 16K RAM. SAMD20 supports USB Device. SAMD21 supports USB Device and Host. A larger version of the SAMD21 is used in the Arduino Zero.
STM32 (ST Microelectronics) – STM32F070CBT6 $2.66. ARM Cortex-M0. This chip runs at 48 MHz, while others in the STM32 family support speeds to 120 MHz. Supports USB Device. STM32L0 family is Cortex-M0+, STM32F1 and STM32F2 families are Cortex-M3. I did a review of an STM32F1 board a few years ago.
MSP (Texas Instruments) – It turns out this isn’t a 32-bit ARM Cortex-M, but a 16-bit microcontroller based on something called CPUXV2. I’m not sure how it got into the ARM part of my brain.
Launchpad (Texas Instruments) – This isn’t a microcontroller at all, but a series of development boards based on the various TI parts. More brain pollution.
Stellaris (Texas Instruments) – A family of higher-end ARM Cortex-M3 microcontrollers priced at $6.50 and up. This is probably not what I’m looking for, but it’s impressive how well TI’s marketing department has infected my brain with their brands. Does TI actually make a low-end ARM Cortex-M microcontroller? I couldn’t find one.
LPC (NXP) – LPC11E67JBD48E, $4.30. ARM Cortex-M0+. 50 MHz. 20K RAM. Other members of the LPC family are Cortex-M0 and Cortex-M3, with speeds up to 200 MHz.
Kinetis (Freescale) – MKL16Z256VLH4, $4.05. ARM Cortex-M0+. 48 MHz. This device actually has 256K Flash and 32K RAM. Smaller Kinetis devices with 128K Flash are available, but are only offered in QFN packages.
PSoC (Cypress Semiconductor) – CY8C5267AXI-LP051, $6.50. ARM Cortex-M3. PSoC 5 @ 67 MHz. 32K RAM. The PSoC 4 is a Cortex-M0 device and also worth considering, but the models with 128K+ program Flash memory are only offered in QFN packages. The interesting thing about the PSoC 4 and 5 families is that they include configurable logic blocks in addition to the standard CPU core and peripherals. It’s like having a tightly-integrated CPLD built into the microcontroller.
Wikipedia has a small reference table that describes some of the differences between the M0, M0+, and M3 cores.
Suggestion to semiconductor manufacturers: if you want to sell a line of microcontrollers that people will actually remember, give it a pronounceable name, instead of some random acronym. Only TI and Freescale did this.
Software and Tools
OK, maybe that list wasn’t too enlightening, other than to identify the major players. If I’m choosing a microcontroller family to invest time in learning, I’m probably less interested in a $0.60 price difference or 4K of extra RAM, and more interested in the quality of the vendor tools, documentation, peripheral libraries, and examples. I’ll also be interested in the availability of low-cost development boards, and the size of the developer community. How do the options listed above compare in those respects? Who’s got a nifty extra feature, like a built-in bootloader, or some cool hardware peripheral, or a software library that doesn’t suck? Leave a comment and tell me your experiences.
The “elephant in the room” when it comes to ARM Cortex-M development is setting up the toolchain. I described this in my old STM32 review, and it wasn’t a fun process, but that was four years ago. Maybe things have gotten better since then? I know some people are perfectly comfortable installing Eclipse along with a plugin from here, a compiler and linker from there, and a debugger from some other place. Personally I’d rather use an IDE that has as many pieces as possible bundled together, so I can skip the setup and spend more time working on my actual project. Since my earlier review, CooCox seems to have remained popular, and something new called Em:Blocks has also grown in popularity, but most people appear to still be rolling their own toolchain.