The other day I was discussing the Microchip (formerly Atmel) SAM devices with a friend. These processors are Cortex-M based machines and there is a whole ecosystem of components from which to choose. I have never had the opportunity to work with the SAM devices on a project, but they have always been interesting. Since I was generally interested in the device family I decided to head over to arrow.com to search for available devices. I found the ATSAML10E14A for a total unit cost of $0.97! So, I wondered, could I design a controller for a total BOM cost under $10?
The Rules
What would a challenge be without some ground rules. So, I built a few rules to serve a a rough set of requirements to design the controller board.
Must be self-contained system: The only stuff required to use the card is power and a programming cable (SWD/JTAG programming pins).
Must communicate with host PC without external components.
Must be useful: The controller needs to be able to be connected to sensors, motors, and other devices.. in other words, the controller must be able to do real stuff.
The BOM (components + PWB) must be $10 or under.
Basic Concept
Since I found the ATSAML10E14 device that fits quite nicely into the BOM cost target, a board concept can be built around that device. The SAML10 integrates a Coretex-M23 ARM core with RAM, Flash and an internal clock source that can be used as the primary clock for the device. The device by itself represents a mostly self-contained system, but I’ll still need power conversion to create the 3.3V required to power the device, and a way to connect to the host system for communications.
Enter the STMicroelectronics LDK320AM33R.
This device is a LDO (Low-Dropout Regulator) that will buck convert with only 100mV of dropout. That provides an input voltage range of about 3.4 V to 5.5 V, which is perfect for use with a USB interface. This is a great device because it is also real cost effective ($0.31 unit cost for the STO23-5 package), which makes it fit into our cost target quite nicely.
The SAML10, however, does not have a native USB interface. So I’ll use the FT234XD (FT234XD-R, the DFN version for reasons that will be later revealed:) ). This is a USB to serial converter device that will connect to the host computer in serial emulation mode a.k.a. Virtual Com Port (VCP).
So, now we’ve got a processor, memory, a clock, a power source and the interface with the host computer. And with the versatile IO ports on the SAM device, the processor can connect to SPI, I2C, UART, Digital IO, Analog in, and there are even a few Analog output channels. The SAM even packs some PWM output into the 32-pin device that are great for driving servos or motor controllers.
But will it be less than $10!?!?!
The Preliminary BOM
Now that I’ve identified the major blocks in the design it’s time to generate a preliminary bill of material and check our progress against the cost target of $10 in material. I put together a preliminary schematic and looked up all of the components that we’ll need to build this controller.
The MCU, and USB interface make up most of the BOM cost, with the next most expensive component being the total of the 8-pin connectors. Connectors can get expensive pretty quickly, and since our budget is pretty tight ($10 for a single controller) I had to search around a little to identify a connector that was both usable in a breadboard (read: 0.1” spacing, pin headers or sockets) as well as inexpensive.
As you’ll probably notice from the above BOM, the total roll up of BOM cost after the schematic design is just over half the total cost target, leaving us a mere $4.33 for ordering the PWB.
Next time, I’ll delve into the PWB layout, and share the schematics.
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