Prototype to Production – How Microcontrollers Are Programmed at Scale
If you’re developing an electronic product, chances are you’ve programmed your prototype manually using a development board or an external programmer.
That part is easy. But what happens when you’re ready to build hundreds or even tens of thousands of units?
How do you reliably get your firmware into every microcontroller on every board without holding up production or blowing your budget?
It’s one of the most common questions I get and today I’ll walk you through every stage, from prototyping to production, and explain the real-world methods used to program MCUs at scale.
Prototyping Stage
In the early stages, you’re probably using a development board something like an Arduino, STM32 Nucleo, or ESP32 DevKit.
These boards are built for convenience. They usually have a USB interface and come preloaded with a bootloader, so you can just connect them to your computer and flash new firmware without any special tools.
A bootloader is a small piece of code that runs immediately when the microcontroller powers on. Its job is to check whether it should enter programming mode waiting for new firmware or jump straight into running your application code
Many MCUs include a factory bootloader that listens on USB, UART, or SPI. This lets you program the chip without a debugger just a serial connection or USB cable.
It’s a convenient option, especially in prototypes or low-volume runs.
Now, if you’re building a custom board with a bare MCU, you may or may not want to rely on the built-in bootloader.
For example, if the microcontroller supports it then programming via a direct USB connection is the simplest option.
For example, some STM32 models support the official USB DFU (Device Firmware Update) protocol, whereas some ESP32 models include an Espressif bootloader that isn’t compliant with the USB DFU protocol.
But, you might instead use an external USB programmer or debugger like an ST-Link, J-Link which connects using debug interfaces like SWD or JTAG depending on the chip
That method is more robust for development, especially when debugging firmware or working with chips that don’t have a convenient bootloader.
However, these external programmers can be expensive and commonly cost a few hundred dollars, although cheaper knock-offs are usually available.
Either way, just make sure your board gives you access to the programming interface.
That could be a standard header, or even just test pads, but it needs to be there from the very first revision.
Small Batch Production
Once you’re building more than just a handful of units maybe for beta testing, crowdfunding backers, or pilot production you likely need a better process than flashing each board by hand.
One common approach here is to build a pogo pin jig. These are simple test fixtures where pogo pins make contact with programming pads on the PCB.
You just drop in the board, close the lid, and a script flashes the firmware.
It saves time plugging and unplugging each board, and avoids soldering headers on every board just for programming.
Another option if your firmware is stable is to pre-program the microcontroller chips before assembly.
This can be done with a chip socket programmer or small-scale programming service.
Just keep in mind that if your firmware changes after the chips are programmed, you’re stuck either reprogramming or scrapping them.
So this approach makes the most sense when you’re confident in the firmware version.
Also, always remember to include a readable firmware version number in your software, maybe accessible via serial or USB.
It’s a small thing that can save massive headaches when you’re trying to figure out which units have which firmware loaded.
High-Volume Production
At higher production volumes you have two main paths: use pre-programmed chips, or in-circuit programming during production.
Let’s start with pre-programmed chips, which is generally the easiest option for you.
In this case, you send your final firmware binary to a third-party programming service this could be a large distributor like Arrow or Avnet, or a specialized service bureau, or sometimes even the microcontroller manufacturer.
They load your code onto each microcontroller before it’s shipped to your contract manufacturer.
Often, they’ll also apply labels, laser markings, or track firmware versions for you.
This method is fast, scalable, and keeps your assembly line simple.
The factory doesn’t have to do any flashing it’s just “place the chip and go.”
But there are trade-offs. First, your firmware must be finalized.
That doesn’t just mean feature-complete it means tested, versioned, and ideally with some security protections.
You also need to coordinate carefully with both your contract manufacturer and the programming service.
If your firmware is still evolving, or if you want to combine flashing with functional testing, then you’ll go with in-circuit programming on the production line.
In this setup, the factory has a test jig with pogo pins or spring-loaded connectors, similar to what you may have used for small batch production.
After the PCB is assembled, each unit is placed into the fixture. It gets powered up and flashed automatically.
This process is slower than pre-programming but gives you flexibility of flashing the latest version and running board-level tests.
To make this work, your board design needs dedicated test points or pads that line up with the jig.
It’s also good practice to add ESD protection and filtering on those lines, since they’ll be exposed during manufacturing.
Post-Production: Firmware Updates in the Field
After your product ships, things change again. Now you’re thinking about how to update firmware in the field – whether to fix bugs, add features, or patch security holes.
There are a few ways to handle this:
If your product has a USB port, you can implement firmware updates using DFU mode or a custom bootloader. This is great for end users who can update firmware via an app or tool.
Or, if your product has Wi-Fi, Bluetooth, or cellular, you can implement Over-the-air (OTA) updates.
But OTA isn’t a free feature for you and it’s a full subsystem. This kind of bootloader is more complex and must be more robust than ones used for factory programming.
It needs to sit between the hardware and your main application, with the ability to manage firmware partitions, validate digital signatures, and recover from failed updates.
For many simple products, OTA is overkill. But if your product is connected and in the field long-term, it can add massive value.
And even if you never plan to update the firmware, you still need to think about how to recover a non-functional locked up device or apply a critical patch.
So keep a backup way to reflash the firmware even if it’s just exposed test pads inside the enclosure.
