Tear Down: Redmi Watch 3

November 01, 2023

Blog

The Redmi design team was able to capture small size and long run times with minimal BOM.

When designing any product, there are always tradeoffs that must be made. In the case of a consumer device, such as a smart watch, those tradeoffs often have to do with a limited bill of materials (BOM), the size of the end device, and the amount of power that can be burned which results in a longer run time between charges.

The Redmi Watch 3 smart watch, the object of this Tear Down, does a good job at pushing the envelope in each of these areas. For example, the end selling price is below $100 (actually way below), so it’s obvious that the design team figured out how to minimize the BOM. The watch fits nicely on a not so big wrist, like that of a woman or child. And the battery life? 12 days.

Redmi is a sub-brand under the Xiaomi name. It appears that Redmi is targeting younger consumers with its more fashionable features and lower prices. The Redmi Watch 3 first debuted in China, then later became available globally.

The Low-Power Brain

The key component in the watch and the reason it’s able to tackle the cost, size, and battery life issues, is an ambiq Apollo4 Plus MCU, which is a pure MCU version of the company’s previous-generation processor, the Apollo4. This is not the first collaboration between the two companies. In fact, at least three different watches from Redmi carry an ambiq CPU.

The Apollo4 Plus processor is a high-end variant of ambiq’s Apollo4 family. It’s designed with a 32-bit Arm Cortex M4 core and has a maximum frequency of 192 MHz. It also integrates a graphics processing unit (GPU) and a high-performance display driver. Up to 2 Mbytes of MRAM (for extreme low power) and 2.75 Mbytes of SRAM are available. From a low-power perspective, the Apollo4 Plus can execute from the MRAM at just 4 μA/MHz.

That non-volatile memory is primarily used to store things like the operating system and all the algorithms that make it a smartwatch, such as the fitness tracking, the sports features, the gesture recognition, and so on. The SRAM is also used as a display buffer, as the watch sports a pretty elaborate display. It’s relatively large active-matrix OLED (AMOLED) display, with a brightness of 600 nits, measuring 1.75 in. That display features a screen-to-body ratio of 70%. To drive a display of that size, you need a fairly large SRAM and a SPI connection. The result is an eye-pleasing user interface.

“I believe the Redmi team is taking advantage of the entire non-volatile memory that’s aboard the Apollo4 Plus,” says Renton Ma, an FAE manager for ambiq China. “But customers always want more, no matter how much we offer.” The watch design also makes use of external memory, as you can see on the board.

For Bluetooth communications, the Redmi 3 watch employs a dual-mode transceiver from Realtek. Phone calls are also supported, and that’s executed via Bluetooth.

The watch takes advantage of ambiq’s secureSPOT technology, which enables a set of security features, including secure boot, secure key management, secure OTA data transfers, anti-rollback, secure patching, and recovery.

You can also see from the photos that the battery takes up pretty much the entire backside of the board, which is fairly common for wearable devices. Generally, the designers try to leave as much room as possible for the batteries to maximize run time.

Looking at the various key components in the watch, I would have expected that the display is the biggest power consumer. In reality, it is and it isn’t. When it’s on, actually displaying something, it would be deemed the largest consumer. However, the design is such that the display is mostly in the off mode.

The time when the watch is consuming the most power, ironically, is in the standby mode, which is when you’re not looking at the watch. But just because you're not looking at it doesn't mean it’s not doing anything. It's still pretty busy calculating your heart rate, tracking your steps, and constantly detecting your wrist gestures to see if it that should trigger the display to turn on. And of course, the watch is constantly connected to the user’s phone, ready to receive and display a message or answer a call.

Design Teams’ Wish List

We often ask the design team what they would have included if they were permitted to have a larger bill of materials. In this case, there would have likely been more memory and more sensors, and possibly some other forms of wireless communications. Higher amounts of memory would be supported by the Apollo4 Plus’ multi-bit SPI and eMMC interfaces.

Unfortunately, as you can also see from the photos, my Redmi Watch 3 will never operate again. But it was (almost) worth it to see what made this device tick.