November 01, 2023
weblog
The Redmi design team was able to capture the small size and long lead times with minimal BOM.
When designing any product, there are always trade-offs that need to be made. In the case of a consumer device, such as a smartwatch, these tradeoffs often relate to the limited bill of materials (BOM), the size of the final device, and the amount of power it can burn, resulting in longer times between charges.
The Redmi Watch 3, the target of this Tear Down, does well in pushing the envelope in each of these areas. For example, the final selling price is under $100 (much lower, actually), so obviously the design team has figured out how to keep the BOM to a minimum. This watch fits well on a small wrist such as a women’s or a child’s wrist. And battery life? 12 days.
Redmi is a sub-brand under the Xiaomi name. Redmi seems to be targeting younger consumers with sleeker features and lower prices. Redmi Watch 3 was first launched in China and then made available globally.
weak brain
The key component in the watch, and the reason it can overcome cost, size and battery life issues, is an Apollo4 Plus ambiq MCU, which is a pure MCU version of the company’s previous-generation Apollo4 processor. This is not the first collaboration between these two companies. In fact, at least three different Redmi watches have ambiq CPUs.
The Apollo4 Plus processor is a high-end variant of the Apollo4 ambiq family. It is 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 MB of MRAM (for very low power) and 2.75 MB of SRAM are available. From a power-saving perspective, the Apollo4 Plus can run from MRAM at just 4 µA/MHz.
This non-volatile memory is mainly used to store things like the operating system and all the algorithms that make it a smartwatch, such as fitness tracking, sports features, motion detection, etc. SRAM is also used as a display buffer, since the clock has a very complex display. It’s a fairly large active matrix OLED (AMOLED) screen with 600 nits brightness, 1.75 inches in size. This display has a screen-to-body ratio of 70%. To drive a display of that size, you need a fairly large SRAM and an SPI connection. The result is an eye-catching user interface.
“I believe the Redmi team is using all the non-volatile memory that is on the Apollo4 Plus,” said Renton Ma, director of FAE Ambique China. “But customers always want more, no matter how much we offer.” As you can see on the board, the watch design also uses external memory.
For Bluetooth communication, the Redmi Watch 3 uses a Realtek dual-mode transceiver. Phone calls are also supported and are made via Bluetooth.
The watch is powered by ambiq’s safeSPOT technology, which enables a suite of security features including secure boot, secure key management, secure OTA data transfer, anti-rollback, secure patch and recovery.
You can also see from the photos that the battery takes up almost the entire back of the board, which is fairly common for wearables. In general, designers try to leave as much space as possible for batteries to maximize uptime.
Looking at the various key components in the watch, I would have expected the display to be the biggest power user. Actually, it is and it is not. When it’s on, it actually displays something, it’s considered the biggest consumer. However, the design is such that the display is mostly off.
Ironically, the time when the watch consumes the most power is in standby mode, which is when you are 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 very busy calculating your heart rate, tracking your steps, and constantly detecting your wrist movements to see if that movement causes the screen to turn on. And of course, the watch is constantly connected to the user’s phone and is ready to receive and display messages or answer calls.
Design teams’ wish list
We often ask the design team what they would include if they were allowed a larger bill of materials. In this case, there would probably be more memory and sensors, and possibly some other form of wireless communication. Higher amounts of memory are supported by the Apollo4 Plus multi-bit SPI and eMMC interfaces.
Unfortunately, as you can see from the photos, my Redmi Watch 3 will never work again. But it was (almost) worth it to see what made this machine tick.
Richard Nass’ key responsibilities include setting direction for all aspects of OSM’s ECDs portfolio, including digital, print and live events. Previously, Nas was the brand manager of Design News. Previously, he led the content team for UBMs medical device group and all custom features and events. Nass has been in the engineering OEM industry for over 30 years. In previous stints, he led the content team at EE Times, Embedded.com and TechOnLine. Nass holds a BSEE from NJIT.
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Image Source : embeddedcomputing.com