The ADXL345 is a MEMS accelerometer made by Analog Devices. It’s a popular device among hobbyists because of its low cost, easy availability and rich feature set. But apparently you should be careful about where you buy them: one of my readers ended up with a bunch of ADXL345s that had significant offsets, measurement axes that didn’t work at all, and an inoperative freefall detection mode. After spending lots of time trying to get them to work, he decided to send them to me instead and hopefully find out what was wrong with them.
Most electronic systems that need an accurate clock, which is to say most microprocessor-based systems, use a quartz oscillator. You’ll typically see a metal package somewhere near your chip that contains a slice of quartz which resonates at a certain frequency thanks to the piezoelectric effect.
Quartz crystals are cheap and provide a very accurate clock frequency, but they take up quite a bit of space and are sensitive to shocks. To deal with those two problems, fully on-chip oscillator systems have been available since about 2010. These use micro-electro-mechanical systems (MEMS) technology, which involves the manufacture of tiny moving structures on a chip. Their price is typically higher than that of a quartz crystal however, and their frequency stability and phase noise performance are often a bit worse. Today we’ll have a look at a few different MEMS oscillator chips and see what they look like inside.
First up is the Si501 by Silicon Labs. It’s an 8 MHz oscillator built using what Silicon Labs call CMEMS technology, which means that they integrate the MEMS bit on the same chip as the rest of their circuits. The package looks rather anonymous, with just a cryptic part number and no manufacturer’s logo. Silicon Labs have since sold their MEMS oscillator business to Skyworks, so future versions of this chip might have a different marking.
The LIS3DH is an accelerometer, designed and manufactured by STMicroelectronics. Like most accelerometers today it is a MEMS device (Micro-Electro-Mechanical Systems), which means the sensing function is made from silicon and integrated into an IC process. Special etching techniques are used to create tiny moving parts that can bend in certain directions, along with sensors that can detect that movement.
MEMS accelerometers are used in many electronic devices: your phone that detects whether you’re holding it horizontally or vertically, your games console that can sense which way you’re moving the controller, or your car that can detect the speed and direction in which you’ve just crashed so it can properly deploy the airbag.
The LIS3DH is housed in a little LGA package that measures just 3 x 3 mm2. Oddly there’s no ST logo or marking on the package. The “ON5nn” production code could actually trick you into thinking this is an ONSemi part.
