NXP’s MMA8451QR1

MicroCrystal RV-8803 – A Really Accurate RTC June 17, 2019- RTC chips traditionally were the domain of Maxim and NXP. Switzerland-based MicroCrystal recently made quite a buzz with its RV-8803, which offers simple integration, an I2C interface and extreme accuracy. Let us start out by looking at a system overview. Figure one shows how this I2C RTC is made up internally – interested observers note […]
Analog Devices’ LTM2887 – If a Digital Bus Needs Galvanic Isolation June 15, 2019- In theory, digital buses are intended to connect things to one another. Sadly, one sometimes wants but a logical connection – in many cases, galvanic isolation helps out. Should you ever feel like isolating very fast buses, definitely give the LTM2887 a chance. From a technical point of view, Linear Technologies attempt at entering the […]
AVAGO APDS-9008 or APDS-9005-020 – Keeping an Eye on Ambient Light June 14, 2019- Recent times saw quite a bit of interest for part numbers such as the APDS-9008 or APDS-9005-020 – they are a very interesting type of ambient light sensor. One of the oldest questions in regards to light sensors involves their spectral sensitivity. If a part is to output but one voltage level, the conversion between […]
STMicroelectronics’ LIS2DH12 – Low Power Motion Sensing June 12, 2019- Apple’s iPhone introduced gyroscopic sensors to a wider range – before that, accelerometers were not commonly found in consumer hardware. STM sells excellent parts which, however, get little media attention – time to take a look at the LIS2DH12. Integrating the part into your circuit as an MPU6500 alternative or MPU9500 alternative is not difficult. […]
Panasonic AMG8833 – FLIR for cheap June 7, 2019- FLIR made infrared thermal sensors cool – sadly, their parts are all but cheap (and suffer from export control problems). Panasonic’s Grid-EYE family is a low-resolution alternative which comes at significantly lower cost. From a technical point of view, Panasonic’s image sensor could not be simpler. We are looking at an 8×8 sensor with an […]
DipTronics DTS-62R-V – Small Switch, Very Big (and Very Cheap) June 5, 2019- When yours truly was but a lowly cadet, various US manufacturers produced (pricey but quite reliable) switches. Chinese vendors recently started to catch up – seeing the DTS-62R-V top our customer interest list shows that the run stage of the crawl-walk-run cycle is reached. DipTronics has been around the Asian switch market for ages, exhibiting […]

Accelerometer ahoi!

Process computer designs usually start out with an accelerometer. As reliability problems creep in, making the switch to a more “professional” solution is beneficial.

Accelerometers are based on the MEMS principle: instead of being a purely solid-state affair, the inside of the chip contains a micromechanical motion sensor along with a small mass. In addition to that, a set of transducer circuits are provided, leading to the block diagram shown in figure one.

Accelerometers can be pretty complex

…Accelerometers can be pretty complex

Adding the part to an application circuit, also, is not difficult. Figure two shows the I2C interface along with a few decoupling capacitors – if all of this is in place, the circuit is ready to rumble.

Tug along a few capacitors, and be happy with your accelerometer…

Tug along a few capacitors, and be happy with your accelerometer

Sadly, the MMA8451QR1 is yet another 3V6-limited part, and can not live in a 5V domain. This is problematic, as the LDO voltage regulators tended to be responsible for most of the component failures yours truly saw when importing breakout boards from China.

Advanced features

NXP is among the most experienced designers of accelerometer circuits. The I2C interface lets you define all kinds of interrupts, which get fired off even if the main CPU is not monitoring the data stream transmitted via the I2C. Of course, four different G levels can be programmed into the chip, thereby letting you choose between higher accuracy and a larger measurement range.

Processor load is minimised via two nifty design features. First of all, the 14bit resolution can be artificially reduced to 8 bits per channel, thereby cutting bus congestion in half. If that is not enough, a 32 value FIFO buffer allows your CPU to reuse the bus temporarily.

Sadly, the very difficult-to-prototype part does not play well with others. It supports but two I2C addresses, thereby making the deployment of large sensor networks difficult.

Is it worth it?

If your system finds itself in a high reliability environment, designing a custom board is a sure-fire way to eliminate accelerometer-borne troubles. However, the money issue remains.

Even if you let OEMSecrets minimise your parts cost, be aware that the competitor from China is fierce and hungry. Projections showed that a 20% failure rate still leaves China in front – if you have the time and capacity to perform thorough burn-in tests to weed out unreliable voltage regulators.