Week 15: Moo 2.0 PCB Design

Last week, I met with Noah to review some last minute schematic changes that we had discussed before beginning the PCB design process.  He suggested that I move towards using the ADXL362 digital accelerometer in place of the analog ADXL330 on the current Moo. While the digital accelerometer may be slightly more complex to set-up in software, I would have to agree with Noah that this move is worth making.  Attached are the datasheets for both the ADXL330 and the ADXL362.

ADXL330 Datasheet

ADXL362 Datasheet

Pros of ADXL362:

  • nearly 100x reduction in active mode current consumption (~2uA at 2.0 V supply)
  • digital SPI interface.  Less susceptible to RF noise.  Our current Accel data is pretty noisy.
  • built-in digital temperature sensor
  • smaller package (3mm x 3.25mm x 1.25 mm)
  • user selectable measurement ranges (+/- 2g, 4g, 8g)
  • will be used on the Wisp 5.0 design.
  • avoids issue associated with 1.2V reference voltage for ADC on the MCU
  • motion-detection interrupts above a certain threshold

Cons of ADXL362:

  • never used before = requires new firmware development
  • Digital accelerometers are more complex than analog ones.  May be more prone to bugs in software.
  • one less serial communication bus available for external SPI peripherals
  • changing multiple components (MCU and accelerometer) can potentially increase the chance of error in new design.

Moo 2.0 PCB Design

I have begun laying out the PCB board with the Wolverine chip.  I moved some of the existing components on the board (e.g external Flash memory) to make the routing with the new MCU easier to manage.  The layout process was slow at first, requiring a good understanding of what had been done already, but has gotten easier as time progressed.  I currently have about half the connections set-up with the new MCU and will continue to work on the remaining connections this week.  If we do decide to switch to using the new accelerometer then I will need to make a schematic/PCB component and swap out the existing accelerometer.  This shouldn’t be too difficult because the new accelerometer uses a pretty straightforward SPI interface along with some separate connections for I/O Power (use GPIO), interrupts (may ignore for now or connect with 0 ohm resistors to MCU), and ground.

Note about the power supply requirement on ADXL362: “The ADXL362 does not require any particular startup transient characteristics, except that it must always be started up from 0V. When the device is in operation, any time power is removed from the ADXL362, or falls below the operating voltage range, the supplies (VS, VDD I/O, and any bypass capacitors) must be discharged completely before power is reapplied. To enable supply discharge, it is recommended to power the device from a microcontroller GPIO, connect a shutdown discharge switch to the supply (Figure 47), or use a voltage regulator with a shutdown discharge feature,such as the ADP160.” – ADXL362 datasheet pp. 37


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