Add your comments about the Circuit Note found at www.analog.com/cn0274
The link is wrong, when clicked it takes me to CN0150 not CN0274. I need to design a system similar to the circuit note, but the MCU to configure the ADXL362 is part of my load. That puts me into a chicken and egg situation - How do i configure the sensor without the MCU. Any ways to solve it?
The ADXL362 will default to a particular configuration on power up. If this is not exactly the mode you want, you will need to configure the part using the MCU. Is your MCU powered down until the switch closes? You might be able to manually stimulate the ADXL362 and quickly program the part while the switch is closed. This is definitely a chicken and egg situation.
Do you have a block diagram you could share?
On power up, the activity/inactivity functionality is disabled, but the chip boots is default mode (Link/Loop bits being 00). This is not useful anyway, let me know otherwise. If you guys designed this chip to be used for a motion activated switch, then this is no use really. What is the point in paying such a huge price per chip and not being able to achieve the desired application? The only possibility which can solve this is to map the AWAKE bit (Status Register) to either INT1 or INT2 pin on boot up. The AWAKE bit comes alive with a value 1, which when mapped to either of the INT pin can solve this issue (The default values of reg 0x2a or 0x2b must be 0x40, not 0x00). I hope AD incorporates these changes in the existing chip immediately.
I can get this solved either by a mechanical switch or a tiny PIC MCU always powered - both of which is not at all elegant (increases cost+complexity).
Below is the block diagram.
We appreciate your input.
There are issues with what you suggest, as well:
If the AWAKE bit comes alive with a value 1, the microcontroller is on, as you say, and can configure the accelerometer. However, during register configuration, the accelerometer must be in Standby. As soon as the micro puts the accel in standby, the AWAKE bit is tri-stated and now the micro may lose power again.
So even with your suggestion, we end up with a chicken-and-egg situation.
In our boards we have included a very small PIC microcontroller. It does need to always be connected to power, but it can put itself into a low power sleep mode as soon as it has finished configuring the accelerometer. It's cheap, low cost, and low power.
That being said, we know that even the smallest increase in power, size and cost are not ideal, and we are exploring more elegant solutions, as you say.
I hope this is helpful. If you have any other suggestions, please don't hesitate.
MEMS Applications Engineer