Micro System Design for Data Acquisition, Storage and Transfer

This article explains the design of a micro system for data acquisition, storage and transfer. It covers the IM2 standard for connecting sensors, data storage in internal memory, and data transfer using USB. The article also discusses circular memory allocation and timestamping of data packets.

MICRO SYSTEM DESIGN
A typical embedded system has three components, the input which is usually a
set of sensors and user interface devices. A controller running the software to
make sense of inputs and generate suitable outputs. The final part is output, a
bunch of actuators and user interface devices again. Given the specification,
the micro system is a small stand-alone device connected to sensors.

Data acquisition:
The Intramodule Multielement Microsystem IM2 standard would be preferred
way to connect the sensors as only a set of wires is required to connect the
devices and multiple devices can be connected to the system using same wires
simultaneously.
The IM2 protocol supports signal NSDET which is pulled low by the device until
it has registered itself with the controller. This is done by controller asking for
the device ID and acknowledging the ID being received.
The protocol supports set of signals NIOE and NACK that help set up the data
transfer between sensors and controller using DCLK and DIN and DOUT buses.
This sums up how up to 255 sensors can connect to the controller over same
set of bus with plug-n-play support.
Data Storage:
Once controller has acquired the data, from sensors, it must retain it in
internal memory. This data is only useful if it can be stored in a relative
manner. The sequence of data captured from sensor must not be altered for
individual sensor. But this does not guarantee relationship between data from
different sensors being maintained. So all data stored must consist of
timestamp with it. This is termed as data packet for that particular type of
sensor.
Data is stored in a separate memory section of the system that is protected by
the OS against access by non-authorized process. The memory area is allocated
to each sensor depending upon its requirement. Counting the size in terms of
packet size for each sensor. The memory size is limited, so it is advised to
arrange memory as circular array. In case controller runs out of memory, it

overwrites the oldest packet with latest data packet. This will make sure most
recent possible data is available for the analysis.
Data transfer:
Once the external system connects to the microsystem. The stored data can be
read one sensor at a time. The storage is byte sized but each transfer should be
counted in terms of packet. Once all packets are transferred and
acknowledged by the external device, the memory area can be returned to the
OS as free memory area for future use. The memory is arranged as circular
linked list and reading packets should start from oldest valid data packet.
Since the volume of data to transfer is large, a high speed interconnect is
preferable. USB is better choice as it supports plug-n-play facility without much
setup. Also this is standard port available on a laptop/ desktop machine. Using
any proprietary protocol may be faster but not comfortable. Also USB is a
serial interface, it saves on number of pins the controller needs for
interconnect with the external device making it best suited for the application.
Bibliography
Center, D. T. (2001). DTIC ADA401759: Intramodule Multielement Microsystem (IM2) Bus and
Interface Circuit. Retrieved from https://archive.org:
https://archive.org/details/DTIC_ADA401759/page/n19
Mason, J. Z. (2002). Communication Buses and Protocols for Sensor Networks. sensors, 14.