Optical Sensors

Battery-Powered Imaging Applications Require Unique Strategies

February 17, 2017 By: Ted Marena, Microsemi

The Smartphone Camera

The smartphone image sensor is often the logical product choice for portables. Let us look at a few product examples and their specific challenges.

First, let's consider the thermal image camera design for a hand-held industrial application. As stated previously, a good starting point may be the application processor—it can drive a display, connect to wireless network modems, and interface to on-board storage for saving images or video. However, an application processor cannot directly connect to the majority of thermal sensors.

The smartphone image senor has a second generation Camera Serial Interface (MIPI CSI-2) that exists on an application processor, but a thermal sensor most commonly uses a CMOS parallel or LVDS interface. A small, low-power FPGA could be designed to bridge CMOS parallel to CSI-2, but it would have to generate so little heat that the thermal sensor is not disrupted. Also, the CSI-2 interface of the application processor is not designed to interface to pixels that read temperatures as the thermal sensor does. Instead, the application processor expects color pixel data. This mismatch means that another device has to implement the image processing for the thermal sensor.

A low-power FPGA would be a good choice if it consumes little power, generates very little heat, and can interface to the application's processor with an adequate bus speed. The most common processor interface with good throughput is the PCIe bus.

Selecting a low-power FPGA with PCIe that does not generate heat to disrupt the thermal sensor is paramount. A good choice is the IGLOO2 flash-based FPGA, which has much lower power consumption than SRAM-based FPGAs, do not generate much heat, and have built-in PCIe Gen2 cores in hard logic. In addition, this single chip FPGA is offered in a package as small as 11 mm x 11 mm.

In this application, the IGLOO2 FPGA interfaces to the thermal sensor and performs the necessary processing (figure 2). It converts the temperature data to corresponding colors from a selected palette that can be stored in the on-chip flash or embedded memory blocks. Each line is processed and then formatted to be sent out through the PCIe to the application processor. After the image is received, the processor can then drive it out to the display, save it to local storage, or send it out through a wireless modem. The combination of the FPGA and other smartphone chips can build a compelling thermal camera for industrial, medical, military, and other markets.

Fig. 2:  Block Diagram of a Thermal Image Camera
Fig. 2: Block Diagram of a Thermal Image Camera

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About the Author: Ted Marena

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