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Issue #205 August 2007
Handheld Multifunction Scope
by Jingxi Zhang & Yang Zhang
If you need to monitor signals and inspect target devices, you can design your own LM3S811-based handheld multifunctional oscilloscope. This system supports several modes: Oscilloscope, Voltmeter, Ohmmeter, Capacitance Meter, Inductance Meter, Frequency Counter, Logic Probe, and Pulse Generator.
Start | Circuit Description | Software Developmentt | Improvements | Sources & PDF
Many of our projects (e.g., robotic control applications) involve only audio or low-frequency analog signals, so we thought it would be handy to have a small pocket-sized oscilloscope for testing purposes. When we received an evaluation kit for the Luminary Micro DesignStellaris contest, we were happy to find that we could use the compact LM3S811 evaluation board to build a tiny multifunctional oscilloscope. The evaluation board had almost everything we needed. The only parts we had to add were the components for the analog signal conditioning circuit. In this article, we’ll explain how we built our handheld system (see Photo 1).
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a) b)
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| Photo 1a—The handheld multifunction oscilloscope is housed in a small plastic box. The front view of the device is Luminary Micro’s LM3S811 evaluation board. The dual-channel input ports are at one end of the device, while the USB port is located at the other end. Slide switches for mode selection are visible at one side of the device. b—This is the PCB for the analog signal-conditioning circuit and batteries. The PCB and the LM3S811 evaluation board are stacked together back-to-back on their solder sides. |
The small LM3S811 microcontroller, at the center of the evaluation board, is a powerful 50-MHz 32-bit CPU; with 8 KB of RAM and 64 KB of flash memory. The microcontroller is fully loaded with on-chip peripherals that are suitable for building a variety of functions for mixed signal processing and control. The built-in peripherals include a 500-ksps multichannel 10-bit ADC, which can capture the input analog signal for oscilloscope display, a flexible analog comparator, three 32-bit (or dual 16-bit) general-purpose timers with input event capture, six channels of PWM output, a synchronous serial interface (SSI), which can work as a Freescale SPI, National Semiconductor MICROWIRE, or Texas Instruments synchronous serial interface protocols, an I2C, two channels of UART, and plenty of general-purpose I/O ports (GPIO). There is a small OLED display on the evaluation board. This 1" bright yellowish panel is a fine-pitch 96 × 16 graphic display connected to the LM3S811 through the I2C interface. Unlike most LCDs, the OLED display has a very fast response time, which is ideal for displaying rapidly changing oscilloscope waveforms. A USB controller is also available on the board for JTAG control and serial port emulation. The USB serial port emulation connection can be used to deliver the captured waveform to a host PC and show the waveform on a larger, higher-resolution PC screen.
Finally, we developed a small battery-powered multifunction device with the following functions: A dual-channel oscilloscope supports 250 ksps for each channel. Each channel has seven gain settings, which allow the full-scale display to be independently set to ±10 V, ±5 V, ±2 V, ±1 V, ±500 mV, ±200 mV, or ±100 mV. The trigger method includes Free-Running (Auto) mode, Channel 1 Edge-triggered (± Edge) mode, and Single-Shot mode. An AC/DC voltmeter can autorange from 100 mV to 10 V. The DC polarity and AC can be automatically detected and indicated in the display. An ohmmeter autoranges from 0 to 1.36 MW, with audible sound for connectivity. A capacitance meter is autoranged to measure the capacitances from 1 pF to 100 mF. An inductance meter measures inductors in the millihenries to henries range. A frequency counter counts the frequency of low-level signals with a sensitivity of 150 mV. A logic probe senses logic high, logic low, open circuit (tri-state), and pulse. A pulse generator generates the pulse repeat frequencies from 5 Hz to 5 MHz and duty cycles from 1% to 99%. The generator also supports single-pulse outputs.
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