Using digital scopes to analyze serial data buses
Rick Nelson, Chief Editor -- Test & Measurement World, 4/1/2005 2:00:00 AM
| Read other articles from this issue:Table of contents, April 2005
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You can readily meet many challenges if you employ a standardized serial data bus for your PCB designs. Such a bus occupies a smaller area on your board than would a parallel bus, it requires fewer wires, it lets you employ standard ICs, you don't need to spend time designing your own bus, and the instrumentation you employ may have built-in capabilities for addressing a few common serial data bus design problems related to noise, timing, and waveform quality.
In the Webcast "Serial Databus Analysis—CAN, I2C, SM, and SPI," sponsored by Test & Measurement World and Yokogawa Test & Measurement Division and originally broadcast February 16, Boyd Shaw, product manager for Yokogawa's family of digital oscilloscopes, describes techniques for using a general-purpose digital scope to troubleshoot serial data bus problems. An archive of the Webcast is available on the T&MW Web site.
As indicated by its title, the Webcast specifically addresses four buses: the CAN (Controller Area Network) bus, initiated by Bosch for automotive and other applications; the Philips-developed I2C (Inter-IC) bus, for bidirectional communication between ICs in consumer-electronics equipment such as TVs and VCRs; the related SM (System Management) bus, which uses the I2C bus protocol and finds use in variety of applications, including intelligent batteries; and the SPI (Serial Peripheral Interface) bus, developed by Motorola as a general-purpose serial data bus for communication among MCUs and peripherals.
Problems you might encounter while designing with these buses center on noise caused by crosstalk from other signals, radiation from other electronics products, switching noise from a power supply, white noise in the system under test, or—most likely—a combination of these sources. Boyd presents an overview of the instruments available for serial-data-bus troubleshooting (see the
). Scopes, he contends are versatile performers that excel at measuring waveform quality. He relates key scope specifications including bandwidth, memory length, and sampling rate to serial-bus applications, and he describes special features tailored for in-depth serial-data-bus analysis, including the ability to trigger on a specific address, to decode acquired data, to search for a specific address or data value, and to save acquired waveform data and bus information.
His conclusion: "It is necessary to check waveform quality in order to maintain product quality!"
To learn more about waveform-quality measurements for the buses described, view the complete Webcast at www.tmworld.com/webcasts.
Data bus test equipment comparison
| Noise | Waveform quality | Bus timing | Protocol analysis | Versatility | Approximate cost | |
| ¹ Exc.= Excellent |
||||||
| Digital scope with bus analysis | Exc.¹ | Exc. | Exc. | Good | Exc. | $6700+ |
| Digital scope | Exc. | Exc. | Exc. | Fair | Exc. | $6000+ |
| Logic analyzer | Poor | Poor | Exc. | Fair | Good | $6000+ |
| Dedicated protocol analyzer | Poor | Poor | Good | Exc. | Poor | $4000+ |
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