There are two types of waveforms used with USB devices and Oscium’s handheld oscilloscope gives users the capability to study them conveniently and in detail. This picture above is a screenshot of the analog and digital waveforms used with a simple USB drive. There is an analog voltage (the green trace in the picture) and two digital voltage waveforms (yellow and purple) in the picture. The characteristics of these voltages will be discussed briefly in the following paragraphs.
Depicted in the following image is a USB cable that's been gently "modified" to allow probe access. Oscium's iMSO-104 tool allows users up to four digital channels for the oscilloscope. Below, the purple and yellow leads are connected to the differential digital lines and correspond to the color of the traces displayed on the the oscilloscope's screen. NOTE: Do not forget the tie the digital return to a reference (ground) point before making any measurements. Without a proper reference, the measured digital voltages will be "floating", resulting in inaccurate measurements.
Voltage #1: Analog +5 Vdc
There is a five volt supply in the host device used to power the USB drive. The USB protocol specifies the required voltage & current characteristics for the sourced analog voltage. For example, a USB powered device can draw a total of 5 Amps from the source. The 5 Amps is partitioned between a “charging circuit” current draw of 1.5 A for a battery charger application, like an iPod, with the remaining current budget dedicated to the data transfer requirements. Apple product users may be confused that their iPad and iPad Mini will not charge from the computer’s USB port and wonder why. Well, the answer is the larger battery in these iOS devices pull more current when charging than the USB port can handle.
Voltage #2: Digital Differential Pair "D+" and "D-"
The pulsing signals in the picture are the digital lines that transfer data between the host and the memory device. USB protocol specifies a differential signal pair for this function. Pins 2 and 3 of the USB connector are signals “D-“ and D+”, respectively. The yellow trace is the probed Pin 3, so it’s the positive side of the differential pair and Pin 2, the negative side, is therefore represented as the purple trace.
Logic Analyzer for Detailed Analysis of Digital Communication Signals
Oscium has another tool for electronics enthusiasts that allows for in depth analysis of these digital signals, called the Logic Analyzer. It’s the most recent addition to the array of tools Oscium offers in its iOS Test product line. The logic analyzer tool allows users to decode the signal traffic into groups: the synchronization sequence, the data packet, and the end of packet sequence. Click here to go to Oscium’s Logic Analyzer product page and learn more about using that tool for decoding USB signals.