An application module lets the Tektronix MDO3000 Series oscilloscope decode and trigger on any of the principal serial bus types. For a parallel bus, the MDO3MSO option is required. Serial bus types include Audio, CAN, LIN, FlexRay, I2C, SPI, MIL-STD-1553,
RS-232, RS-422, RS-485 and UART.
Each bus type brings up a horizontal menu, all of which are substantially the same except for the fourth menu item, which permits the user to either Include R/W in Address, Configure, select Bit Rate or select Response Time. The user can assign a different bus to B1 and B2. The next step is to press Menu in the Trigger section to the left of the number pad. Choices are TTL, 5.0-V CMOS, 3.3-V CMOS, 2.5-V CMOS, ECL AND PECL. Alternatively, the user can enter a threshold. The second soft key, Define Inputs, permits the user to choose Clocked Data, for which Rising Edge, Falling Edge or Both may be chosen. Pressing the third soft key permits the user to select the number of data bits by turning Multipurpose Knob a. The range is 1-20. Pressing the fourth soft key permits the user to turn Multipurpose Knobs a and b to define bits for each digital channel.
Returning to the horizontal menu, the sixth soft key, Bus Display, opens the vertical Display menu. The Bus Only, Bus with Waveforms and Hex or Binary may be selected. The seventh soft key, Event Table in the vertical menu, lets the user toggle Event Table on and off. When it is on, time-stamped file details are shown and if present may be saved. Any combination of Channels 1 through 4 and D0 through D15 can be used for most bus sources. For some bus sources, R1 through R4 and Math can also be used.
I2C is a widely used bus and consequently it is often appropriate to use an oscilloscope to look at I2C data transmission. I2C was developed by Philips Semiconductor in 1982 to facilitate connection of peripheral integrated circuits to processors and microcontrollers for short-haul communication within a circuit board. Competing manufacturers including Siemens AG, NEC, Texas Instruments and Motorola subsequently introduced I2C products. Now this popular protocol exists in home, offices, factories and motor vehicles world-wide. Advantages include simplicity and low cost. Distance is limited to several feet due to total bus capacitance. High-impedance and low-noise immunity require a common ground potential, restricting usage to a single PCB or closely associated separate PCB’s, typically in the same enclosure.
Within an I2C are master and slave nodes. The master generates a clock signal and initiates communication with slaves, which receive the clock signal and respond when addressed by the master. I2C is a multi-master bus. Many master nodes can operate and master and slave roles can be reversed between messages, after a Stop is transmitted.
To acquire data from an I2C bus in the MDO3000 Series Oscilloscope, press Define Input so that the appropriate vertical menu appears. You can assign the predefined SCLK input or SDA input to the channel connected to the signal. Press Include R/W in Address. Then press the desired vertical menu button. This determines the way in which the oscilloscope shows the I2C addresses in bus decode traces, cursor readouts, Event Table listings and Trigger Settings.
If YES is selected, the oscilloscope displays addresses as eight bits, where the eighth bit (LSB) is the R/W bit. Ten-bit addresses are displayed as 11 bits. The third bit is the R/W bit.
If NO is selected, seven-bit addresses are displayed as seven bits and ten-bit addresses as ten bits.
The Serial Peripheral Interface (SPI) Bus is used in synchronous serial communication for applications such as secure digital cards and liquid crystal displays, primarily over short distances as in embedded systems. Communication is full-duplex master-slave, with a single master. SPI in some respects resembles Synchronous Serial Interface (SSI) protocol, which in contrast, however, uses differential signaling with a single simplex communication channel.
The SPI Bus employs a single master device and one or more slave devices. Using a single slave device, the SS pin may be connected to logic low. To initiate an action, some slaves look for a falling edge. When there are multiple slaves, an independent SS signal is required for each slave device. Oscilloscopes offered by most manufacturers have triggering and protocol decoding for SPI, and they support two, three and four-wire SPI. These signals can be accessed through analog or digital channels.
In the Tektronix MDO3000 Series Oscilloscope, begin by pressing the B1 and/or B2 button(s) in the front panel. Then press the soft key associated with Bus in the resulting horizontal menu. Using Multipurpose Knob a, scroll to SPI and press the soft key associated with Define Inputs and press the appropriate vertical menu choices. You can set framing to SS or Idle Time. Predefined SCLK, SS, MOSI or MISO signals can be assigned to any channel.
Press the soft key associated with Configure to bring up the vertical SPI Configuration menu. Then press SCLK to set the edge of the signal to match the SPI Bus being acquired. Set the level of the SS, MOSI and MISO signals to match the SPI Bus. An Active High signal is active when the signal exceeds the threshold value. An Active Low signal is active when the signal is less than the threshold value. Use Multipurpose Knob a to set the number of bits of the SPI Word Size. Bit Order can be set by pressing either of the soft keys associated with Bit Order.
RS-232 Bus a few years ago was a familiar serial protocol for connecting peripheral devices to computers. Because of its ease of use and reliability, USB has replaced RS-232 in most applications, although RS-232 is still seen in industrial equipment. Compared to USB, RS-232 is characterized by lower transmission speed, large voltage swings, over-size connectors, lack of multipoint capability and limited multidrop ability. For these and other reasons, it has been largely displaced by RS-422, RS-485, Ethernet and USB. RS-232 is capable of both synchronous and asynchronous transmission. Besides data, control circuits are fully supported, and for that reason RS-232 is still commonly used on the factory floor and elsewhere.
In the MDO3000 Series Oscilloscope, press the B1 and/or B2 button(s) in the front panel vertical section so the horizontal Bus menu appears. Press the soft key associated with Bus and use Multipurpose Knob a to scroll to RS-232. After defining inputs and setting thresholds, press the soft key associated with Configure. The vertical Configuration menu appears. Use Normal polarity for RS-232 signals and inverted polarity for RS-422, RS-485 and UART buses.
Press Bit Rate and turn Multipurpose Knob a to select the appropriate bit rate. Press Data Bits and toggle seven, eight or nine data bits. Press Parity and toggle None, Odd or Even. Press Packets and toggle On or Off. Press End of Packet and use Multipurpose Knob a to select the correct symbols. Pressing Bus Display in the horizontal menu brings up the vertical Display menu and soft keys permit the user to select appropriate formats. Event Table in the horizontal menu brings up the vertical Event Table menu so that it can be turned off or on, File Details can be displayed and files if present can be saved.
Controller Area Network (CAN) Bus permits individual microcontrollers and devices to establish a network without a host computer. It was developed in an automotive and truck context beginning in the 1980s and soon expanded into industrial equipment, nautical, aeronautic and aerospace applications. Standards, specifications and whitepapers are available online.
CAN is a multi-master serial bus for connecting otherwise autonomous nodes. Nodes are connected through two-wire media, which are twisted pair having 120-Ω characteristic impedance. LIN Bus and FlexRay are CAN Bus variants. LIN Bus requires one wire and is lower cost. FlexRay requires two or four wires and is more costly.
To set up CAN Bus in the MDO3000 3000 Series oscilloscope, press the B1 and/or B2 button(s) in the front panel vertical section so that the horizontal Bus menu appears. Press the soft key associated with Bus and use Multipurpose Knob a to scroll down to CAN. After defining inputs and setting threshold, press the soft keys associated with Bit Rate and Bus Display. Use Multipurpose Knob a and soft keys to enter selections in the two vertical menus. Press Event Table in the horizontal menu. When Event Table is toggled on, File Details becomes active and if present can be saved.