WavePOD as a Lab Tool

In Basic Electronics courses it is often part of a Lab course to demonstrate various classic electronic circuit waveforms. Instead of building the actual circuits, it is possible with WavePOD to have a library of standard waveforms, and using BitScope, actually display these real-time on the DSO as if they were from an actual circuit. This approach is better than software simulation since it requires actual DSO settings as required for the real experiment. The use of "canned" waveforms can simplify the setup of lab experiments and provide a much more reliable outcome.

Waveform examples:-

• Half Wave rectified AC
• Full Wave rectified AC
• Full Wave rectified AC (filtered)
• 1KHz Sine Wave
• Dual Tone control signal
• AM Modulation
• FM modulation
• Noise signal
• Sweep signal

Learning PIC programming with WavePOD

Using WavePOD with BitScope provides a very direct way of learning about PIC programming and the use of microcontrollers in electronics. The PIC assembly code provided here is a safe starting point. Each of the 16 WaveLoop_X labels in the code may be selected as a starting vector via the DIP switch. On reset the PIC will start executing the code there. Any value output to PortB will be visible using BitScope and software loops can be observed by watching the DAC output over time. Almost any code loop will result in some output - providing a student with a very quick feedback.

The simplest code loop in wavePOD is the sawtooth wave where the value in PortB is constantly incremented and wraps from 00 to ff.

WaveLoop_0

	      incf    PORTB,F     ; increment the value into the DAC
	      goto    WaveLoop_0  ; repeat forever

This loop takes 3 instruction cycles (2 for the GOTO) which at 5MIPS is 600ns. The time to step thru all 256 codes (1 whole waveform) is 256 x 600nS = 153.6uS - or 6.510416666KHz

BitScope DSO Reference waveforms

While developing BitScope software, it is often necessary to generate a large variety of test waveforms at various frequencies. The problem also arises with an Open Design like BitScope of comparing different interfaces. WavePOD provides a very simple way of making a Reference Waveform that can be duplicated without any special lab setup. This is a benefit to BitScope programmers who can test their interface with a known waveform.

WavePOD OP AMP Response

The WavePOD circuit includes a socket for an industry standard DIP8 dual OP AMP. This is nominally a TL072 device, however any dual OP AMP with the same pinout may be substituted. By using WavePOD to generate step and pulse inputs, it is possible to observe OP AMP characteristics including:

• SLEW RATE
• OVERSHOOT
• RISE TIME
• COMPENSATION
• OUTPUT LOADING

With an external circuit, similar experiments may be made with various active and passive circuits.

Specialized Waveforms

For some applications, circuits require complex analog signals in order to operate. Examples include Video, SMPTE, BiPhase, FSK and RS232. With a PIC and a DAC, these signals can be programmed as a WavePOD WaveLoop to generate test signals. By using the analog channels of BitScope, it is possible to see responses to these complex signals time referenced precisely to the source (WavePOD).

WavePOD as an Active Logic Probe

Since WavePOD also generates logic signals, it may be used in digital circuits to generate test vectors and stimulus signals. All WavePOD signals are monitored by BitScope, providing a very powerful debug tool. The port used by the WavePOD PIC is completely configurable, so it is possible to make some PIC signals inputs so they are generated by the test circuit rather than WavePOD. If WavePOD is attached to a LP102W cable assembly, the Logic Analyzer inputs may see signals generated by the PIC or the circuit under test.

Example - debugging an SPI DAC circuit using WavePOD and BitScope

1. Use WavePOD with LP102W to connect to circuit
2. Use PB4-7 as SPI control pins
3. Initialize the WaveLoop to use PB4-7 as OUT, PB0-3 as IN
4. Write an in line sequence that places the SPI signal values on PB4-7 for (say) 0A0h
5. Loop back to 4.
6. Use a BitScope Analog input to monitor the output of the SPI DAC

The test rig described will exercise the SPI DAC in your circuit and provide a simultaneous trace of digital and analog operation of various signals. This can be done without having to write test code in you target circuit - in fact, you may be able to test parts of the circuit before the CPU is working.