Aim
- To build a simple storage oscilloscope using the PIC trainer
- To report on the program's design and operation
Equipment
- PIC Trainer and PICkit3 in-circuit debugger
- MPLAB Integrated Development Environment software
- Laboratory CRO and signal generator
Task
Write a PIC program to enable the PIC trainer board to be used as a simple storage oscilloscope.
Basic Operation
The program should work as follows:
- When the "capture" pushbutton (PB1) is pressed, the program begins capturing and storing 1000 samples of the signal connected to the analog input terminal (the BNC connector for AN2) at the rate of 10 kHz (1 sample every 100 µs).
- When the capture process is complete, the program repetitively replays the stored samples (at the same 10 kHz rate) via the analog output terminal (DAC channel A). The beginning of each replay cycle is marked by a brief synchronising pulse (DAC channel B).
You can verify that your program is working correctly using a signal generator to supply input and an oscilloscope to view the output. Connect the signal generator to both the PIC trainer input and to Channel I on the CRO (you'll need a BNC T-connector) so you can monitor the input. Connect the PIC trainer signal output to Channel II on the CRO, and the sync output to the external trigger input on the CRO. Before connecting the signal generator to the PIC trainer, make sure that its output amplitude and offset is adjusted so that the signal voltage is always within the range 0 V to 5 V.
Exceeding these limits could damage the PIC processor chip.
Extensions
It is possible to score close to full marks for this project if you implement only the basic operations described above. However, if your program is working correctly and you are looking for maximum marks, you can attempt one or both of these extensions.
Variable sample rate: The sample rate for both capture and replay is determined by port B DIP switches 1 and 2:
Auto trigger mode: Port B DIP switch 3 (RB4) controls "auto-trigger" mode. If the switch is on when the "capture" button is pressed, the capture does not begin until the input signal crosses the 2.5 V threshold from below. If the switch is off, capture begins immediately.
Hints
Although you can determine the state of PB1 by testing bit 1 of PORTB, it's much easier to write the code if you instead test the INT1 interrupt flag bit (in register INTCON3). The latter approach is simpler because the flag is set when the button is pressed and remains set until you explicitly clear it. To make the flag sensitive to the falling edge (the pushbuttons are active low), you'll need to set the appropriate edge polarity (in INTCON2).
The simplest way to store and retrieve the signal samples is using indirect addressing. At the beginning of a capture or replay cycle, load one of the file select registers (FSR) with the address of the beginning of the data block where the samples are stored, then use the appropriate POSTINC register so that the address is automatically incremented after each reference.
The structure of your program will be simplest if you use interrupts to manage the capture and replay timing. Configure a timer to generate interrupts at the sample rate, then write an interrupt handler that will process a single sample. You'll need to devise a way for the handler to know whether the program is in capture or replay mode.
You'll probably find it most convenient to use timer 2 to generate the timing, because you can use its period register to fine-tune the timeout length to exactly 100 µs.
Assessment
Functionality
1. Sampling: The program samples its input and produces analog output. However, the output is not an accurate copy of the input, or the program does not capture and replay correctly.
2. Storage: The program successfully captures and continuously replays an accurate copy of its input. However, the rate or length of the sampling is not correct.
3. Timing: The capture and replay is correct in every detail, including sample rate and length.
4. Sample rate: The sample rate is correctly controlled by the Port B DIP switches.
5. Triggering: The auto-trigger facility is correctly implemented.
Code
1. Structure: The code is organised logically, with each subroutine having a well-defined role. Registers are used appropriately to store data and pass it between modules.
2. Assembler directives: Appropriate use is made of assembler directions, including constant definitions, data directives, assembly-time expressions, and macros.
3. Documentation: The program has an introductory comment listing its function, creation date, and authors. Each subroutine has a comment describing its purpose and explaining where it expects to find any input it needs, and where it will put any output it produces. Instructions whose purpose is unclear include a brief end-of-line comment to clarify their function.
4. Presentation: The code is attractively presented and easy to follow. Indentation is consistent and follows established norms. Labels and names are good descriptors for the entities they denote.
5. Use of interrupts: The interrupt system is used appropriately to manage the real-time aspects of the program. Timers are used where appropriate to determine time-dependent behaviour.
Report 1. Completeness: The report is complete and contains all necessary information. The length of the report is within expected guidelines.
2. Presentation: The report is attractively presented, with few spelling and grammatical errors, appropriate use of fonts, and consistent page layout.
3. Writing style: The report is written in a clear and direct style, and flows smoothly. The level of discourse is appropriate for the audience.
4. Technical details: The report uses appropriate terminology and forms of expression. Discussions of a technical nature are factually correct.
5. Use of figures: The report makes effective use of figures and other illustrative content. Figures are well integrated into the text, and contribute significantly to the understanding of the material.