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Building a MPC555-Based Oscilloscope
by Daniel Mihalik

Start ı Software Description ı Oscilloscope Operation ı Enhancement Opportunities ı Sources and PDF

SOFTWARE DESCRIPTION

The program is simple by design. This project is intended as a dedicated oscilloscope, so the entire programıs main loop is devoted to waiting for the signal to begin transmission. Features such as interrupts are not used.

The QADC can run independently of the core to ensure that samples are taken and converted at precise intervals. It contains an instruction queue called the conversion command word (CCW) table, made up of 64 words, each of which contains an instruction. Each instruction can tell the QADC to sample a pin, pause, or stop. When activated, the QADC begins executing the CCWs sequentially. When in Continuous Scan mode, the QADC continues until either a pause is encountered in the queue or the main program disables the QADC.

The QADC also has a data queue called the result word table, which stores the results from the matching commands in the CCW table. For example, if the thirty-third entry in the command queue is an instruction to sample AN0, then AN0 is sampled and the result is stored in the thirty-third element of the data queue. When the QADC reaches the end of the command queue, it starts again from the beginning.

The program initializes the QADC by filling the CCW table with the same instructionısample the AN0 pin (see Listing 1). Every time 32 samples are taken, the software copies the results of those samples from the queue into the RAM to prevent them from being overwritten when the QADC loops around.

The SCI is a part of the queued serial multi-channel module (QSMCM). Data is written into the data register, automatically serialized, and sent out the serial channel. The software checks the status of the data register to make sure that itıs clear before writing the next byte to it. while(QSMCM.SC1SR.B.TDRE == 0); tells the program to wait in a loop until the register is clear. This prevents data from being written from the buffer to the data register faster than it can be sent out the serial port.

The other half of the oscilloscope is implemented on a personal computer, with the front end developed with Visual Basic (see Listing 2). This front end uses the MSComm object for communication, which handles communication through the COM1 port. It has a function that runs automatically when activity is detected on the serial port and is used to handle receiving information from the MPC555. The graph is made using the picture box object. Lines are drawn from one point to the next until the graph is completed.

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