ELEC60013 Embedded Systems - Music Synthesizer Documentation

The following document outlines the design, implementation and real-time dependecies of a music synthesizer for the 2nd coursework of the Embedded Systems module.

System tasks

The sytem at hand executes several real-time tasks, including:

• Reading the key matrix
• Updating the display
• Generating sound

Reading the key matrix

Purpose:

• Decode the key matrix, obtaining the states of the keys, knobs and joysticks
• Obtain the currently selected note from the decoded key matrix
• Select the appopriate step size for the currently selected note
• Transmit the currently selected note, along side its state (pressed or released) using CAN
• Obtain any changes in the knobs from the decoded key matrix, corresponding to volume, octave or waveform

Function: code(void scanKeysTask(void *pvParameters))
Implementation: Thread, executing every 50ms
Minimum initiation time: TO BE COMPLETED.
Maximum execution time: TO BE COMPLETED.
Priority: Highest (1), as playing the appropriate sound is more important than updating the display.

Updating the display

Purpose:

• Display the currently selected note (string)
• Display the current volume (int)
• Display the current octave (int)
• Display the current waveform (XMB icons)

Function: code(displayUpdateTask(void *pvParameters)
Implementation: Thread, executing every 100ms. While a refresh rate of 10Hz does not sound impressive; the display is largely static. Therefore, 100ms was perfectly accetable, without any noticeable delays.
Minimum initiation time: TO BE COMPLETED.
Maximum execution time: TO BE COMPLETED.
Priority: Lowest (2).

Generating the sound

Purpose:

• Adds the sample rate to the accumulated value, corresponding to the desired frequency
• Sets the output voltage, by using the accumlated value and the desired volume
• Writes to analogue output, generating the desired sound

Function: code(sampleISR())
Implementation: Interrupt executing with a frequency of 44.1kHz
Minimum initiation time: TO BE COMPLETED.
Maximum execution time: TO BE COMPLETED.

CAN

Purpose:

• TO BE ADDED

Function:
Implementation:
Minimum initiation time:
Maximum execution time:
Priority:

Critical instant analysis of the rate monotonic scheduler

TO BE ADDED - After system is completed.

CPU Resource Usage

TO BE ADDED - After system is completed; see this link: https://edstem.org/us/courses/19499/discussion/1300057

Shared data structures

Shared data structures:

• code(currentStepSize), safe access guaranteed using code(std::atomic<uint32_t>)
• code(keyArray), each element within the array is of type code(std::atomic<uint8_t>)
• msgInQ, handled by FreeRTOS
• code(RX_Message), handled by code(std::atomic_flag)

It was desiced to use C++ code(std::atomic), as it is easier to use and implement, while providing the same functionality as a mutex. According to the documentation: "Each instantiation and full specialization of the std::atomic template defines an atomic type. If one thread writes to an atomic object while another thread reads from it, the behavior is well-defined (see memory model for details on data races). In addition, accesses to atomic objects may establish inter-thread synchronization and order non-atomic memory accesses as specified by std::memory_order."(https://en.cppreference.com/w/cpp/atomic/atomic)

TODO - Expand on the memory model for std::atomic.
TODO - Explain how FreeRTOS handles msgInQ.
TODO - Once CAN is completed, expand on the protection of RX_Message

Analysis of inter-task blocking dependencies