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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