Definition

Real-time: is doing something within a specified time constraint

• Hard real time : missing the dead-line consists a system failure
• Soft real time : like video streaming where some buffering is okay (also called firm real time)

Types of real-time system

• hardware :

  • ASIC
  • PLD
  • FPGA

  → Down sides are expensive hard to develop, complex

• Bare metal (firmware):

  →Code that has total control of all aspects of the hardware, this type of implementation excels when there is a small number of relatively simple tasks (or one monolithic task)

RTOS-Based firmware:

• Give the illusion that each task has the processor to itself.

RTOS-Based software:

• Software running on a full OS that has an MMU and a CPU.
• The advantage of ruing a full OS is all of the capabilities that come with it.

Jitter

What is Jitter?:

Is the delay between an event being triggered and being detected.

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Note that if a system has a maximum amount of jitter the system is considered determistic

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Interrupts

• Interrupts are signals sent to the MCU or CPU indicating that an event has taken place and directly execute specific code (called an interrupt service routine ISR).

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In ARM cortex-M, when two interrupts occur one before the other finish executing, the CPU does not restore context to the pre-interrupt state, it continues in the same state and executes the next ISR.

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• An ISR should be as small as possible (to avoid the risk increasing delay) so generally, an ISR sets a flag in a register or writes a value to an array in RAM.

DMA: Direct Memory Access