Embedded systems

Often for a single application, low price, highly reliable, resource efficiency

Embedded system pipeline

Intellectual Property Core (IP Core): Reusable unit of logic/functionality (eg. network / video / bus protocol / … chip) $\to$ produced in high volume
Field-Programmable Gate Array (FPGA): Reconfigurable/programmable integrated circuit for specific functionality $\to$ better performance
MicroController Unit (MCU): Scaled-down microprocessor sending/receiving commands to connected components $\to$ needs to be programmed
Sensors: Measure physical properties - convert them to electric signals

• Specifications: accuracy, sensitivity, sensing range, sampling interval, type (digital / analogue)

Actuators: Cause events to occur in the environment

• Driven by embedded system’s signals, often require extra power

Conversions

Analogue-Digital Converter (ADC)

Analogue input (eg. sensor) $\to$ digital output
Limited number of bits $\to$ quantization error:

$$E_{RMS}=\sqrt{\frac{1}{\Delta }\ast {\int }_{-\frac{\Delta }{2}}^{\frac{\Delta }{2}}{x}^{2}dx}=...=\frac{\Delta }{\sqrt{12}}$$ $$\Delta =\frac{range}{2^{bits}}$$

ADC sampling rate - frequency of conversion and sending data - needs to be high enough

Digital-Analogue Converter (DAC)

Digital input (from processor) $\to$ analogue output (eg. actuator)
Pulse-Width Modulation: simulating analogue output

Designing Embedded System Applications

Selecting hardware

Datasheets for complex components
MCU selection based on requirements: word length, clock speed, IO pins, timers, …

• storage (cost vs speed tradeoff)
• registers (fast, single location)
• register file (a set of registers)
• memory:
  • cache: fast, cheaper than registers (used for data and instruction cache)
  • main memory: connected to CPU via bus $\to$ slow but large
  • EEPROM: survives power off, but small and slow (used for configuration data)
  • flash: large, faster reads but slower writes than EEPROM, survives power off

Programming MCU

MCU (machine language) vs programmer (high level language)

• Cross compilation: programme on PC, deploy to MCU
• Toolchain: development tools

Embedded operating systems

Communication, managing tasks/processes and resource allocation
Process: abstraction of processing tasks, usually 1 process per app

• Separate memory space $\to$ slower inter-process communication

Task: smallest unit of execution, multiple tasks per process - fast switching

• Shared memory space $\to$ direct inter-task communication (shared variables)
  

OS is optional: embedded devices are often single-purpose

• consumes resources (processing, memory, energy)

OS is useful:

• easier programming and deployment (provides libraries)
• task-resource allocation support (for complex apps)

Real-Time Operating System (RTOS): event driven OS with a scheduler for time sharing (eg. round-robin / priority preeemptive scheduling / … )