Multi-purpose Embedded System

Hardware Architecture

MCU

• MCU can be 8-bit/16-bit/32-bit
• MCU should have sufficient RAM (i.e. no external RAM required)
• MCU should have program Flash (i.e. no external program flash required)

Data Memory

• Use serial bus (I2C/SPI) to expand data memory to save I/O pins
• Need a file system to access the data
• EEPROM for small block of re-writable data
• Flash for large block of re-writable data

Communication

• RS232 for short distance communication with PC
• RS485 for longer distance communication (upto 1200m) over a set of devices
• USB for USB communication with PC
• Ethernet for Internet connectivity
  • Use of Ethernet controller (e.g. DM9000A (requiring software TCP/IP stack)
  • User of TCP/IP chip (e.g. W5300)
• GPIB for automated test equipment

User Interface

• Graphical displays (LCD, VFD, etc.)
• Keypad for input

Sensory Control

• ADC for analog feedback signals
  • Requires anti-aliasing analog filters and op-amp circuits
    • Use desktop software (e.g. FilterLab) to design circuits
  • May incorporate digital filter to improve reduce noise
• DAC for analog control signals
  • Refer to here if modulation is needed.
• GPIO for digital feedback and control signals
  • Option 1: directly controlled by MCU
  • Option 2: indirectly through D-type flip-flops (e.g. 74LV154) for low pin counts MCU
  • Option 3: indirectly through shift register flip-flops (e.g. 74HC165 for input, TPIC6595 or 74HC595 for output ) for extremely low pin count MCU

Software Architecture

RTOS

               +-----------+-----------+----------------------------------------------+
 Application   |  Task 1   |  Task 2   |    ...                                       |
               +-----------+-----------+------------+----------------------+----------+
 Standard API  |          POSIX System Call         |     POSIX Thread     |  C-Lib   |
               +------------------+-----------------+----------------------+----------+
   Kernel      | Power Management |                 |    Task Scheduler    |
               +------------------+                 +----------+-----------+ 
               |               Drivers              | FreeRTOS | Coroutine |
               +------+-----+-----+-----+-----+-----+----------+-----------+
   Hardware    | UART | ADC | DAC | NVM | PWM | ... |  TIMERS  | 
               +------+-----+-----+-----+-----+-----+----------+

• The task scheduler is based on FreeRTOS V4.1.3 incorporating coroutine developed by Simon Tatham. The scheduler API is wrapped by the POSIX Thread API.
  • e.g. pthread_create(), usleep(), etc.
• Software drivers are developed to allow users at Application Level to use the hardware (e.g. ADC, DAC, UART, NVM etc) through the POSIX System call API.
  • e.g. open(), write(), read(), ioctl(), lseek(), etc.
• The most up-to-date development can be found at repository freertos_posix

GUI

                      +---------------+----------------+
 User Applications    | Application 1 | Application 2  |
                      |               +----------------+------------------+
   Middlewares        |               |    Toolkits    |  Window Manager  |
                      +---------------+----------------+------------------+
   GUI Library        |            Graphics Engine (inc Font)             |
                      +-----------+----------------------+----------------+
   POSIX API          |           |  POSIX System Calls  | POSIX Threads  |
                      |           +-----------+----------+----------------+
 Drivers & Scheduler  |  Display  |  Pointer  |  Keypad  | Task Scheduler |
                      +-----------+-----------+----------+----------------+

TCP/IP Stack

                  +---------------------------------------------+--------------------+-------+
 Applications     |            Ethernet Application             |  GUI Application   |  ...  |
                  +--------+------+------+----------+------+----+                    |       |
 Web Libraries    |  Web   | DHCP | SNMP |   DNS    | SMTP |... |                    |       |
                  | Server |      |      | Resolver |      |    |                    |       |
                  +--------+------+------+----------+------+----+                    |       |
 Socket API       |            Linux Socket Wrapper             |                    |       |
                  |          +----------------------------------+                    |       |
 TCP/IP Stack     |          |               uip                |                    |       |
                  |          +----------------------------------+                    |       |
                  |          |       Ethernet Thread            |                    |       |
                  |          +---------------------------+------+--------------------+-------+
 POSIX API        |          |    POSIX System Calls     |            POSIX Threads          |
                  +----------+---------------------------+-----------------------------------+
                  |               Drivers                |            Task Scheduler         |
                  +----------+---------------------------+-----------------------------------+
                  |   H/W    |
                  |  TCP/IP  |
                  |  Stack   |
                  +----------+

• See here for description