esp-protocols/esp_modem/docs/internal_design.md

Internal design

Design decisions

• Use C++ with additional C API

  esp-modem in C	esp-modem in C++

    +2282
    -3908

• Use exceptions
  • Use macro wrapper over try-catch blocks when exceptions off (use abort() if THROW())

exceptions off	exceptions on

Difference is counted as <CURRENT> - <REFERENCE>, i.e. a positive number means that <CURRENT> is larger.
Total sizes of <CURRENT>:                                                 <REFERENCE>     Difference
 DRAM .data size:   10996 bytes                                                 10996
 DRAM .bss  size:   12552 bytes                                                 12488            +64
Used static DRAM:   23548 bytes ( 157188 available, 13.0% used)                 23484            +64 (    -64 available,      +0 total)
Used static IRAM:   54145 bytes (  76927 available, 41.3% used)                 54093            +52 (    -52 available,      +0 total)
      Flash code:  603979 bytes                                                595551          +8428
    Flash rodata:  187772 bytes                                                177852          +9920
Total image size:~ 869444 bytes (.bin may be padded larger)                    850980         +18464

exceptions on	RTTI on

Total sizes of <CURRENT>:                                                 <REFERENCE>     Difference
 DRAM .data size:   10996 bytes                                                 10996
 DRAM .bss  size:   12552 bytes                                                 12552
Used static DRAM:   23548 bytes ( 157188 available, 13.0% used)                 23548                (     +0 available,      +0 total)
Used static IRAM:   54145 bytes (  76927 available, 41.3% used)                 54145                (     +0 available,      +0 total)
      Flash code:  605331 bytes                                                603979          +1352
    Flash rodata:  196788 bytes                                                187772          +9016
Total image size:~ 879812 bytes (.bin may be padded larger)                    869444         +10368

• Initializes and allocates in the constructor (might throw)
  • easier code with exceptions ON, with exceptions OFF alloc/init failures are not treated as runtime error (program aborts)
  • break down long initialization in constructor into more private methods
• Implements different devices using inheritance from GenericModule, which is the most general implementation of a common modem
  • Internally uses templates with device specialization (modeled as DCE<SpecificModule>) which could be used as well for some special cases, such as implantation of a minimal device (ModuleIf), add new AT commands (oOnly in compile time), or using the Module with DTE only (no DCE, no Netif) for sending AT commands without network

DCE collaboration model

The diagram describes how the DCE class collaborates with DTE, PPP and the device abstraction

Terminal inheritance

Terminal is a class which can read or write data, and can handle callbacks when data are available. UART specialization is provided implementing these method using the uart driver.

CMUX terminal

The below diagram depicts the idea of using CMUX terminal mode using the CMux class which is a terminal (it implements the basic read/write methods) interfacing arbitrary number of virtual terminals, but at the same time it is also composed of another terminal, the real terminal, which is multiplexed.