simple_cmux_client example of modem-v1.0.2 does not work with SIM800c and ESP32 (IDFGH-11134)

[Hossein-M98]

Answers checklist.

• I have read the documentation for esp-protocols components and the issue is not addressed there.
• I have updated my esp-protocols branch (master or release) to the latest version and checked that the issue is present there.
• I have searched the issue tracker for a similar issue and not found a similar issue.

General issue report

Hello
I am using esp-idf-v5.1.1 and esp-protocols-modem-v1.0.2.
I compiled the simple_cmux_client example and programmed my board, but it seems there is a bug in the esp-protocols-modem-v1.0.2 release that DCE can't enter the CMUX mode.
I just changed the UART pins in sdkconfig to match my board. Likewise, I also changed the DCE device to SIM800.
Debug logs are available here:

I (0) cpu_start: App cpu up.
I (504) cpu_start: Pro cpu start user code
I (504) cpu_start: cpu freq: 160000000 Hz
I (504) cpu_start: Application information:
I (509) cpu_start: Project name:     simple_cmux_client
I (515) cpu_start: App version:      1
I (519) cpu_start: Compile time:     Sep 26 2023 08:52:30
I (526) cpu_start: ELF file SHA256:  46dbbb0d3508d699...
I (531) cpu_start: ESP-IDF:          v5.1.1-dirty
I (537) cpu_start: Min chip rev:     v0.0
I (542) cpu_start: Max chip rev:     v3.99 
I (546) cpu_start: Chip rev:         v3.0
I (551) heap_init: Initializing. RAM available for dynamic allocation:
I (559) heap_init: At 3FFAE6E0 len 00001920 (6 KiB): DRAM
I (564) heap_init: At 3FFB52E0 len 0002AD20 (171 KiB): DRAM
I (571) heap_init: At 3FFE0440 len 00003AE0 (14 KiB): D/IRAM
I (577) heap_init: At 3FFE4350 len 0001BCB0 (111 KiB): D/IRAM
I (583) heap_init: At 4008D924 len 000126DC (73 KiB): IRAM
I (591) spi_flash: detected chip: generic
I (594) spi_flash: flash io: dio
I (602) app_start: Starting scheduler on CPU0
I (603) app_start: Starting scheduler on CPU1
I (603) main_task: Started on CPU0
I (613) main_task: Calling app_main()
I (613) uart: queue free spaces: 30
I (623) cmux_example: not set_flow_control, because 2-wire mode active.
W (1083) CMUX: Restarting CMUX state machine (reason: 4)
I (1083) CMUX: Protocol recovered
E (1753) cmux_example: Failed to configure multiplexed command mode... exiting
I (1753) main_task: Returned from app_main()

[david-cermak]

Thanks for the quick report. Indeed, that latest change did break the CMUX protocol for SIM800. Added a lot of consistency checks to make the CMUX more strict (and robust), but missed the MSC frame.

Should be fixed in 8d5947e