Description of Change

Current String PPPClass::cmd(const char *at_command, int timeout) function can return empty string in 3 cases:

• when timeout occurs
• when OK AT response is received
• when ERROR AT response is received.

That is because esp_modem_at() from esp-modem does not return basic AT responses (OK/ERROR) as output strings, only as esp_err_t returned by a function.

This PR adds bool PPPClass::cmd(const char *at_command, String &response, int timeout); function and its bool PPPClass::cmd(String at_command, String &response, int timeout) variation.

These functions allow to return command response in more detailed way:

• When PPP is not started or timeout occurs: Function returns false; response is empty (not touched)
• When AT error response is received: Function returns false; response contains ERROR or detailed AT response
• When AT success response is received: Function returns true; response contains OK or detailed AT response

Example output from the functions:

• Success of AT+CFUN=1 returns true; response contains OK or AT+CFUN=1 if command echo is enabled
• Success of AT+CFUN=4 returns true; response contains +PPPD: DISCONNECTED. Response is caused by SIM7600E sending +PPPD: DISCONNECTED before sending the OK response.
• Success of AT+CPSI? returns true; response contains +CPSI: LTE,Online, [...]

Test Scenarios

Tested on arduino-esp32 v3.3.4; ESP32 DevKit; SIM7600E modem

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