I couldn’t find any clear reference design for using the UART to connect to a microcontroller, does one exist, I’m not quite sure what to make of the DTR? The mangOH does have a schematic but the DTR signal tracks to a header with no information how it’s meant to be used.
HL7692 spec has this to say about the DTR signal: Prevents the AirPrime HL7692 from entering sleep mode, switches between data mode and command mode, and wakes the module up.
Checking some other threads, it appears that this is the (only) way to wake up the module if it’s in sleep mode, fair enough.
However the AT command interface guide says this: DTR/AT&D is only applicable to the USB AT port; it has no effect on UART1
Wut? So when I’m not using USB, DTR is strictly used to keep the module awake with no other function?
In AirPrime_HL76xx_AT_Commands_Interface_Guide_Rev11_0.pdf：
2.12. &D Command: Set Data Terminal Ready (DTR)
DTR/AT&D is only applicable to the USB AT port; it has no effect on UART1
In my test, DTR/AT&D works for both UART1 and USB AT port to switch to command mode. So the doc may need to be updated.
e.g. Test with UART1 and USB AT port:
AT&D1 → PPP connection (ATD99**1# CONNECT) → DTR inactive → switch to command mode.
In AirPrime_HL7692_Product_Technical_Specification_Rev7_0.pdf:
5.36. +KSLEEP Command: Power Management Control for UART
Notes: This command only controls UART power management, and does not affect the USB AT command port
For HL7692, To wake up from sleep you can use DTR or send any characters, e.g. (UART1 ONLY)
DTR signal be used to wake up module
AT+KSLEEP=1 → enter sleep → Togle DTR → module wake up.
AT+KSLEEP=0 → DTR inactive → enter sleep → DTR active → module wake up.
Wake up by any character:
AT+KSLEEP=1, AT&K0 → enter sleep → send any character → module wake up