PCB combining Atmega32u4, MTK3339 GPS module, and RN2903 LoraWAN module.
I wanted to make a single PCB that had some GPIO, communication interfaces, a module to transmit LoraWAN packets, and a way to get current location. The LoraWAN stack is offloaded from the 32u4 and rests in a PIC microcontroller inside the RN2903. This allows a simple interface of simply passing the payload from the 32u4 to the RN2903 without having to worry about the LoraWAN stack inside your project.
The LoraWAN-Stick incorporates the LiPo charging circuitry common to the Adafruit Feather line of dev boards, as well as follows the same pinout for attachment of Adafruit Featherwings. Because both the RN2903 and GPS module use serial communications, serial communications with other nodes may be difficult; the GPS module uses the hardware UART, and communication to the RN2903 uses software serial. From my experience, having multiple software serial interfaces tends to work in unexpected ways, and should probably be avoided without thorough testing.
Pinout follows Adafruit Feather format as I wanted to be able to add compatability of Adafruit Featherwings to this project. The 32u4 only has a single hardware UART, hard wired to the TX/RX of the GPS module. These pins are broken out on the header.
The RN2903 is controlled by serial, but because the 32u4 has only a single UART, a software serial interface must be used. The RN2903 is hardwired with its RX on pin 5 and TX on pin 10. Reset for the RN2903 is hard wired to pin 6 and should be pulled high when not in reset state. The RN2903 Pickit3 programming header is also broken out to update the firmware on the RN2903 directly.
#uC Info The 32u4-AU microcontroller that I used here comes preconfigured to use the internal oscillator, thus the C7, C8, and XTAL pads can be left empty. When configuring the microcontroller for the first time, it will not show up as a USB device until the bootloader has been flashed to it via ISP. Once the bootloader has been flashed, it should show up as an Adafruit Feather 32u4 under Device Manager, and can be programmed/configured in the same manner as any of the Adafruit 32u4 Feathers.
#Radio Interface The RN2903 by default communicates on 56000 baud. The SoftwareSerial library is more reliable at slower speeds; to set the baudrate in the RN2903, you must send a low signal on the TX line for at least two character lengths, followed by 0x55 at the new baudrate requested. Using the Arduino libraries, this might look like:
digitalWrite(10, LOW);
delay(500);
digitalWrite(10, HIGH);
rn2903.begin(9600);
rn2903.write(0x55);
#Fuse Settings The bootloader included configures the 32u4 in 8 MHz in the same configuration as the Adafruit 32u4 Feathers. The uC can be configured to run with an internal oscillator or an external oscillator. If you use the 32u4-AU variant, the built in oscillator can be used in lieu of an external oscillator. The fuse settings to configure the uC as such would be something like:
avrdude -c dragon_isp -p m32u4 -U lfuse:w:0xe2:m -U hfuse:w:0xd8:m -U efuse:w:0xfb:m -U flash:w:Caterina-Feather32u4.hex -v
https://github.com/lostangles/LoraWAN-Stick/blob/master/LoraWAN-Stick.xlsx
Total cost: $52.52
U3 RN2903-I/RM095 RN2903-I_RM095:XCVR_RN2903-I%2fRM095 Microchip U2 ATmega32U4-AU Housings_QFP:TQFP-44_10x10mm_Pitch0.8mm U1 FGPMMOPA6H FGPMMOPA6H:FGPMMOPA6H P1 USB_OTG Connect:USB_Mini-B R1 22 Resistors_SMD:R_0603_HandSoldering R2 22 Resistors_SMD:R_0603_HandSoldering U4 SPX3819 adafruit:adafruit-SOT23-5L R3 100k Resistors_SMD:R_0603_HandSoldering C1 10uF Capacitors_SMD:C_0603_HandSoldering C2 10uF Capacitors_SMD:C_0603_HandSoldering C3 1uF Capacitors_SMD:C_0603_HandSoldering D1 DIODESCH Diodes_SMD:D_0805 CN1 JST_2PIN-SMT adafruit:adafruit-JST-PH-2-SMT C4 1uF Capacitors_SMD:C_0603_HandSoldering SW1 SPST open-project:SW_PUSH_SMD U5 MCP73831 arthurc:SOT23-5 D2 LED-BAT Diodes_SMD:D_0805 R4 1k Resistors_SMD:R_0603_HandSoldering R5 10k Resistors_SMD:R_0603_HandSoldering C5 10uF Capacitors_SMD:C_0603_HandSoldering R6 100k Resistors_SMD:R_0603_HandSoldering R7 100k Resistors_SMD:R_0603_HandSoldering C6 10uF Capacitors_SMD:C_0603_HandSoldering JP3 PINHD-1X12 adafruit:adafruit-1X12 JP1 CONN_01X16 Pin_Headers:Pin_Header_Straight_1x16 JP2 PINHD-1X6 adafruit:adafruit-1X06 ANT1 CON-SMA misc:CON-SMA-EDGE D3 LED-RED Diodes_SMD:D_0805 R8 1k Resistors_SMD:R_0603_HandSoldering D4 LED-POW Diodes_SMD:D_0805 R9 1k Resistors_SMD:R_0603_HandSoldering
Add ICP header for 32u4