Extension interface

Custom devices can be built and used together with CanSat. These can be used to make interesting projects, which you can find ideas for from our Blog.

CanSat's extension interface features a free UART line, two ADC pins, and 5 free digital I/O pins. Additionally, SPI and I2C lines are available for the extension interface, although they are shared with SD card and the sensor suite, respectively.

The user can also choose to use the UART2 and ADC pins as digital I/O, in case serial communication or analog to digital conversion is not needed in their solution.

Pin number	Pin name	Use as	Notes
12	GPIO12	Digital I/O	Free
15	GPIO15	Digital I/O	Free
16	GPIO16	UART2 RX	Free
17	GPIO17	UART2 TX	Free
18	SPI_CLK	SPI CLK	Co-use with SD card
19	SPI_MISO	SPI MISO	Co-use with SD card
21	I2C_SDA	I2C SDA	Co-use with sensor suite
22	I2C_SCL	I2C SCL	Co-use with sensor suite
23	SPI_MOSI	SPI MOSI	Co-use with SD card
25	GPIO25	Digital I/O	Free
26	GPIO26	Digital I/O	Free
27	GPIO27	Digital I/O	Free
32	GPIO32	ADC	Free
33	GPIO33	ADC	Free

Table: Extension interface pin lookup table. Pin name refers to library pin name.

Communication options

The CanSat library does not include communication wrappers for the custom devices. For UART, I2C and SPI communication between CanSat NeXT and your custom payload device, refer to Arduino's default UART, Wire, and SPI libraries, respectively.

UART

The UART2 line is a good alternative as it serves as an unallocated communication interface for extended payloads.

For sending data through the UART line, please refer to the Arduino

       CanSat NeXT
          ESP32                          User's device
   +----------------+                 +----------------+
   |                |   TX (Transmit) |                |
   |       TX  o----|---------------->| RX  (Receive)  |
   |                |                 |                |
   |       RX  o<---|<----------------| TX             |
   |                |   GND (Ground)  |                |
   |       GND  o---|-----------------| GND            |
   +----------------+                 +----------------+

Image: UART protocol in ASCII

I2C

Use of I2C is supported, but the user must bear in mind that another subsystem exists on the line.

With multiple I2C slaves, the user code needs to specify which I2C slave the CanSat is using at a given time. This is distinguished with a slave address, which is unique hexadecimal code to each device and can be found from the subsystem device's data sheet.

SPI

Use of SPI is supported as well, but the user must bear in mind that another subsystem exists on the line.

With SPI, the slave distinction is instead made by specifying a chip select pin. The user must dedicate one of the free GPIO pins to be a chip select for their custom extended payload device. The SD Card's chip select pin is defined in the CanSatPins.h library file as SD_CS.

Image: the CanSat NeXT I2C bus featuring several secondary, or "slave" subsystems. In this context, the Sensor suite is one of the slave subsystems.

Image: the CanSat NeXT SPI bus configuration when two secondary, or "slave" subsystems are present. In this context, the SD card is one of the slave subsystems.