Data Path Diagram
The image above shows the CubeSat data path. In the following text, each channel/bus will be explained in detail.
- BUS 0 - Main I²C Bus of the Satellite
- BUS 1 - OBDH and Solar Panels (SPI)
- BUS 2 - OBDH and Microcontroller 1 on the Antenna Module
- BUS 3 - OBDH Microcontroller and TT&C Microcontroller
- BUS 4 - OBDH and Main Radio Bus
- BUS 5 - Beacon Microcontroller and Microcontroller 2 on the Antenna Module
- BUS 6 - EPS Microcontroller and Beacon Microcontroller communication
- BUS 7 - Radio Signals to the Antennas
- BUS 8 - OBDH and Solar Panels (Analog Channel)
- Debug Buses - External Interface Busses
The main purpose of this bus is to exchange data between the OBDH, EPS and the payloads.
It will use I²C, at 100 kbps. The OBDH will be the Master and the other modules will be slaves.
OBDH <--> EPS
The OBDH will send the Start Condition every second, the EPS will return 69 Bytes. The bytes sequence is the following (big-endian):
- ADC solar panel current - 12 bytes
- ADC solar panel voltage - 6 bytes
- ADC mppt output voltage - 2 bytes
- ADC regulators input voltage - 2 bytes
- ADC beacon/eps current - 2 bytes
- ADC MSP internal temperature - 2 bytes
- Batteries average current - 2 bytes
- Batteries monitor temperature - 2 bytes
- Batteries voltage - 4 bytes
- Batteries current - 2 bytes
- Batteries accumulated current - 2 bytes
- Batteries monitor protection register - 1 byte
- Batteries monitor status register - 1 byte
- Batteries monitor cycle counter register - 1 byte
- Batteries remaining active absolute capacity register - 2 bytes
- Batteries remaining standby absolute capacity register - 2 bytes
- Batteries remaining active relative capacity - 1 byte
- Batteries remaining standby relative capacity - 1 byte
- 7 RTDs measurements via ADS1248 - 21 bytes
- EPS status - 1 byte
The main purpose of this bus is to exchange data between the OBDH and the three solar panels: -X, -Y, -Z. The OBDH will read from the temperature sensors.
It will use SPI, at 8 Mbps to read from the temperature sensor in the solar panels. The full description of the pins and wires can be found in the OBDH section.
These data will be used to map the solar panels temperature.
In this bus the OBDH will send the commands to the microcontroller 1, to deploy the main radio antennas, read the deploy status and other sensors located at the antenna module. The antenna microcontroller 1 controls the deploy circuit of the two antennas of the main radio.
It will be used I²C, at 100 kbps. The OBDH will be the Master and the microcontroller 1 the slave. The OBDH will start the communication once, on the first boot (deploy), and after each reset to read the deploy status. It will not have a deterministic frequency of transactions.
The communication process is To Do.
This SPI bus has three main purposes:
- When a shutdown command is received, the OBDH must warn the TT&C to do the same, both have to sleep for 24 hours.
- To perform the handshake (synchronization/mutex) between the OBDH microcontroller and Beacon microcontroller allowing only one radio to transmit at a time
- Transmit some data to the TT&C microcontroller send these data through Beacon Radio.
SPI at 2 kbps will transfer the bytes from the OBDH microcontroller (master) to the TT&C microcontroller and vice-versa.
If It is a data transfer the OBDH will send the following data:
| Information | Bytes Quantity | Description |
|---|---|---|
| Vbat1, Vbat2 | 4 | Batteries Voltage |
| Tbats | 6 | Batteries Temperatures |
| QBat | 2 | Total charge of batteries |
| Isolarpanel | 12 | Solar Panels Currents |
| Vsolarpanel | 6 | Solar Panels Voltages |
| FSat Status | 2 | Overall Status of cubesat |
| IMU | 12 | Accelerometer and Gyroscope |
| System Time | 4 | Time since boot |
| Reset Counter | 2 | Number of OBDH resets since launch |
In a shutdown request the TT&C microcontroller must return an acknowledge for the OBDH.
In a RF channel request the OBDH asks the TT&C if the main radio can do a transmission. The TT&C must respond with Free or Busy. Every new transmission on the Transceiver the OBDH must do a RF channel request.
In this bus the OBDH will send and receive data to the main radio at the TT&C board.
SPI at 8Mbps will transfer the bytes from the OBDH (master) to the Radio and vice-versa. The GPIO is to enable the PA.
Once per second the OBDH will read the radio buffer, to see if the cubesat received a telecommand. When this happens it will read, decode and interpret. If the telecommand is a downlink, the OBDH will:
- Packet the data (NGHam).
- Take the mutex (Transmission permission) from the beacon.
- Enable the PA regulator.
After its done will:
- Disable the PA regulator.
- Release the Mutex (Transmission permission).
Telecommands are asynchronous, thus the GPIO will not have a deterministic frequency of transactions only the SPI bus, as stated above.
In these bus the Beacon microcontroller will send the commands to the microcontroller 2, to deploy the beacon antennas, read the deploy status and other sensors located at the antenna module. This microcontroller controls the deploy circuit of the two antennas of the beacon radio.
It will be used I²C, at 100 kbps. The Beacon microcontroller will be the Master and the microcontroller 2 the slave. The Beacon microcontroller will start the communication once on the first boot (deploy), an then after each reset to read the deploy status. It will not have a deterministic frequency of transactions.
The communication process is To Do.
This bus has a redundancy purpose. It will send EPS data (Batteries voltages) to the Beacon microcontroller.
It will be used UART (9600 bps) , where the EPS will interrupt the Beacon microcontroller every 10 seconds.
The EPS will send 31 bytes in this order:
| Information | Bytes Quantity | Description |
|---|---|---|
| Vbat1, Vbat2 | 4 | Batteries Voltage |
| Tbats | 6 | Batteries Temperatures |
| QBat | 2 | Total Charge of Batteries |
| Isolarpanel | 12 | Solar Panels Currents |
| Vsolarpanel | 6 | Solar Panels Voltages |
| Energy level | 1 | Overall Status of Satellite |
Analog cables to send the signal to the antennas, to spread it.
There are three SMA connector present in the TT&C board, two of them for UHF operation and one for VHF operation. The final solution will use only one or two cables to connect the TT&C to the antennas, depending on the engineering model tests results.
The main radio will use 2 antennas and and the beacon other 2. The antenna module contains two UHF monopoles and two VHF monopoles.
The main purpose of this bus is to exchange data between the OBDH and the three solar panels: -X, -Y, -Z. The OBDH will read analog data from the photodiodes.
The photodiodes will only use 2 wires to circulate its current. The full description of the pins and wires can be found in the OBDH section.
These data will be used to map the light distribution on solar panels.
Debug the satellite and update the firmware.
Four Wires for the Spy Bi-Wire (JTAG), 2 wires for debug for all the three modules.
The 2 wires for debug will use the UART(@ 9600 bps). The spy bi-wire is defined here.