Hardware
The OBDH board is composed by the following main components:
- MCU
- Non-volatile memory
- IMU
- Voltage monitor with watchdog timer
- Current drivers
- Radio interface
- EPS interface
- Beacon interface
- Payloads interface
- Current sensing circuit
The MCU used in the FloripaSat-I's OBDH is the Texas Instruments' microcontroller MSP430F6659IPZ. The family MSP430 is a 16-bit RISC architecture designed for low power consumption. It contains ADC with 12-bit resolution, six universal serial communication interfaces (set as: three I2C, two SPI and one UART), 16-bit timers and up to 74 I/O pins. The operating clock of the OBDH module is 16 MHz. The software description can be seen here. There is also a Voltage Reference IC (REF5030A-Q1) to ensure a clean reference for the ADC readings.
The OBDH uses a SD card and memories IC's. The micro SD Socket is the Molex 5031821852 and the three IC's are 128MBit Nor Flash (IS25LP128-JBLE) memories. A non-volatile memory is essential to retain a large amount of data and prevent loss in the event of a reboot. Also, it is important to note that these memories store the data generated by the entire satellite. So, a 2GB SD Card (current size, but could be increased) has been chosen to guarantee the complete storage of generated data and the memories IC's are used for ensure redundancy in critical data.
The IMU module is composed by two 9 axis inertial measure (a 3-axis ace units for redundant readings. An Invensense MPU9250 and a Bosch BMX055. Both have a 3-axis accelerometer, a 3-axis gyroscope, and a 3-axis magnetometer that capture relevant information about the satellite's rotation and the earth's magnetic and gravity fields. These data are transferred to the MSP430 via an I 2 C bus and are useful for the Attitude Control and Determination System (ADCS).
Together with the microcontroller there is an External Voltage Monitor with Watchdog Timer (TPS3823-33QDBVRQ1) to ensure a certain level of voltage to power up the system and to provide a redundant Watchdog (besides the MSP430's internal Watchdog Timer).
The current drivers used are two H-bridges (DRV8833). They control the Magnetorquers from Gomspace's NanoPower P110 solar panels according ADCS logic control.
To interface the MSP430 with the NiceRF's RF4463 radio there are 4 pins that compound the SPI bus (MISO, MOSI, SCLK, CSn) and more 4 GPIOs to monitor interruptions, reset the radio and assume other generic functions. The OBDH controls the regulator, located on EPS board, that supply the radio through another GPIO. All this 9 connections are made over the PC104 connector.
OBDH is powered by the EPS module, when the energy level is high enough. It is supplied at 3,3 V via PC104 pins. Beyond that, there is an I2C bus, also via PC104 pins, to transmit some data generated in EPS to OBDH. This data is stored and part of it is forwarded to send through the Beacon.
In order to have a connection between the MCUs of OBDH and TTC, there is 4 GPIOs that compound a full duplex bus.
To interfaces with the payloads, it is used the same I2C bus that OBDH shares with EPS. The OBDH controls the regulator, located on the EPS board, that supply the payloads through GPIOs (1 to each payload).
The Current Sensing module have three Rail to Rail Op Amps (TLV341AIDBVR) for the sun sensors readings. And a Current Sense amplifier (MAX9934TAUA+) for the OBDH total current consumption.