SEDSAT-2 CDH Design Notes 20080422

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CDH Design Review II

Current state of development

Due to exams and end-of-term projects the prototype will delayed, but this is the next step in the development. We've also gotten a new member in our subsystem group, Indian student Smriti Jha.

Progress since Design Review I

  1. Established how the microcontrollers will switch control between each other
  2. Started work on system state diagrams
  3. Worked on PCB design

MCU Coupling

For added redundancy we are designing our CDH subsystem to include two microcontrollers so that if one fails there will be a backup controller to take over. The way this is going to happen is as follows;

  1. The system starts up when powered on
  2. The master checks the Q output of the SR-gate. If 0 it will shift a 4-bit sequence to the logic unit to set the SR gate.
  3. The slave will poll the master trough a SPI link for a healthy response. If not recieved within a given timeframe and it fails to give a response a given amount of times - the slave will set the SR gate to 0 by shifting a 4-bit sequence to the logic unit.
  4. The slave will check the not Q output of the SR-gate. If the output is 1 it will know it's in control and will operate on a normal basis.

The output of the SR gate will decide which MCU is in control by redirecting the USART and payload memory signals. So if one MCU malfunctions it will not be able to distrupt the other MCU's communication.

Communication Model

Before CDH starts transmitting data over USART it has to send a message to Comms by I2C telling them if the data is important or not. If the data is important Comms will buffer the data and not delete it until it has been confirmed received by a ground station. If the data is not important it will just be sent. Typical non-important data will be housekeeping data. (Housekeeping data is status data collected by CDH from the different subsystems and sent every ~30 seconds or so to ground. Housekeeping data packets will be numbered and stored onboard so ground can request spesific packets to analyse the state of the cube at some spesific point in time.) Typical important data will be from the payload, ie: when we are downloading an image. The USART communication between Comms and CDH will be controlled by hardware handshaking signals (Request To Send / Clear To Send). So whenever CDH wants to transmit something it will set the RTS signal and wait for a CTS before transmitting. This way file downloads will function seamlessly between passes.

Preliminary Figures

  1. Operating voltage: 3.3 VDC
  2. USART Interface to Comms: RS-232 style @ 3.3 VDC w/ hardware handshaking
  3. Control interface to other subsystems: Philips I2C with CDH as Master
  4. Power Consumption: 80 mW
  5. Mass: 50 gr
  6. Dimensions: Unknown
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