The High Altitude Balloon Experiments in Technology (HABET) program is coordinated by Dr. John P. Basart and is funded by the Iowa Space Grant Consortium (ISGC). The ISGC is a group of Iowa universities that receive research funding from the National Aeronautics and Space Administration (NASA).
In accordance with NASAs efforts to provide funding for innovative projects with a relevance to NASAs mission and in conjunction with their creedo of " faster, cheaper, better...", HABETs goal is to continually perform various experiments at altitudes approaching 100,000 feet. Since each HABET flight is unique in its goals and implementation, a highly adaptive systems design has been implemented for gathering data and command-and-control of the spacecraft. One of the objectives is to be able to capture in-flight video. To accomplish this, HABET has decided to implement a device to capture large amounts of data so that video (or any other types of data) can be stored on board the spacecraft. They currently have no such capability.
The Expandable Memory System (EMS) team has been tasked with designing a high-capacity, expandable memory system that can be versatile enough to capture any kind of serial data that HABET would wish to store on-board their spacecraft.
HABET has specified the following guidelines pertaining to the design of this memory system. First, the new memory board must match the design of their current system for their Heavy Spacecraft Bus (HSB-1). This means that the circuit board design must adhere to a physical size of 4 inches by 6 inches to fit into their passive backplane design for the rest of their circuitry. It will also have to have a 50-pin connector to mate with the system bus. Second, the memory must be expandable to meet varying demands of different flight requirements. This means that some flights may use more memory than others. Since weight is an issue for any HABET mission, the minimum amount of hardware is always flown. Third, the circuitry and memory system must be able to withstand the extreme cold temperature at 100,000 feet of altitude and be sturdy enough to handle all of the jarring and buffeting of the spacecraft during a normal flight.
The EMS circuitry will consist of two main components: the memory cells and the control unit with its own chip decode logic. Each of these components have been thoroughly investigated by the EMS team. There are many options available for use, all of which have their good and bad points. Analysis of the memory components and controller components are listed in the following paragraphs of this section.
A number of solutions for mass data storage are available for general purpose use. Many of these items were considered and evaluated for use by this project. Some of these are evaluated below.
There are many microcontroller/microprocessors available for use as the control unit. The main limiting factor is the number of input/output (I/O) lines available for addressing and control of individual memory chips. To be able to address a large amount of memory, as many as twenty eight address lines might be needed. This would give an addressing capability of approximately 250 Megabytes. The microcontroller would also need to have some individual control lines for reading and writing control to the memory chips and possibly some others that may be needed by the particular Flash Memory ICs that are being used. The microcontroller would also need to have a serial port available to receive data and then eight additional lines for data transmission to the memory cells. The total number of pins that will be needed will be nearly 40.
The EMS Circuit Block diagram is shown in Figure 1. The 50-pin main bus connector is shown on the left side of the diagram. This connector will allow the EMS board to connect to HABETs HSB through its passive backplane. The board will be supplied with the necessary power through this connection. It will also be used for the serial transmission of data from any of the other components onboard the HSB. The PIC 17C756 and its chip select logic is shown next. The microcontroller is described in the previous section. The chip select logic allows the EMS team to expand the number of Flash Memory cells available for data storage. The chip selector will be a Programmable Electrically Erasable Logic (PEEL) device. Since each Flash Memory chip needs to have one of its inputs pulled high for "chip select" before it can store a byte of data, the EMS team decided to use a PEEL for decoding which cell was selected. The last five (most significant) bits of the address are used as the chip select logic. This gives the EMS board a total capacity of 32 Flash Memory cells for data storage. The center portion of Figure 1 shows the Flash Memory cells. There is enough space for eight on the main circuit board. The other Flash Memory cells will be installed on a daughter card that connects to the EMS board via the 50 pin connector on the right side of the diagram.
The Daughter Card for the EMS memory board will be designed to mate directly on top of the EMS board. It will connect through the Memory Expansion Card Connector interface on the right side of the EMS block diagram. The daughter card will consist of a smaller circuit card that only has the Flash Memory ICs and a PEEL for the Chip Select Decoding. It will have the capacity for as many as 24 Flash Memory ICs. This will allow the EMS Board to have as much as 64 Megabytes of Flash Memory.
There are two main areas of software design for the EMS project. First, the software that must developed for the microcontroller to allow it to reliably store data into the Flash Memory cells. This programming will be done in C with mixed Assembly language and is currently under development by the team. Second, the software that will allow HABET to interface the EMS board directly with a PC computer to download the data that has been stored for a given mission. This will consist of a Graphical User Interface (GUI) of some sort that will guide the user through the process of downloading data from the EMS board.
Though a good start has been made on this project in the form of research of basic components, the real construction of the EMS board will take place next semester. The goal of the EMS team is to have a functioning EMS main circuit board by the end of April 1999. Some of the work to be completed is detailed in the following paragraphs.
Layout and design of the EMS circuit board will begin in January. Preliminary layout of the components has been completed. Circuit traces for the individual bus lines will be added. Then the circuit board will be etched by the EMS team in HABETs workshop. The actual circuit will be laid out by MicroGraphix Designer software. After etching, the individual components will be mounted and soldered onto the circuit board. This will be Mr. Cooks primary responsibility for Spring 99.
The software development will begin at the same time as the circuit board manufacture. A Microchip PIC-specific C compiler will be used for coding the microcontroller. The microcontroller will have basic read, write, erase, verify, and test functions available through the PICs serial interface. Borland C/C++ will be used for coding the PC interface of the EMS board. The PC will have a GUI interface to download, upload, test, and erase the contents of the EMS memory cells. The PIC code will be the primary responsibility of Mr. Hermanto and the GUI will be the responsibility of Mr. Porter.
The testing and debugging will involve several steps. The first step will be to test the basic functioning of the EMS board. The board will be visually and then electrically inspected. The second step will be to create low-level code to test basic functioning of the memory, addressing, and data bus. The third step will be the testing of each individual function in the software. Fourth, the entire system will be tested in the lab. Finally, the EMS board will be flown on a HABET mission to capture telemetry data that will also be sent back to the ground via HABETs radio telemetry receiver station. After recovery of the spacecraft, the two sets of data will be compared for any errors.
An attached budget of human effort and material costs is attached. These budgets are current through 8 December 1998. HABET is providing the additional funding for the hardware purchases of this design project.
An attached Gantt chart shows the proposed schedule of work throughout the life of this project. Included are all design and implementation phases, along with classwork. The chart has been updated to include completion of all work through 8 December 1998.