Switch analog muxes Direct ADC’s to sample and convert Direct ADC’s to release converted data Store data in RAM

Process incoming asynchronous GPS data

Detect and synchronize with incoming GPS data Store data in RAM

Time stamp incoming data

Format data into packets

Send packets to transmitter and memory

Science and flight data packets transmitted "on-the-fly"

Science and flight data packets written to NVM in 512 byte blocks GPS data transmitted and at the same time stored in NVM

External Interfaces

The flight computer is broken up into two blocks, an analog-to-digital conversion (ADC) block, and a processing, storage, and communications (PSC) block. The ADC block’s external interfaces are all incoming analog signals. All other inputs and outputs interface to the PSC block. The PSC block contains the microcontroller, which handles all the processing, the CompactFlash, which is used to store all data that comes through the microcontroller, the serial communications hardware, which handles driving serial communications between the microcontroller and an outside computer, and a latch, which handles all incoming status switch signals.

Incoming Analog Signals

The ADC block is broken down into three sections, the 16-bit, -2.5 to +2.5 V section, the 16-bit, 0 to 5 V section, and the 12-bit, 0 to 5 V section. The three sections are named sixt2.5, sixt5, and twel5, respectively. Each section contains a different number of ADC component sets. Each set is composed of an analog multiplexer and an ADC. The only difference between sets from each section is a different ADC chip. The sixt2.5 section contains one ADC set with the ADC chip handling 16-bit conversion of -2.5 to +2.5 V input signals, with 8 channels available. The sixt5 section contains three ADC sets with the ADC chips handling 16-bit conversion of 0 to 5 V input signals, with 24 channels available. The twel5 section contains five ADC sets with the ADC chips handling 12-bit conversion of 0 to 5 V input signals, with 40 channels available.

Incoming Digital Signals

Incoming digital data consists of asynchronous GPS data and status switch data. The GPS data is 0 or 5 V level serial stream coming in on one line at 9600 baud. The status switch data comes in as 0 or 5 volt level signals into a latch on the flight computer. There are eight channels available on the latch.

Umbilical Communications

Umbilical communications consists of an RS-422 serial communications link between the flight computer and a PC in the block house. Umbilical communications is desired for prelaunch testing of the flight computer, data retrieval, and if necessary, for reprogramming the microcontroller. Four RS-422 lines are required through the umbilical plus one microcontroller flash programming control line.

Output to Transmitter

Output to the transmitter is a high speed, 0-1 V serial data stream through an SMA connected coaxial cable.

Power Connections

The flight computer requires power inputs of +5 V, +15 V, and –15 V.

Hardware

The following summarizes the major hardware components that are used in the flight computer.

MH68HC916Y1 Microcontroller

The HC16 controller is the brains of the flight computer, controlling the timing of the ADCs, formatting all data into packets, and storing and transmitting those packets. It is a 16-bit device operating at 16.78 MHz with 48 kB of flash EEPROM and 2 kB of RAM.

CompactFlash Nonvolatile Memory Card

The CompactFlash memory storage card is a popular memory card used in such products as PDA’s and digital cameras. It is available in sizes beginning at 4 MB to over 256 MB. With SRP-4’s expected data budget and flight time, it is expected 6 MB of storage space will be required. The CompactFlash requires 512 byte block transfers for each write cycle, thus the 512 byte packet format, as discussed in the software section.

ADS7815, ADS7820, and ADS7821 Analog-to-Digital Converters

The ADC’s are successive approximation converters with 250 microsecond sample-andconvert periods and 16-bit parallel output.