Issue 98, September 1998
Smart Rockets - Data Acquisition in Model Rocketry

SOFTWARE

The code was written in PIC assembly language. We used Microchip’s MPASM assembler and PICStart 16C programmer as development tools. A block diagram of the software is shown in Figure 4.

On powerup or after coming out of a reset, the software initializes the I/O facilities and internal resources of the PIC (DIO, ADC, SPI interface, USART, and Timer 0). The USART operates at 9600 bps, and the SPI clock runs at 1 MHz.

The program then polls RB0, which is normally high. A trigger signal pulls RB0 low and causes the program to check the mode and execute the appropriate code. After the selected mode executes, the program loops back to polling RB0 for another trigger signal (see Figure 4a).

Mode A is the real-time data-acquisition portion of the program. It is entered if pin RB0 is jumpered low. The ADC is read during an interrupt service routine that is governed by Timer 0, which provides an interrupt every millisecond, thus setting the data-acquisition rate (see Figure 4b).

How data is acquired is dictated by the behavior of the Xicor 25F128 serial flash-memory chip. It receives data in 256-bit packets via its SPI interface and stores it in one 32-byte sector of its flash memory.

Transferring 256 bits takes only about a quarter of a millisecond at an SPI clock rate of 1 MHz. The programming operation, however, takes 10 ms. Fortunately, this occurs automatically after the bits are received and it requires no further intervention of the PIC.

The program in mode A reads the ADC 32 times at a 1-kHz rate, storing these values in 32 bytes of buffer RAM on the PIC chip. Between the thirty-second and thirty-third A/D conversions, it shoots the contents of the buffer to the Xicor chip through the SPI.

It then proceeds to fill the buffer RAM again. During the 32 ms that the buffer is being refilled, the Xicor chip has plenty of time to write the data it just received into its flash memory.

In addition to sending acceleration data, the PIC sends the sector address that indicates where to store the data in the Xicor chip. The sector address is incremented after every SPI transfer.

This cycle repeats itself 256 times until 8 KB of data are stored on the Xicor chip. The program then ends mode A and goes back to polling the trigger pin.

The Xicor chip holds up to 16 KB of data. But, we found that 8 KB of data, corresponding to 8 s of time, was enough to capture a rocket flight from launch to recovery phase.

Triggering the system in mode B presumes that a host computer is connected to the USART of the PIC via the interface box. The PIC reads a single byte from the Xicor chip, converts it to a three-byte ASCII decimal number, and transmits it to the host computer (see Figure 4a).

Each transmitted number is appended with the ASCII codes for carriage return ($0D) and line feed ($0A). This cycle repeats itself until all 8 KB of data are transmitted to the host.

The user must set the host computer to accept this ASCII stream before the PIC is triggered into mode B. There is no handshaking between the PIC and the host. As with mode A, on termination of mode B, the program loops back to polling the trigger input.

In the field, we used a laptop, running a terminal emulator set in screen-capture mode, as the host computer. The captured file is a 1-column ? 8192-row matrix of ASCII decimal numbers.

As soon as the rocket was recovered and the data uploaded to a laptop, we plotted the data to ensure that the system worked properly. Thus, we could correct problems on site, instead of discovering them back in the lab. Being able to read the data in the field using a simple editor more than offset the extra time needed to transfer ASCII as opposed to binary data.