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FIGURE 15.1 Using a PC and an external data acquisition system for recording data.
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DESIGN DESCRIPTION FOR THE SNIFFSTICK 257
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FIGURE 15.2 An autonomous data acquisition system. 1. Small size with minimum power consumption.
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Keeping in mind the limited I/O offered by single-chip microcontrollers, we have gone in for components that have simple serial interfaces. Usually, memory devices such as EEPROMs are available with the two-wire I2C interface, while ADCs are available with the SPI/Microwire (which requires three wires) interface. It would be nice to have ADCs with IIC interfaces as well, but unfortunately, there aren t many ADCs with IIC interfaces.
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15.3 Design Description for the SniffStick
Figure 15.3 illustrates the block diagram of SniffStick, our low-power data acquisition system. It uses an AT90S2343 (or Tiny22) processor in an internal RC oscillator mode. It takes all five I/O port lines from the controller to connect all the peripherals. A docking port with connection to the controller reset is used to configure the system prior to data acquisition, and after the data is acquired, the docking port is used to read out the data. The docking port is connected to the PC parallel port. The PC parallel port holds the controller under reset condition and takes control of the SDA and SCL lines of the AT24C512 serial EEPROM and reads out the entire memory (Figure 15.4). The operation of the SniffStick is best understood by breaking up the design in three parts:
1. Interface to the ADC 2. Interface to the EEPROM 3. Interface to the docking port
258 AVR PROJECT 6: ASTRODAT: A STAND-ALONE DATA ACQUISITION SYSTEM
(Controller) AT90S2343/Tiny22 RST
(Memory) AT24C512
(ADC) MAX187
Analog Input
Docking Port ACQUIRE
Docking Port is connected to the Host PC for configuring the SniffStick as well as for data readout
FIGURE 15.3 Block diagram of SniffStick.
100 Ohm C0 74HCT244 D0 SDA Reset
C1 74HCT244 S7 Signals on SniffStick DAS
Parallel Port Signals
Dout SCL Gnd Vcc
Gnd (To power the 74HCT244)
FIGURE 15.4 Block diagram of the PC parallel-port-based docking port for the SniffStick DAS.
DESIGN DESCRIPTION FOR THE SNIFFSTICK 259
Let us see how this design works. The ADC I have chosen for this system is a tiny 8pin, 12-bit ADC MAX187 from Maxim. The ADC performs a 12-bit conversion in about 10 s. The ADC offers a serial three-wire SPI/MICROWIRE interface. We have connected three I/O lines from our processor to the ADC. One of the signal lines to the ADC, the CS* line, has been shared with the SCL signal of the EEPROM. The serial EEPROM has an IIC interface and thus requires two I/O lines from the processor, one of the signals being shared with the CS* signal of the ADC. This arrangement is possible as the shared signal between the ADC and the EEPROM requires complementary logic signals for each device, i.e., when this signal line is 1, the EEPROM is active and the ADC is inactive, and when this signal is 0, the EEPROM is inactive (provided that the SDA signal is held to 1 by the processor) but the ADC gets selected. A switch is connected to the available I/O line for triggering an acquisition cycle. Figure 15.5 illustrates the circuit schematic for the SniffStick, and Figure 15.6 is a photograph of the SniffStick circuit board.
FIGURE 15.5 Circuit schematic for the SniffStick.
260 AVR PROJECT 6: ASTRODAT: A STAND-ALONE DATA ACQUISITION SYSTEM
FIGURE 15.6 Photograph of the SniffStick under fabrication.
15.4 Using the SniffStick
How is the SniffStick intended to be used All data acquisitions need to acquire a data at a certain rate. The processor for SniffStick is clocked at the internal RC oscillator clock, which has a nominal value of 1 MHz at 5 V The EEPROM can only be written a byte every . 10 ms unless one writes the EEPROM in page mode in which a whole page of memory can be written in 10 ms, which could increase the average byte write speed. The SniffStick is configured in such a way that the EEPROM is not only used to store the acquisition data but also the information about the data acquisition rate. I choose to reserve the first two locations in the EEPROM (at address $0000 hex and $0001 hex) for an acquisition time multiplier. This 16-bit number is used to multiply the basic rate of 10 ms to achieve any desired rate. Let us see how this works. Suppose you want to acquire data at the rate of 200 ms, then (TimeMultiplier) * 10ms therefore, TimeMultiplier 20 14 (hex) : 200ms
Thus the first two bytes in the EEPROM are stored with the following numbers: Address $00: Data $00 Address $01: Data $14 These two numbers are programmed into the EEPROM through the docking port at the time of system configuration at the base station. After that, the SniffStick can be ported to the field and used rightaway. To start acquisition, the Acquire key is pressed. The program will then read the first two locations of the EEPROM to determine the acquisition rate and program the internal timer appropriately. After that the data from the ADC is acquired and stored in the EEPROM at locations starting at address $0002. Each sample is two bytes
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