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Low Power Data Logger

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					Low Power Data Logger

              By
       Jedidiah McClurg
         David Simon
        Amber Thiesen
        Brian Washburn
     Customer Requirements
 Professor Kruger
 Records an analog signal for a month to a
  single SD card
 Stored using FAT32/FAT16
 Extract a low frequency signal from noisy
  data
 18-bit A/D precision
 Low Power
               Objective
 Design a data logger which receives data
  through an RS232 port
 Employ a microcontroller to manage data
  input, conversion, and recording
 Sample the data at a regular intervals and
  use a 24-bit A/D converter to digitize it
 Write the results to an SD card using
  FAT32 formatting
                 Block Diagram
 µC: 18LF4520
 A/D: AD7714
       24-bit and
        20Hz
   RS232:
    ADM3222
   µC: 18LF452
   All devices have low-power or shut-off modes
             Data Loggers
 Most data loggers run at 1Hz or less
 Prices vary between $30 and $200
 Applications include wind speed, soil
  moisture, road traffic, or temperature
 Memory is a must
 Loggers are not for use in real-time
  systems
      Microcontroller Selection
     We searched for a microcontroller with the following
     characteristics…

   Simple/familiar architecture
   Hardware SPI module and USART
   Reasonable amount of data RAM for buffering
   Enough program memory to store our firmware
   Low power consumption



           Microchip
           PIC18LF4520
      A/D Converter Selection
    We needed an analog-to-digital converter which met the
    following requirements…

   Low power consumption
   A single-supply configuration (operating at
    3.0V)
   High precision
   SPI interface


       Analog Devices
       AD7714
             Hardware Setup
   ADM3222: low-
    power RS-232 line
    driver
   SD card directly
    connected to the
    PIC’s SPI bus
    Firmware Development
The software was built and tested using the
following tools…

   Microchip’s MPLAB IDE
   Microchip’s C18 compiler
   Microchip’s PICkit 2 in-circuit device
    programmer
   Oscilloscope
   Variable power supplies
        SD Card Software
We designed a set of routines to communicate
with the SD card.

    Card sizes up to 2 GB are supported
    All types of FAT formatting (FAT32, FAT16,
     etc.) are supported

Card reads and writes happen one-sector-at-a-
time (512 bytes), and these typically take 15ms
per sector.
                Data Rates
   A/D sampled at a rate of 40 Hz
       Gives the FAT software uninterrupted 25ms
        blocks of time

   A/D has programmable notch filter (digital
    low-pass)
       First notch set to 100 Hz
       This sets bandwidth to 26.2 Hz
       Maximum possible bandwidth while retaining
        18-bit resolution
                A/D Settings

   A/D reference voltage is 1.235V
       Supplied by a precision voltage reference
   A/D internal gain is set to 16
       This gives a full-scale range of 154 mV (-77
        mV to +77 mV)
Social/Environmental Concerns
   Green Technology
       Low Power Consumption
       Higher initial cost Vs. lower cost of use


   Accuracy
       Real Time Systems
    Testing Power Requirements
   Current Measurements
     Static current consumption is about 3 mA, as
     desired
     Measured using an oscilloscope and a 1-ohm
     current-sense resistor in series with one of the
     power wires
                Testing Accuracy
We monitored the following 1 Hz audio signal with the data logger, and
achieved the accurate digital representation shown in the lower graph.
                     Results
   Met customer’s design specifications
       Low-power operation
       Capable of logging data to an SD card for a
        month
       18-bit effective A/D
        precision captures
        low-freq. signals
       Accurate
                            Price/Parts List
Name                Mfgr.              Descrip.                                Price     Num
AD7714ANZ-3         Analog Devices     IC ADC 24BIT SIGMA-DELTA 24-DIP           17.06     1
PIC18LF4520-I/P     Microchip          IC MCU FLASH 16KX16 40DIP                  8.30     1
DM1AA-SF-PEJ(21)    Hirose             CONN MEMORY SECURE DIGITAL SMD             4.67     1
NH15VP              Energizer          BATT NIMH AA 1.2V 2500MAH BULK             4.08     4
ADM3222ANZ          Analog Devices     IC TXRX RS232 3.3V W/SD 18-DIP             2.54     1
5227169-7           Tyco               CONN JACK BNC VERT 50 OHM PCB AU           2.10     1
164X11769X          Conec              CONN DSUB 9POS RECEPT CRIMP                1.73     1
2477                Keystone           HOLDER BATTERY 4CELL AA PC MNT             1.43     1
FOXLF024S           Fox                CRYSTAL 2.4576 MHZ 32PF                    1.09     1
                    National
LP2950CZ-3.0/NOPB   Semiconductor      IC VREG 3V MICRPWR TO-92                   1.04     1
ECS-40-20-4XDN      ECS                CRYSTAL 4.000 MHZ 20PF 49US                0.80     1
LM385Z-1.2G         ON Semiconductor   DIODE REG MICROPWR 1.235V TO-92            0.75     1


                                                                        Total = $57.83
              PCB Design
   Compact layout increases portability
   Protective device housing
   Would add to
    current costs
      Possible Improvements
 Complete PCB assembly
 Use batteries
 Write PC software to collect/analyze data
  from SD card
Questions

				
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posted:1/23/2011
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