CSE466 Lab 2 - Section D (Extra Credit): “Light Meter” 

NOTE: If completed, this Extra Credit section is worth an additional 20% of the total possible points for your Lab2 grade. This Extra Credit section has an ESTIMATED completion time of approximately 2-3 hours.

Objective:

Create a dynamically calibrated light meter by reading the analog value coming from the photoresistor. The values may not swing all the way from 0 to 0xff, but you will calibrate it to match the light scale listed in Step 2. One of your buttons should toggle between the raw ADC output displayed in hex (8 MSBs), and the scaled Weston light meter units. The other button should toggle you through a state machine that allows you to calibrate a minimum light level and a maximum light level.

In this section you will learn the following:

• how to use the A/D converter including using the A/D complete interrupt
• how to scale data without using floating point math

Helpful Hints

• When interpolating, you usually use floating-point arithmetic to calculate intermediate values. Unfortunately, the ATmega16 does not have hardware support for floating point operations. gcc does provide software emulation of floating point, but it's extremely inefficient on this chip (1000+ cycles for a 16-bit divide). Because of this, you are not allowed to use floating point on this assignment. However, you can use fixed-point arithmetic since the numbers coming from the ADC are 10 bits wide, and there are 16-bit integers, you can left shift the values to get up to 6 bits of fractional precision. Remember that the Weston meter works on a logarithmic scale, as opposed to the linear scale output by the ADC. A look-up table might be a good idea here. You'll need more precision on one end of the scale than on the other.

            Scaling should be simple:

                ADC value - Min Value      Index of output value
                ------------------------------- = ------------------------------
                Range (Max - Min)              Size of Light Scale

            Do not use floating point math!

Suggested Steps:

1. Build a calibration state machine using two buttons.

Button 1 will be used for the calibration sequence.

• The first button push displays "LO" in hex display to indicate the system is ready to calibrate the low light point.
• The second button push captures the low calibration point, and displays "HI", indicating it’s ready to capture the high calibration point.
• The third push of the button captures the high calibration setting, and switches to calibrated display mode.

Button 2 toggles between raw data and calibrated/scaled data.

2. Implement a lookup table with the following values to be displayed on your 7-segment LED display. The values in this table should the only values displayed when in calibrated/scaled data. Values of the calibrated scale are:

0.1       Min. value

0.2

0.3

0.4

0.6

0.8

1.2

1.6

2.4

3.2

4.9

6.5

9.5

13

19

25

38

50

75

99        Max. value

3. Write a function to scale the ADC data bounded by the calibrated range (Step 1) to the table of values in Step 2. From the maximum and minimum values obtained during the calibration (Step 1) a range of reading values is established (possible examples are 5-200, 51-98, 110-114, 215-255, etc). A conversion should automatically occur to convert the raw ADC values and output the calibrated values in Step 2 based on the calibration information collected in Step 1.  Here are the specifics:

• The minimum value obtained during the calibration sequence should correspond to the minimum of the Weston light scale, which is 0.1.
• The maximum value obtained during the calibration sequence should correspond to the minimum of the Weston light scale, which is 99.
• Any value between the minimum and maximum should be scaled so that it corresponds to one of the 20 values listed. (See hint)
• If the raw ADC value goes higher than the maximum established in Step 1, it should still only display the max value. The values should also not go below the minimum.

4. If values are noisy, average 8 or 16 readings.

Deliverables:

1. Demonstrate your light meter to a TA.
• The user should be able to calibrate the light meter using the method listed in Step1. The user should then be able to toggle between raw hex values output by the ADC and the calibrated display based on the light values obtained during the calibration sequence. The values that should be displayed are listed in Step 2.
• The values should not roll over (overflow) when the meter saturates.
2. Turn in a hardcopy of your C code
• Do not use floating point arithmetic anywhere in your code.
• Keep your interrupt handlers small.