An Evaluation System for the TSL2550 Ambient Light Sensor The TSL2550 contains two ADC registers (channel 0 and channel 1). Each of the two ADC value registers contains seven bits. Bits 6 through 0 contain the 7-bit code representing the ADC count value that is proportional to the photodetector current. Each ADC register contains two components to determine the logarithmic ADC count value: “Chord” bits and “Step” bits. The Chord bits correspond to the most significant portion of the ADC value and specifies a segment of the piece-wise linear approximation to a log function. The Step bits correspond to the least significant portion of the ADC count value and specify a linear value within a segment. The transfer function is broken into 8 Chords of 16 Steps each. This function is very similar to mu-law encoding used in audio compression and differs in that it does not have a sign bit and it is not inverted. When Chord and Step bits all equal to 0, this condition corresponds to a light level that is below the detection limit of the sensor. When Chord and Step bits all equal to 1, this condition corresponds to an overflow (i.e. saturation). Tables 1 and 2 show the relationship between the Chord and Step bits and the Chord and Step numbers and values. Chord Bits (B6, B5, B4) Chord Number Chord Value Step Value 000 0 0 1 001 1 16 2 010 2 49 34 011 3 115 8 100 4 247 16 101 5 511 32 110 6 1039 64 111 7 2095 128 Table 1. Chord Numbers and Values Step Bits (B3, B2, B1, B0) Step Number 0000 0 0001 1 0010 2 0011 3 0100 4 0101 5 0110 6 0111 7 1000 8 1001 9 1010 10 1011 11 1100 12 1101 13 1110 14 1111 15 Table 2. Step Numbers and Values
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