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provided by this command. These are the Threshold, the Hysteresis, and the Mode. These
coefficients are the same as described above in the "TSL1401-DB Monitor Program" section. The
sequence of bytes required by the Set Binary Coefficients command is:
Where SETBIN is a constant having the value $EC (mnemonic "enter coefficients").
Threshold is the value which determines whether an acquired pixel is a one (light) or a zero (dark).
Hysteresis is the width of the band above and below Threshold, which, in effect, creates two
thresholds: an upper and a lower. A pixel must be at least as high as the upper threshold to cause a
transistion from dark to light; and it must be lower than the lower threshold to cause a transistion from
light to dark. The separation between the two thresholds (the hysteresis band) is twice the value entered
for Hysteresis.
The Mode byte includes flags that determine the binary acquisition mode, along with the floating
threshold Filter value. It's format is:
Bit 7
Bit 6
0: Fixed
0: Level
1: Floating
1: Window
When FIXED thresholding is selected, each pixel is compared to the same, constant, upper and lower
thresholds, defined by Threshold and Hysteresis. The first pixel is compared to Threshold alone to
determine the initial state (0 or 1). After that, each pixel is compared with either Threshold +
Hysteresis, if the last comparison yielded a 0, or Threshold – Hysteresis, if the last comparison
yeilded a 1. Both Threshold and Hysteresis can range from 0 to 255. When their sum is greater than
255, 255 is used as the upper threshold. When their difference is less than 0, 0 is used as the lower
threshold.
When FLOATing thresholding is selected, the value given for Threshold is assumed to be a signed byte
that ranges from –128 ($80) to 127 ($7F). It is treated as an offset, which is added to an internal
floating parameter (Float) that changes, depending on the values of the pixels that preceded it. For the
first pixel, Float is simply assigned the value of that pixel. From there on out, Float is modified after
each pixel is processed, according to the formula:
Therefore, when Filter equals 0, Float takes on the value of the pixel itself. When Filter is a larger
number, Float becomes a moving average whose time constant increases as Filter increases. The new
value of Float is used to process the
Each pixel is then compared with Float + Threshold + Hysteresis, if the previous pixel evaluated to a
0, or with Float + Threshold – Hysteresis, if the previous pixel evaluated to a 1.
The preceding discussion assumes that Level thresholding is in effect. If Window thresholding is
selected instead, the upper and lower thresholds are computed as described above, but each pixel is
assigned a 0 if it's value lies between the two thresholds, and a 1 if it's value lies above the upper
threshold or below the lower one (i.e. outside the window formed by the hysteresis band).
© Bueno Systems, Inc. • TSL1401-DB (2009.10.01)
SETBIN, Threshold, Hysteresis, Mode
Bit 5
Bit 4
Reserved
Reserved
Float = Float + (Pixel - Float) / (1 >> Filter)
next
pixel.
Bit 3
Bit 2
Filter value for floating threshold:
Reserved
0 (no filtering) to 7 (maximum)
Bit 1
Bit 0
Page 28 of 52
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