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The following PBASIC code fragment (taken from the complete program template shown later in this
section) can be used to read a single scan from the TSL1401-DB. It consists of five lines of code, which
are shown and discussed individually:
SHIFTOUT SI, CLK, 0, [1\1]
SI and CLK are defined in the larger program's preamble as PINs and connect to like-named ports on
the TSL1401-DB. This statement clocks out SI as a single bit of synchronous serial data, which starts a
new exposure interval.
PWM CLK, 128, 1
We want to clock through all 128 pixels as fast as possible, since we're just timing an exposure here, not
reading data. The PWM statement fits the bill perfectly. The 128 in this statement is the duty cycle
(50%), not the number of pulses. The number of pulses is given by the 1, which represents the length of
time to output the PWM signal. For the BS2, this is nominally 1mS. (Actually it's more like 1.2mS and
consists of about 150 cycles.)
PULSOUT SI, exp >> 1 MIN 1016 - 1016
This statement sets the exposure time. exp can be either a constant or a variable and represents the
length of the exposure in microseconds. The reason for using PULSOUT instead of PAUSE, say, is that
the timing resolution is so much finer. And the reason for sending the pulse on SI is that it's not used for
anything else durng this time, and, so long as we don't clock the pulse with CLK the sensor chip is
unaffected. The value subtracted from the pulse width (1016) represents the timing overhead from the
PBASIC program, minus the start-of-integration delay to the 18
means that the minimum exposure time will be about 2.032mS.
Note: Timings for BASIC Stamps other than the BS2 will vary, and the value subtracted will need to
be adjusted accordingly.
SHIFTOUT SI, CLK, 0, [1\1]
This clocks out the SI pulse again to end the exposure and begin actual data readout.
SHIFTIN AO, CLK, LSBPRE, [pdata(0)\16, pdata(1)\16, pdata(2)\16, pdata(3)\16]
SHIFTIN A0, CLK, LSBPRE, [pdata(4)\16, pdata(5)\16, pdata(6)\16, pdata(7)\16]
These two statements read 128 bits of thresholded pixel data from the AO pin into an eight-position
word array, declared pdata word(8), least-signficant bits first. Doing it "inline" like this is faster than
doing it in a loop.
Here are the waveforms from the above acquisition routine, with the various sections annotated:
IDLE
SHIFTOUT
PWMOUT
SI
CLK
1
Tri-
Stated
AO
© Bueno Systems, Inc. • TSL1401-DB (2009.10.01)
PULSOUT
129
150
Tri-
Stated
th
clock in the PWM statement. This
SHIFTOUT
SHIFTIN16 x 8
1
2 3 4 5 6 7 8 9 10
Thresholded Data
2V Input Threshold
. . .
129
Tri-
Stated
Page 6 of 52
IDLE
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