Metrologic Instruments IS4921 - Manual (Page 33)

27
Decode Timing
Engine image acquisition or decoding can occur from either the Idle Mode or the Sleep Mode. The process is
initiated by asserting the nTrig signal (or serial command when in the Idle Mode). Once the trigger signal is
received, the image sensor is reset and image acquisition begins. During image acquisition, the illumination
LEDs are enabled for a time determined by the FirstFlash circuitry on the non-decode engine. The image is
then transferred to the processor and decoded. Upon decoding the image, the processor asserts the
nGoodRead signal (low) and beings transmitting the decoded data. When the decode engine receives a trigger
signal while in the Sleep Mode, an additional delay is needed for the processor exit Sleep Mode and
reconfigures the sensor.
Figure 25 and Figure 26 show the amount of time required for decoding when a nTrig signal is asserted in both
the Idle Mode and Sleep Mode.
Notes: The total image acquisition / decode time can be approximated by measuring the time from the nTrig
signal going low to the nGoodRead signal going low. This time will vary slightly based on several
factors including code quality, code type, and distance from the engine. The following waveforms
show a typical condition.
The nTrig signal must be kept low for at least 20msec.
Figure 25. Decode time after receiving nTrig signal in Idle Mode.
Figure 26. Decode time after receiving nTrig signal in Sleep Mode.