Measurement Computing USB-QUAD08 - Manual (Page 22)

USB-QUAD08 User's Guide Functional Details
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Asynchronous scanning
When read asynchronously, counters can be configured so that they get set to 0 after each read, count up or
down repeatedly, or count until the 16, 32, 48-bit, or a user-set limit has been reached. Refer to the counter
mode descriptions below.
Debounce mode
The USB-QUAD08 has debounce circuitry which eliminates switch-induced transients that are typically
associated with electro-mechanical devices including relays, proximity switches, and encoders.
All debounce options are software selectable. You can select a debounce time, debounce mode, and rising-edge
or falling-edge sensitivity. Each channel can be debounced with 16 programmable debounce times in the range
of 500 ns to 25.5 ms.
Two debounce modes (trigger after stable and trigger before stable) and a debounce bypass are shown in
Figure 17. The signal from the buffer can be inverted before it enters the debounce circuitry. The inverter is
used to make the input rising-edge or falling-edge sensitive.
Figure 17. Debounce block diagram
Edge selection is available with or without debounce. In this case, the debounce time setting is ignored and the
input signal goes straight from the inverter or inverter bypass to the counter module.
The two debounce modes are trigger after stable and trigger before stable. In either mode, the selected
debounce time determines how fast the signal can change and still be recognized.
Trigger after stable mode
In the trigger after stable mode, the output of the debounce module does not change state until a period of
stability has been achieved. This means that the input has an edge, and then must be stable for a period of time
equal to the debounce time. Refer to Figure 18.
Figure 18. Trigger after stable mode
T1 through T5 indicate time periods. In trigger after stable mode, the input signal to the debounce module is
required to have a period of stability after an incoming edge, in order for that edge to be accepted (passed
through to the counter module.) For this example, the debounce time is equal to T2 and T5.
 T1 – In Figure 18, the input signal goes high at the beginning of time period T1, but never stays high for a
period of time equal to the debounce time setting (equal to T2 for this example.)