Several Maxim GPIO devices can detect signal-level changes and keep a record for the corresponding ports even if the signal has returned to its original level. For these devices either the rising or falling edge of a signal change can trigger the capture of the event. With some applications only the leading edge indicates a monitored event; the falling edge corresponds to the resolution of the event and should not draw the attention of the monitor. For these applications, detection of an edge-sensitive transition is needed.
The Maxim devices that detect signal transitions can be given edge sensitivity by using a few RC components and a Schmitt trigger device as shown in Figure 1. The RC circuit captures the first transition in the normal logic signal-level range and puts the following transition either above or below the range. The Schmitt trigger circuit subsequently filters out the event corresponding to the second transition. Although there are also two transitions at the output of the Schmitt trigger, the Maxim parts do not distinguish the changes since the duration is much shorter than an I²C-read command.
Figure 1. Circuit for edge-sensitive transition detection.
Several Maxim devices feature the transition detection feature and can be used with the circuit in Figure 1: MAX7302
, and MAX7323
. The Schmitt trigger devices that can be used in the circuit of Figure 1 are the single-buffer SN74LVC1G17 and dual-buffer SN74LVC2G17.