November 20, 1997

Reset ICs create sequenced or long resets

Doug Anderson, Electronic Technology Corp, Ames, IA

Reset-generator ICs offer an inexpensive way to generate reliable reset signals for µC and µP applications. Certain applications, such as multiple-processor systems, require multiple resets that occur in a predictable sequence. You can use multiple-reset generators together to address the needs of these applications (Figure 1). The Reset output of IC₂ connects to the MR input of IC₂. In this configuration, the reset time-out period of IC₂ does not begin until the reset time-out period of IC₁ is complete, thus creating sequenced resets. Also, the Reset output of IC₂ now has a time-out period that is twice as long as that of a single reset generator.

You can similarly connect additional reset generators in sequence to create additional sequenced reset signals or even longer reset time-out periods. The first reset generator in the sequence is the master. You select it to have a higher reset threshold (4.63V for the ETC811L) than those of the slave reset generators (4.38V for the ETC811M). As the supply voltage increases during power-up, it reaches the reset threshold of the slave devices first (Figure 2). At this point, however, the supply voltage does not exceed the reset voltage of the master. Thus, the master holds Reset low and forces all the slaves to remain reset.

As the supply voltage increases further, it exceeds the threshold of the master, and the sequence of resets begins. In a similar fashion, as the voltage decreases during power-down, it first reaches the master's reset threshold. The master brings its Reset low, forcing all the slaves' Reset pins low in rapid succession. If the application requires a manual reset, you can attach a pushbutton switch to the MR pin on the master. The MR pin has an internal pullup to V_CC. When the switch closes, the Reset pins go high in the same sequence as upon power-up. Reset generators are available with other reset thresholds (3.08V for the ETC811Y, 2.93V for the ETC811S, and 2.63V for the ETC811R), so you can use this technique in 3V systems as well. (DI #2112)

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