Cascaded Timers For Long Time Delays

Two uA2240 timers may be cascaded to provide long time delays as shown in Figure 7-43. Each uA2240 counter consists of a time-base oscillator, an eight-bit counter, a control flip-flop, and a voltage regulator. The frequency of the time-base oscillator (TBO) is set by the time constant of an external resistor and capacitor.

The open-collector output of the TBO drives the internal eight-bit counter if the TBO output is connected to the regulator output via a pull-up resistor. Otherwise, an external source can be connected to the TBO terminal to supply the input to the eight-bit counter. The open-collector counter outputs Q0 through Q7 provide a “wired-OR” function. The combined output is low if any of the connected counter outputs is low.

The trigger and reset inputs to the internal control flip-flop determine the uA2240 operational state. Once the positive trigger pulse starts the TBO and resets all counter outputs to a low level, further trigger signals are ignored until a reset pulse is received. The reset input inhibits the TBO output and sets all counter outputs to a high level.

In this application, the TBO frequency of U1 is set at 2 kHz by the time constant of R1 and C1. This provides a circuit time-base period of 0.5 ms that drives the internal eight-bit counter of U1.

Figure 7-43. Cascaded Operation for Long Delays