The uA2240 programmable timer/counter may be operated by an external clock as shown in Figure 7-44. The internal time-base oscillator is disabled by connecting the trigger input (pin 13) to ground through a 1 kΩ resistor. An external clock is applied to the time-base output
(TBO), pin 14. For proper operation, the minimum clock amplitude and pulse-duration are 3 V and 1 µS respectively.
In this application the uA2240 consists of an eight bit counter, a control flip-flop, and a voltage regulator. The external clock triggers the eight-bit counter on the negative-going edge of the clock pulse. The open-collector counter outputs are connected together to the 5.1 kΩ pull-up resistor to provide a “wired-OR” function where the combined output is low if anyone of the outputs is low.
This arrangement provides time delays or frequency outputs that have a period equal to integer multiples of the external clock time-base period. In the astable mode, the uA2240 will free-run from the time it is triggered until it receives an external reset signal. If the monostable mode is selected, one or more of the counter outputs is connected to the reset terminal, and provides the reset at the end of the pulse delay period. For operation with a supply voltage of 6 V or less, the internal time-base can be powered down by open-circuiting pin 16 and connecting pin 15 to VCC. In this configuration, the internal time-base does not draw any current, and the overall current drain is reduced by approximately 3 mA.