CD40 17 is a 5-bit Johnson calculator with 10 decoding output and CP, CR and INH inputs. The Schmitt trigger at the clock input has the function of pulse shaping, and there is no limit on the rising and falling time of the input clock pulse. When INH is low, the calculator counts at the rising edge of the clock; Conversely, the counting function is invalid. When CR is high, the counter is cleared. Johnson counter provides fast operation, 2-input decoding strobe and burr-free decoding output. Locking the door ensures the correct counting order. The decoded output is usually low level, and remains high level only during the corresponding clock cycle. The carry of the CO signal is completed once every 10 clock input cycle, which is used as the down pulse clock of the multistage counting chain.

CD40 17 is widely used as a counter, frequency divider and interlocking electronic switch. This picture is the most common usage of CD40 17, which uses the output of 40 17 to drive LED to emit light. 555 is an oscillation circuit, and its output pulse signal is used as the CP signal of 40 17, so that ten output terminals of 40 17 circularly output high-level driving LEDs to light up in turn. Pin 4 of 555 is the reset terminal. When the voltage of this pin is lower than 0.4V, the output pin (pin 3) is at a low level regardless of the state of pin 2.6. When the switch SB is closed, the four pins of 555 are grounded, the oscillation circuit stops oscillating, the CP pulse required by 40 17 is no longer output, and the running lamp stops circulating. Resistors R3 and C3 provide a stable high potential to terminal 4 to ensure the stable operation of the oscillator.