Output expansion concept using 74HC595

Hi,
I am running out of I/O pins in my microcontroller. But i need to control 24 relays. Basically i am switching the relays using ULN2803A. So i thought of using 74HC595 shift register IC to trigger the bases of ULN. Any relay can be triggered at any time say one at a time, two at a same time and upto 24 at a same time.

My doubt while using this shift register is when i want to output new value, the bits which had a state same as current state will experience a momentary fluctuation or not?



For example, In my circuit diagram, when current output state is as follows
U5(QhQgQfQeQdQcQbQa) U6(QhQgQfQeQdQcQbQa) U7(QhQgQfQeQdQcQbQa) = 00001101 01000011 00101010

and i want to give the new value as 00001001 01000111 00100010.

Here i changed the 3 bits out of 18 bits. But other 15 bits are to be maintained at the same state. Will these 15 bits experience any fluctuation or stay solid?

Thanks,
Murugesh

Murugesh_89 said:

My doubt while using this shift register is when i want to output new value, the bits which had a state same as current state will experience a momentary fluctuation or not?

Click to expand...

Not.

The 74HC595 is equipped with a storage register and 3-state outputs, the storage register is controller by Register Clock (RCLK) or Storage Register Clock Input (SCTP) line, depending on the manufacturer.

While a new state value is being shifted into the device the RCLK/SCTP is held low, once the new state value is in position the RCLK/SCTP line is pulled high storing the new value state with the storage register.

The Master Reset (MR) or Storage Register Clear (SRCLR) line, once again depending on the manufacturer, can also be utilized to initialization the device into a known cleared state.

Murugesh_89 said:

Here i changed the 3 bits out of 18 bits. But other 15 bits are to be maintained at the same state. Will these 15 bits experience any fluctuation or stay solid?

Click to expand...

No, the 15 bits which remain unchanged should not experience any fluctuation of state.



Several 74HC595 can be easily cascaded by connecting the QH line of the first 74HC595 to the SER of the second 74HC595, and so on.

Using this technique several 74HC595 can be cascaded while only requiring three lines from the microcontroller.

The following tutorial demonstrates many of the features of the 74HC595, although the tutorial employs an Arduino, the techniques can be easily ported to any microcontroller.

Can you move over? The 74HC595 8 bit shift register



TI SN54HC595/SN74HC595 Datasheet


BigDog

Thanks BigDog.