multiplexing devices which do not have chip select

[Shishir K Chakravarthy]

Hi all, need some help here

Say I have a microcontroller for an application, and only two IO pins are remaining. I have four other IC’s that don’t have CS ( Chip select ). Now I need to select one chip at a time using those two pins. One idea that came to my mind is a 2 to 4 decoder. Now my doubt is that if I can select one of these four chips by connecting the outputs of the decoder to power supply? ( Refer diagram )

 

If yes, then can I use IC74139 ( 2:4 decoder ), which is active low to power the gnd pin? Also, what happens to an IC if +5V is given to both the VCC and GND pins? What happens if VCC and GND pins are supplied with 0V?

 

Diagram:

 

 

Thanks in advance

 

[Ashutosh Bhatt]

your question is very silly.

you can not short both biasing pins Vcc and GND. they are proviede for internal circuit functioning. 

 

which 4 ICs you want to select using this decoder chip.

do these 4 chips have any pins like Enable, reset etc

 

if you want active high outputs to select chips then use other 2 to 4 decoder chip that give active high output

[Shishir K Chakravarthy]

@A M Bhatt, Can u then suggest a way to power only one of the four devices at a time, so that when I give 00 on the input, device 1 must be powered, 01, device 2 must be powered and so on…The devices in question are RF link transmitters and receivers ( 433Tx, 433Rx, 315Tx, 315Rx ). At a given point of time i need to power only one of the modules. They dont have EN, etc pins. Please forgive me for my ambuigity in the earlier post.

[AJISH ALFRED]

I understand this as single transmitter – multiple receiver configuration. A simple method is to use HT12 encoder-decoder IC.

http://www.engineersgarage.com/electronic-circuits/wireless-rf-remote-control-circuit#tabset-tab-3

 

The receiver will response only if the address bit matches. 

[Shishir K Chakravarthy]

@Ajeesh: I understand that, and have implemented it too before and even in this project too, 8 address lines and 4 data lines, that has been taken care of. The thing is that I need only one of the 4 units working at a time so that they all dont load the antenna at the same time. I want them working cyclically, which is why I would require powering the 4 units on and off separately. The address and data bus run through both HT12E and HT12D. I need to find a way to send/receive the serial data to/from the encoder/decoder to specific RF modules, not all at a time.

[Ashutosh Bhatt]

in that case you have to use 2 to 4 decoder/demux.

but one more addition is you have to switch on/off relays using output lines of this dec/demux

and from relay contact you give Vcc supply to your transmitters.

[AJISH ALFRED]

Hi Shishir,

“I want them working cyclically, which is why I would require powering the 4 units on and off separately”

 

Is it possible to use CD4017 ring counter to cycle the power switch? 

 

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