Decoder logic chip recomendations?

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
I need to select 4 chips with a low latch enable.

I am using a Pic 18f14k22 and I don't have enough outputs to select all 4 chips with individual pins.

On had I have a 74LS155 which would do the job but I would be working at it's supply voltage limits of 4.5V. My supply will be 3 AAA cells in series for 4.5V.

Can someone recommend a decoder that would work?

Since I have an mcu, can this be down some other way? Perhaps borrow from the idea of Charlieplexing and create my own decoder with some diodes?
 

MMcLaren

Joined Feb 14, 2010
853
Hi Spin',

Isn't the 18F14K22 a twenty pin chip? If so, what are you doin' with all those pins?

Since you're thinking about adding an IC at this point anyway, why not check to see if an additional IC could be used to save even more I/O pins. For example, if you're currently using ten pins to drive (A) an LCD display, (B) some push button switches, and (C) a few output pins, then consider using a circuit (below) that might save you 5 or 6 I/O pins, which could then be used for the four chip select lines you need.



This circuit could be modified easily to use eight I/O pins to drive the LCD, four outputs, and a 4x4 keypad. That's normally what someone might use just to drive the 4x4 keypad.

Here's another circuit that uses just three I/O lines to drive an LCD in 8-bit interface mode and read up to eight switches;



Food for thought.

Happy Holidays and cheerful regards, Mike, K8LH
 

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Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
Found on thanks everyone. It's from the CMOS line CD1556 I think. I really wanted to use these 7 segment displays that I have.
 

MMcLaren

Joined Feb 14, 2010
853
I really wanted to use these 7 segment displays that I have.
Is this the same Refrigerator Thermometer/Alarm project that you wanted to run on 3 volts with a Charlieplexed display? If not, can you tell us what you're trying to build? What are you connecting to the I/O on the 18F14K22?
 
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Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
Is this the same Refrigerator Thermometer/Alarm project that you wanted to run on 3 volts with a Charlieplexed display? If not, can you tell us what you're trying to build? What are you connecting to the I/O on the 18F14K22?
The displays I have won't be bright enough to multiplex. Pins I will use, 3 switches, 1 for one wire, 2 positive / negative indicator, 2 select pins, 1 for F /C indicator.

I may have missed one or two,
 
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