Question about chip enable circuitry on 3-to-8 decoder

nick112147

I have a question about decoders. Lets assume for the remainder of this post and all subsequent posts in this topic that I am using a 3-to-8 decoder/demux with with standard A, B and C chip select inputs but also 3 enable inputs labeled G1(drive high), G2A(drive low) and G2B(drive low).
The enable inputs must be driven low or high·according to what I said above, and according to their schematics, you should use·additional·address bus lines to drive the lines. Wouldn't it·be better to conserve·precious address lines by simply using the VCC line to drive the enable inputs high and connecting the enable inputs that·need to be driven low by grounding them? If my description was to confusing, here is the datasheet of the product I plan on using as an address decoder:

http://focus.ti.com/lit/ds/symlink/74ac11138.pdf

One of the reasons I came up with that was that VCC is considered logical 1, or high while VSS is considered logical 0, or low.

Also, as a side question, is it alright on the PCB·design and schematic·to connect multiple chips to the same wire for data transfer to the Propeller? E.g. connecting 3 serial EEPROMs in a chain to the pins I program to accept serial data on the processor or the address lines in a chain to the processor? Thanks,


Nick

Mike Green

G1, G2A, and G2B are intended for simplifying the decoding when cascading these decoders. The fact that the documentation on the device shows them used as additional address lines doesn't mean that that's the only way to use the part. You could just connect them to either Vss or Vdd although I'd suggest you use a pullup or pulldown resistor rather than directly connecting them to Vdd or Vss respectively.

On your side question. You could connect multiple chips on the same line as long as they're meant for that sort of use. They would need to be open collector/drain or perhaps tri-state-able with the output enable lines controlled somehow so only one chip can attempt to drive the line at one time. If more than one chip can drive the line in opposite directions, you'll burn out one or both of them with the short circuit.

nick112147

Mr.Green you are a wonder. Tri-state chips with the control line hooked to the eight chip select lines so that if one chip is driving the line, all of the other chips tri-state controllers will block the other chips, ensuring I don't short circuit the whole system. I am glad the Vdd and Vss idea works, but I will be sure to place a pullup·or pulldown resistor on the lines so I don't mess anything up.

I want to use multiple connections to single wires because I want to use a serial "channel" based I/O scheme, e.g.

Channel 1: Program ROM & Media ROM
Channel 2: Main RAM
Channel 3: Second Propeller

So on and so on. RAM and ROM will require Chip enable signals, hence my use of 3-to-8 decoders. I just have to be really careful not to allow 2 devices to drive any "channel" in opposite directions at the same time.

Post Edited (nick112147) : 3/11/2008 2:42:24 AM GMT