Is there such a chip?

[Marks256]

I have an 8bit output coming from a chip(tristate outputs), and i want to have an 8bit led display connected to it. I need a chip(or circuit) that will take that 8 bits and continue displaying data on the leds when the lines go to high impedance.

Kind of like what a flip-flop does, but i can't have a clock (unless i can figure out how to get some sort of clock working with the output chip...).

I know i didn't do such a great job explaining it, but i really am not sure quite what i want....


Thanks.

 

[dknguyen]

Wouldnt it be easier to use a serial coprocessor of some sorts to drive the LED in the way it was last instructed?

If you use something like a flip-flop or latch, you would need one separate I/O line to tell the all the latches/FF to hold state. Why are they going high impedance anyways? It's not like you are rerouting them to another device (if you were you wouldn't be saying that the states go high impedance since when they do, you can't use them for anything).

 

[kchriste]

You could try an octal latch clocked from the signal that controls the tristate function of the original chip. But it beggs the question; why not just wire the output enable line of the original chip so that the outputs never tristate?

 

[SPDCHK]

I used the attached circuit to display real time clock via 4 7-segment LED displays. The IC's 4543N are latching 7 segment display drivers. The IC 4028 was used to clock each chip individually with the new data. Unless the 4543 gets the LD pulse, it will not react to the input on the IA to ID pins.

Sorry for the quality of the schematic. Eagle exports these things as massive files and the re-sizing causes some loss in quality.

 

[kchriste]

Eagle exports these things as massive files and the re-sizing causes some loss in quality.

You can just lower the DPI when you export it...

 

[Sceadwian]

What's the 8bit output from?

 

[SPDCHK]

You can just lower the DPI when you export it...

Ah, never too old to learn something new.

 

[ericgibbs]

hi marks
If I have got your requirement right?.

1.You are driving 8 individual LED's from a 8 bit i/c which has a 'tri-state' capabilty.[ say ic #1]

2. You want a second i/c [ ic #2] that will hold the current output state of ic #1, while you change the data pattern on ic #1.

You will require and octal latch i/c, with a clk/stb pin. The clk/stb for ic #2
can be generated by a 8 bit gate, with the gate inputs wired to the outputs
of ic #1, the output of the 8 bit gate, will be capacitive coupled the the
clk/stb of the octal latch.

Roger that ?.

If you want a diagram, just ask.

Regards
EricG

Hope the 'digit' is much better.
Remember your five a day.

 

[ericgibbs]

hi marks
Attached a sketch of suggested device. Scrap my 8 bit gate strobe idea, its to messy. Use a spare pin on your PIC to latch the 74LS373.

Regards
EricG

 

[Marks256]

I knew that i could use an octal latch, but i didn't want a clock, remember? Oh well, i will figure something out for the clock...

 

[hjames]

So in digital logic land, there are these things called "keeper latches/gates" which are just buffers with a weak (<<CMOS drive strength) feedback resistor from the output to the input. You can hack this by using a '573 octal latch and piggybacking a resistor pack on top of it. (or a '541 octal buffer if you don't mind the input and output lines not being inline)

They're used on tristate busses in order to keep the lines at valid CMOS levels (instead of in the analog/high current threshold region). Otherwise it's not terribly useful, unless you are trying to apply a fix onto some currently existing hardware.

 

[ericgibbs]

Hi marks,

Had a good look around can't find a ready available device for your project.

As a suggestion, if you are low on spare PIC pins,have you considered using
a HEF4094 8 stage shift and store shift register. It only needs 3 PIC pins
to drive it. Have a look at the datasheet. This would give you some spare PIC pins.

Use the 8 S/R outputs pins to drive your LED's, load the bits serially.

EricG

 

[Sceadwian]

I think Marks has more alternatives than he knows what to do with right now =>

 

[Marks256]

Hi marks,

Had a good look around can't find a ready available device for your project.

As a suggestion, if you are low on spare PIC pins,have you considered using
a HEF4094 8 stage shift and store shift register. It only needs 3 PIC pins
to drive it. Have a look at the datasheet. This would give you some spare PIC pins.

Use the 8 S/R outputs pins to drive your LED's, load the bits serially.

EricG

I could, but i don't have any extra pins...

I don't have a project, it is just hypothetical thinking...

 

[Nigel Goodwin]

I could, but i don't have any extra pins...

I don't have a project, it is just hypothetical thinking...

So why not hypothetically use a PIC?.

 

[Sceadwian]

The weak pullup resistors seem like a good sollution to me.

 

[Marks256]

wouldn't they stay on all of the time then?

Ok, lets say that there is an 8 bit bus. On this bus let's put three devices... how about two tristate output buffers, and our little LED display.
Would there be a chip that allows the LEDs to stay in their set position while the other devices are being accessed?

 

[Nigel Goodwin]

Your little LED display would require an enable signal, just as the tri-state buffers do - it has to know when to read data, and when not to.

 

[Sceadwian]

You bascially need to divide your bus into an address side and a data side. Since you're using a micro controller you should be doing an 8 bit data bus and an 8 bit address bus, that gives you a limit of 256 8 bit devices. You can do more but it requires extra twiddling with the address bus, such as feeding the address bus 8 bits, triggering a strobe feeding in the next 8 bits and then the data on the data bus becomes valid. It's basically an external memory interface and can be implemented with various latches and registers and the right micro controller, either manually, or by a native external memory interface.

 

[Marks256]

Your little LED display would require an enable signal, just as the tri-state buffers do - it has to know when to read data, and when not to.

Yes, i know. I could use a tristate buffer, yes?

You bascially need to divide your bus into an address side and a data side. Since you're using a micro controller you should be doing an 8 bit data bus and an 8 bit address bus, that gives you a limit of 256 8 bit devices. You can do more but it requires extra twiddling with the address bus, such as feeding the address bus 8 bits, triggering a strobe feeding in the next 8 bits and then the data on the data bus becomes valid. It's basically an external memory interface and can be implemented with various latches and registers and the right micro controller, either manually, or by a native external memory interface.

Yep. Lets say we have an 8bit address bus(we could go 16bit if you want, as that would be what? 65536 devices?), as well as the 8bit data bus. So, could i do it with the tristate buffer, and what?