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Pause Mod LED


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#1 Ikrananka ONLINE  

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Posted Thu Feb 7, 2019 4:38 PM

I have previously completed a pause mod on one of my CVs and am now planning on doing this again for another.  This involves placing a switch on the system that when switched to pause pulls pin 24 (WAIT) of the Z80 CPU low (ground).  However, this time I'd like to have an LED illuminate when the system is paused.  I have a low understanding of electronics and so am looking for some help on how to achieve this.  Would the following work?  I have a specific switch in mind so I need to be able to do this with a single pole switch as shown.

 

Pause LED.png


Edited by Ikrananka, Thu Feb 7, 2019 4:39 PM.


#2 Osgeld OFFLINE  

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Posted Thu Feb 7, 2019 5:13 PM

Put a resistor before or after the led 330ohm or more

#3 Ikrananka ONLINE  

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Posted Fri Feb 8, 2019 3:17 PM

Put a resistor before or after the led 330ohm or more

 

Thanks.  So, something like this.  When not paused will the LED part of the circuit interfere with the Z80 WAIT line?:

 

Pause LED.png



#4 ChildOfCv OFFLINE  

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Posted Fri Feb 8, 2019 3:28 PM

Keep using +5V :-o

 

This may give you an interesting result though.  The M1 state pulls the WAIT pin down for a clock cycle, as does the sound chip.  So I think you'll see the LED on all the time, though dimmer when not paused.

 

It might be simpler to use a switch with an integrated LED (well, I *think* they exist), or otherwise use a DPST or DPDT switch and have the LED on the other pole.


Edited by ChildOfCv, Fri Feb 8, 2019 3:31 PM.


#5 ChildOfCv OFFLINE  

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Posted Fri Feb 8, 2019 3:41 PM

Another suggestion is to experiment with using the BUSRQ line instead of the WAIT line.  BUSRQ not only tells the processor to sit tight, but it also tells the processor to completely disengage control of the data and address buses.  So it's WAIT on steroids.  And it's a line that's unused except for the possibility of expansion units such as the Atari module.



#6 omf OFFLINE  

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Posted Fri Feb 8, 2019 4:35 PM

you could do it like that yes but i would isolate the LED from WAIT with a double pole double throw switch.

have one side of the switch for WAIT and one side for the led connecting to +5 via a 220 or 330 ohm resistor, you could use a higher value depending on the LED brightness you want to achieve, you will have to experiment on the brightness you want to use



#7 Ikrananka ONLINE  

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Posted Fri Feb 8, 2019 5:36 PM

you could do it like that yes but i would isolate the LED from WAIT with a double pole double throw switch.

have one side of the switch for WAIT and one side for the led connecting to +5 via a 220 or 330 ohm resistor, you could use a higher value depending on the LED brightness you want to achieve, you will have to experiment on the brightness you want to use

 

I have a specific DPDT switch I'll be using for this but the other pole will be used to turn off the sound when the system is paused.  Hence my desire to try and pause the system and illuminate the LED with a single pole.



#8 SiLic0ne t0aD OFFLINE  

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Posted Fri Feb 8, 2019 8:41 PM

Parkfun and I did some experiments with this a couple years ago.. He made a nice drawing of the pause mod w/sound off circuit and took some pictures as well, which are in his gallery here, IIRC. Maybe it'll be able to help you.

#9 opcode OFFLINE  

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Posted Fri Feb 8, 2019 10:28 PM

That is right, check the LED datasheet. I believe blue LEDs usually use lower resistor values. Also keep anything that connects to Z80 pins as 5V. 



#10 Ikrananka ONLINE  

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Posted Sat Feb 9, 2019 10:17 AM

Parkfun and I did some experiments with this a couple years ago.. He made a nice drawing of the pause mod w/sound off circuit and took some pictures as well, which are in his gallery here, IIRC. Maybe it'll be able to help you.

 

Yeah, I know how to do the pause and sound mod, completed this on a CV a while back.  What I want to do is to be able to add a pause LED but without changing the DPDT switch that I'm using.  I'm absolutely certain that it's possible, just not sure exactly how just yet.



#11 Ikrananka ONLINE  

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Posted Sat Feb 9, 2019 10:18 AM

That is right, check the LED datasheet. I believe blue LEDs usually use lower resistor values. Also keep anything that connects to Z80 pins as 5V. 

 

Thanks - I'll check for the correct resistor value before I proceed to testing and will stick with +5V.



#12 grips03 OFFLINE  

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Posted Sat Feb 9, 2019 12:16 PM

add a Schottky diode



#13 Ikrananka ONLINE  

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Posted Sat Feb 9, 2019 1:47 PM

add a Schottky diode

 

Where and why?



#14 Ikrananka ONLINE  

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Posted Sat Feb 9, 2019 3:38 PM

This is my latest.  I've added the existing +5V to BUSRQ via a 3.3K resistor for context, as well as showing the full DPDT switch with audio.  The switch is shown with the system in normal operation (i.e. not paused).

 

Thoughts?

 

Pause Mod.png  



#15 grips03 OFFLINE  

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Posted Sat Feb 9, 2019 3:43 PM

To prevent LED from always being on.

 

Refer to schematic, post 3.  Add Schottky diode, put anode towards Z80, cathode towards switch (same point as cathode of blue LED).



#16 Ikrananka ONLINE  

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Posted Sat Feb 9, 2019 3:54 PM

To prevent LED from always being on.

 

Refer to schematic, post 3.  Add Schottky diode, put anode towards Z80, cathode towards switch (same point as cathode of blue LED).

 

Does the Schottky go in parallel or in series with the blue LED?



#17 grips03 OFFLINE  

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Posted Sat Feb 9, 2019 4:05 PM

Like this.  I've not tested, but looks like it would work ok.

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  • diode.png


#18 Ikrananka ONLINE  

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Posted Sat Feb 9, 2019 4:15 PM

Like this.  I've not tested, but looks like it would work ok.

 

Great, thank you.  I think I understand (almost anyway  :-D ).  So, with the Shottky diode there am I right in thinking that now it shouldn't matter if I connect to the Z80 WAIT or the Z80 BUSRQ, i.e. if I connect to WAIT the Shottky diode will prevent the LED part of the circuit from affecting the WAIT line at the Z80?



#19 fdr4prez OFFLINE  

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Posted Sat Feb 9, 2019 4:16 PM

use a 3PDT switch if you want the LED circuit will be separate/isolated



#20 Ikrananka ONLINE  

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Posted Sat Feb 9, 2019 4:22 PM

use a 3PDT switch if you want the LED circuit will be separate/isolated

 

Yes, I know that a 3PDT switch would separate things and would be ideal, but I have a specific DPDT switch I want to use and I know it can be achieved (but just not how - although looks like we might have a solution above thanks to grips03).



#21 grips03 OFFLINE  

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Posted Sat Feb 9, 2019 4:28 PM

Might need another 330 ohm resistor in there, between Z80 and and Shottky diode.  Does Wait ever go high? 

 

Shottky diode is to make it so only GND (low) goes to Wait pin.



#22 Ikrananka ONLINE  

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Posted Sat Feb 9, 2019 4:32 PM

What Shottky should I buy?  I'm assuming that I want as low a forward voltage as possible.  Is this suitable?



#23 Ikrananka ONLINE  

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Posted Sat Feb 9, 2019 4:34 PM

Might need another 330 ohm resistor in there, between Z80 and and Shottky diode.  Does Wait ever go high? 

 

Shottky diode is to make it so only GND (low) goes to Wait pin.

 

Isn't Wait held high, i.e. inactive (it is tied to +5V via a 2.7K resistor)?

 

delete.png



#24 grips03 OFFLINE  

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Posted Sat Feb 9, 2019 4:36 PM

I have to admit I've not looked at this enough.  Perhaps something more is needed?

 

If I have time I'll look at it more. 



#25 Ikrananka ONLINE  

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Posted Sat Feb 9, 2019 4:40 PM

I have to admit I've not looked at this enough.  Perhaps something more is needed?

 

If I have time I'll look at it more. 

 

Thank you - I really appreciate your help  :)






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