ZXblast - ZX81 memory extension plus USB

Any discussions related to the creation of new hardware or software for the ZX80 or ZX81
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sirmorris
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Re: ZXblast - ZX81 memory extension plus USB

Post by sirmorris » Wed Nov 08, 2017 6:58 am

Are you thinking what I'm thinking, Andy?? ;)
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

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Andy Rea
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Re: ZXblast - ZX81 memory extension plus USB

Post by Andy Rea » Wed Nov 08, 2017 8:55 am

PokeMon wrote:
Tue Nov 07, 2017 11:18 pm
For half a clock cycle A15 is overridden during M1 in the memory area $8000-$BFFF. As the CPU outputs are not very strong and this a rare situation it wouldn't harm the Z80. ;)
Image
Well this is something... whilst i have no doubt that the technique will work the question remains as to how likely (if at all) damage will occur to the cpu, many different types of cpu are in use in our beloved little black boxes.

regards Andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

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sirmorris
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Re: ZXblast - ZX81 memory extension plus USB

Post by sirmorris » Wed Nov 08, 2017 9:23 am

Then again CPU is pretty easy to replace if it's socketed .. ;)
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

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1024MAK
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Re: ZXblast - ZX81 memory extension plus USB

Post by 1024MAK » Thu Nov 09, 2017 2:26 pm

Then again, if you are overly concerned, you could always cut the A15 track near the CPU and wire a 220 ohm resistor in series, or bend the pin out and fit said resistor between CPU pin and the board...

Mark

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Andy Rea
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Re: ZXblast - ZX81 memory extension plus USB

Post by Andy Rea » Thu Nov 09, 2017 5:54 pm

1024MAK wrote:
Thu Nov 09, 2017 2:26 pm
Then again, if you are overly concerned, you could always cut the A15 track near the CPU and wire a 220 ohm resistor in series, or bend the pin out and fit said resistor between CPU pin and the board...

Mark
That sounds like a great idea.

Regards Andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

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siggi
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Re: ZXblast - ZX81 memory extension plus USB

Post by siggi » Thu Nov 09, 2017 7:40 pm

Andy Rea wrote:
Thu Nov 09, 2017 5:54 pm
1024MAK wrote:
Thu Nov 09, 2017 2:26 pm
Then again, if you are overly concerned, you could always cut the A15 track near the CPU and wire a 220 ohm resistor in series, or bend the pin out and fit said resistor between CPU pin and the board...

Mark
That sounds like a great idea.

Regards Andy
I think, that cutting the track of A15 near the ULA and adding a resistor and 2 diodes is also a great idea(*) :mrgreen:

Siggi

(*) Usually called "M1 NOT circuit" ;)
My ZX81 web-server: online since 2007
http://zx81-siggi.endoftheinternet.org/index.html

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PokeMon
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Re: ZXblast - ZX81 memory extension plus USB

Post by PokeMon » Thu Nov 09, 2017 10:58 pm

Well I am absolutely sure that this is no problem to the CPU. I think you can short A15 for a longer time with a multimeter and will not draw more than 10mA (if at all). So a resistor is useless. A continues short is not recommended - so this is a temporary short only.

As you see in the data sheets a high voltage of 2.4V is guaranteed for the CMOS version with maximum 1.6mA (0.25mA for the NMOS version). So in practice a short won't draw more currency than maybe 10mA. In fact I can measure about 10mA with a multimeter.

This is working in the same way as any pulsed LED which can be overdriven in currency for short moments. This depends mostly on the rate of hi-low ratio and frequency used. An example:

A half clock cycle is 158ns. This can be repeated maximum every 4 clock cycles (1230ns) and give a load of 1/8 or 12.5%. But 4 clock cycles is the shortest instruction while up to 16 clock cycles are also used like LD HL,(xxxx) or similar. So you can expect the ratio to be much lower in the range of 6-8%. In a maximum currency of 10mA this is comparable to a continues load of 0.7mA or so. Nothing spectacular.

Remember that the CPU is specified to deliver currency for a capacitive load of up to 100pF for the address bus. This is in the first 20ns with a currency of 10mA totally out of range but dynamically a normal situation. Guess what happen if 10 address lines changes will draw much more currency or for longer times. Yes, there are different output drivers but all output drivers have to be supplied internally with currency. So internal structures have to handle about 100mA currency for quite a lot of moments. Every capacitor draws currency - this is the life of all electronic devices. So a short is not comparable to other shorts. It depends always on the situation.

The load is only active if code is executed in address range $8000-$BFFF. If M1NOT is not used at all, there is no special load to A15. And people can setup ZXblast not to do so. In my eyes this is harmless. If you want to change the world you need courage and trust. 8-)

Decide now yourself if you want to be a coward or a hero.
May the force be with you ! :mrgreen:

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sirmorris
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Re: ZXblast - ZX81 memory extension plus USB

Post by sirmorris » Fri Nov 10, 2017 8:52 am

PokeMon wrote:
Thu Nov 09, 2017 10:58 pm
If you want to change the world you need courage and trust. 8-)

Decide now yourself if you want to be a coward or a hero.
May the force be with you ! :mrgreen:
:lol: :lol:
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

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