Due to virus outbreak I was burning my time:
It is some type of fetish.
I found PCB from classic 2600 JR. It was forcefully removed from case maybe using hammer. The PCB is bent, some parts torn out and missing switches.
PCB was cracked and several traces were broken. I repaired them, but game didn’t start. Sometimes I noticed some video starting, but only for few frames. I checked all connections using oscilloscope and found that CPU address pin (A7, maybe) is floating. I removed the chip and everything become clear:
Very interesting- chip body is broken, but crystal is OK. Only ONE leg disconnected.
The rest of the story is very simple: ordered some cheap from china, received fake one, opened dispute, received money back. Then ordered some other and system started up.
I just only want to send note to myself. Z80 CPU clock must be 5Vpp and pure TTL levels sometimes are to low for good and stable CPU work.
Even this old CPU is very stable at 5Vpp clock, meanwhile 3.3Vpp was too low. Some CPU versions are working fine, but some are very unstable. 3V CPLD output was good only for CMOS version, meanwhile NMOS was not working- only 2..3 CPU cycles.
This information is not written in all available datasheets.
I was testing various AM radios and noticed, that I don’t have proper AM source. The AM broadcasting station in my region is very scarce and weak. Also, I wanted more options for radio receiver testing.
I though, that the internet is full of simple AM modulator circuits, but…
Typical internet schematics are not ream AM. It only “some sort” of AM modulation, and, yes, radio receiver can decode sound from these circuits. Most of circuit do “1″ and “2″ versions, but none was like real AM.
There is simple and good way to generate quite pure sine wave:
This circuit is not very simple, but results are quite fine: output is pure sine wave. From the other side, this schematics have some minuses: it is hard to change frequence. So this type of generator is not very usefull for variable frequence devices.
For my purpose I am using this generator to modulate RF. Here are some osciloscope shots of real world hardware:
AM modulation in it’s clasic way:
P.S. this device use dual polarity power suply.
After building ZX Spectrum clone in CPLD I ported hardware code to another project and this is a bit simplier, but it uses some mono LCD from old copy mashine.
Now I used only one CPLD, but I added additional SRAM chip. Now I don’t need any memory managment (MMU)- video part of the computer is completelly “write only”. It is shadowed in external SRAM chip and if computer need to read some data it do not get date from video part of the computer. This makes design much easier to implement.
CPLD also monitors all IN/OUT operations and generates all specfic video signals for LCD (it is four pixels at a time interface).
This is 320×240 pixel monochromatic screen, without any gray levels. So I can not emulate any colors using grayscale. For border I used vertical stripes, but for screen part is “only black and any other color is white” mode. So games are unusable. Also, moving parts are blured.
All files to download:
Intel-Altera Quartus CPLD source code for ZX Spectrum LCD version.
Do not forget, that regular Z80 need 5Vpp clock for proper operation. I used CMOS buffer for this.
Ant here is pinout for EDMMPU3BDF LCD:
5. pix CLK
6. Vcc, +5V
8. Vee, -25V
9. D3, pixel data
13. Vee, same as 8.
Sometimes I want to show how old 8 bit computers work for some n00bs. It is nice to load some game from tape, some from disk. But when I want to show more game, best thing is cartridge. And not single universal cartridge, but a pile of them. This looks more “expensive” and cool.
Another reason- cheap PCB and that I wanted to to in this way.
And main reason for cartridge is speed- just put in and power on. I never liked this game.
I found old ZX Spectrum clone made in Soviet Union in 90′s. It was my computer, I wrote several program on it and also, I wrote drivers for Espon printer on this hardware. This model has Soviet chip КР580ВВ55А (Intel i8255) and I used this port for LPT interface and some custom hardware.
I powered it and without results- old crappy Soviet chips are dead. At least 3 chips were bad and quality of PCB prevent me from full repair.
Good fellow donated my collection with new item- Sharp PC-2500. The only big problem- this old computer had caustic batteries inside and they leaked. Alkaline used in the batteries has very ugly feature- it can travel under conformal coating along copper traces. All this made this computer DOA.
In this picture, computer is a bit working. Still missing some parts of the image, one keyboard key is not responsible and software freezes. It is possible, that damage to PCB is not only eroded traces, but there is side effect of CMOS technology- the resistance of the PCB is lowered and it may cause problems with high resistance CMOS circuitry. Any unclean spot in PCB has quite low resistance for low current signals. I was trying to wash parts of PCB, but it is paper based PCB and discoloration shows that alkaline salts are inside board material.
There is quite expensive obsolete chip from various Commodore computers and peripherials- CIA/PIA MOS 6526 and a bit changed 8520 used in Amiga computers. This is “complex” peripherial interface adapter chip for 6502 processor or 68K chipset.
It is quite complex, as it has: 16 individual programmed I/O lines, 2 independent, linkable timers, real time clock timer/counter, shift register for serial interface and some other stuff. As this chip is for “interface”, some of the pins goes outside computer and connects to various other devices or… some strange objects like metal pins or pens. S, short circuit, ESD spikes can kill these chips. When I lived in Soviet Union, I blow Amiga 8520 chip and I had my Amiga shelved for several years!
Now is still possible to buy these chips- sometime NOS (very expensive), sometime good pullouts, but sometimes fake ones. And you have to spend over 25USD for just one chip. Maybe is good variant to buy DEAD computer in hope, that chip may be fine.
I am cleaning, testing and rearranging my old computer collection and I decided to repair some of them because now I have more money and more knowlegde. And I have spare CPLD boards from Russian Igrosoft computers. So, one day I decided to build some test rig and build my own CIA/PIA in CPLD (it is 5V TTL compatible).
Here is the CPLD CIA/PIA workplace:
This computer has both 6526 dead. Left CIA is only partially emulated- so I used smaller CHIP. Meanwhile right one is more complex. This is not only timers, int, but also keyboard interface.