New Logic Analyser Brainstorm

Twin 4040s. 128k static RAM. A line buffer, probably in the region of the number 244. Something related to Arduino in a vague fashion.

8 data lines connected to the buffer, and from there onto the RAM chip. The 4040s provide the address generation.

Press a button or otherwise glean a clue about an appropriate time to start sampling from some logic or other. Reset the addressing logic. While ram not full:- increment address, latch data into the buffer and then write it to the RAM. When RAM full, await instructions to download via serial.

Could work, non?

Alternatively there seems to be a little logic analyser war happening right now - I'd back this regime. If you earn in dollars, then it's offering quite good value for money. If you earn in English pounds then you're laughing.

Programmer Redux

The drop-in centre for stray capacitance was closed by the health and safety after it was shorted by a stream of wee created by a Superintended Nindo. At least I think that's what I heard. I could be wrong. Whatever happened, all the residents must have all come to join with the raving mass I already had, the result of which is that the board resolutely refused to work again after my last witless ramblings.


Pictured: Stray capacitance gathering
ready to frustrate my EEPROMming.

So - I started again!

The code was as much of a lash-up as the board, so I gutted it in favour of a Max6956 IO expander cum LED display driver. 20 gorgeous lines of GPIO, all driven by the Wire library and 2 lines off the Arduino. 12 Address lines, 8 data lines - Pimms o'clock! It's so much cleaner now without having to clock data around scratchy shift registers. I might actually show it to you.

The board came together in about 40 minutes and the code didn't take long at all because I already had a working sketch that drove the beastie. And to my immense relief it worked first time, and reliably every time since. I think I may well solidify this design onto one of the Arduino shield PCBs that I bought from Lady Ada aeons ago 'in case I ever needed one'.



You can see here the new MMC sail, #4 in a series of many. My favorite one so far, this has more than the requisite 6 resistors. I got a smart colleague to recommend a way to get an LED to flash in response to the clock line clocking. He told me that a monostable would do the trick and here it is. Why not just pop an LED on the line? Well, I'd done that already and sometimes that clock line might be left high.

Lash-up!

What a mess! But it works - sometimes :) All that stray capacitance - well someone's got to give it a home.

This is what I'm forced to do because the Stag PP39 EPROM programmer that I was so generously given by a fellow FreeCycler works well, but for its serial connection. Ho hum. Wanting to burn an EPROM to hold the driver code for the Atom MMC interface was the necessity that was the mother of this frightful invention. In a fit of pique I had the idea that I should create a 3-step process: Burn my code into an EEPROM, transfer this to the programmer's buffer, and thence into an EPROM! Why not just use the EEPROM for my project? An astute question, esteemed reader. Well the type of EPROM expected by the Atom is an olde-fashioned marque with a subtly different pin-out to the more contemporary (read: standard) 27x series. Luckily the PP39 can burn the 2532 that was required. Even luckier I suppose is the fact that I had one of these! An adapter board can be made to facilitate the harmonious interfacing of the disparate breeds, but this would involve less lashing-up, you see? And we all need a jolly good lashing from time to time.

I digress. What you see before you (or more correctly above) is an ls299 accompanied by a brace of ls164 shift registers. These in turn are connected to an 8k Atmel EEPROM. In the driving seat you see the Arduino and - naturally - a poor but functional MMC interface. The 164s are in charge of address generation, and the 299 has bi-directional data line duties. It's a simple and effective design which I have referenced before.

I've developed the Atom MMC driver in assembler, naturally. This time I opted for cross assembly. If you saw the code attached to my previous post (my - is that the time?) you may well understand why - the inline assembler is hard work with its terse labelling syntax. I develop and assemble on a PC, using a custom Visual Studio workspace and a freeware 6502 assembler. The resulting binary is debugged as far as possible using Wouter Ras' brilliant though tricksy DOS Atom emulator. When I'm happy with the code it gets put on the MMC card and burned to the EEPROM using a subtle combination of swearing, crossed fingers and sacrificial chickens. The burning process needs to be attempted a fair number of times (the stray capacitance, bless) until the verify step passes and I can be sure the lash-up has worked. Once transferred to the programmer, the code fizzles its way onto a freshly UV-cooked EPROM and then into its warm and welcoming bed - Socket IC24. And so to work:

Presenting - the MMC adapter I built to fit on the venerable machine's expansion port, as visible here.



And in situ:



Goodness - that is the time! Two and a half months this post has taken me! Either that or my post-Sachertorte coma was deeper than usual... Whatever, forgive me. I am off to play Atom Invaders - which now loads in under 3 seconds, a far cry from the original 5 minutes of the tape version!

He's at it again!

I'm getting back into the Atom swing! After a fruitless hour measuring incorrect logic levels at the Atom's expansion port I was on the verge of giving up. I had been prodding the VIA, according to available documentation, and getting nowhere. All I wanted was a toggly bit. Was that too much to ask for? That's when I decided to do some chip swapping.

It takes me longer than most people to get to this point as I'm a software person at heart and so blame that first ;)

I just happened to have a vintage 6522 VIA lying on my desk so I swapped the chips over. Joy! I was flipping bits like a demon. Naturally my thoughts turned to MMC...

10@=0
20!#208=!#208+3
30V=#B800
40W=#FFF4
50T=#80
60DIM LL(13),C(6)
70F.N=0TO13;LL(N)=#FFFF;N.N
80F.N=1TO2; DIM P(-1)
90P."PASS "N',$21
100[
110\ main
120:LL0 LDA @#0
130STA C+1; STA C+2; STA C+3; STA C+4
140LDA @#95; STA C+0
150JSR LL3
160LDA @#40+#0; JSR LL7
170LDA @CH"1"; SBC @0; JMP W
180\ select card
190:LL1 LDA @0; STA V; RTS
200\ deselect card
210:LL2 LDA @#20; STA V; RTS
220\ init hw and go spi
230:LL3 LDA @0; STA V+#C
240LDA @#FE; STA V+2
250LDA @#20; STA V
260LDY @10
270:LL4 LDA @#FF; JSR LL5; DEY; BNE LL4
280RTS
290\ xferbyte
300:LL5 STX T+0; STY T+1; LDY @8
310:LL6 PHA; AND @#80; STA V
320ORA @#40; STA V
330LDX V; AND @#BF; STA V
340TXA; ROR A
350PLA; ROL A
360DEY; BNE LL6
370PHA; JSR #F7FA; PLA
380LDY T+1; LDX T+0;
390RTS
400\ command
410:LL7 STA C+5; LDY @5
420:LL8 LDA C,Y; JSR LL5; DEY; BPL LL8
430:LL9 LDA @#FF; JSR LL5; AND @#FF; CLC; BPL LL10
440DEY; BNE LL9;
450SEC
460:LL10 RTS
470]
480P.$6
490NE.N
500END

If you're an ex-atommer or you've been around BBC micros you might recognise this. If you don't then I can tell you it's some Atom basic with inline assembler. The Atom's Basic dialect was hard on the eyes but lightning fast. By the standards of the day, naturally ;)

The built-in assembler gave the user of this machine an instant boost when learning to program. No fussing with 3rd party apps, loading the assembler or machine code, or the attendant problems with cassettes. Turn on. Assemble. Bam! (Which is as it happens almost quite literally what happened to my 1st Atom over 20 years ago...) Now if only it was that easy for the kids of today - it was much better in the old days - I remember when all this was fields etc. etc.

The above listing is code to bit-bang SPI to an MMC card attached via the simplest of hardware harnesses to the rear expansion connector of the Atom. I'm looking at making a ROM based solution that hooks into the OS's filing system vectors. It's all quite primitive at the moment but I'm hoping it'll solidify. Get in touch if you have any experience of developing this kind of Atom program. I'd love to talk to you.

It's gone all doubley-spaced!

(edit)
The listings are back!

Apologies about the listings, esteemed reader, they appear to have lost their formatting and become double-spaced. I'm looking into it but in the mean time, please bear with me!

Atom Power!

She's arrived! After many years of fussing and prevaricating I've finally taken the plunge and bought myself an Atom. The prices were rocketing in the bay of E, and I reckoned on the time being nigh. This one was advertised as having a broken key and otherwise with little hope of receiving a working box I decided on taking a punt and fixing it up if necessary. I've always loved this machine. I owned one as a boy although it was only a half-populated board without a case. It still did the business though!

I must admit to being a little saddened when I opened the wrapping. It was grubby and half of the keys appeared to be smashed in, sitting a good 6-7mm lower than their neighbours. The 1st thing was to open it up and check the power supply status. The Atom was infamous for overheating and many owners bypassed the internal regulators in favour of feeding it a regulated 5v diet. As was indeed the case with this one. It had received some tweaks in its time, but I was really happy to see the work was all of a high standard. These were all what I call 'magazine hacks' - the electronic equivalent of a one-liner joke. Single wire patches for enabling an eighth bit on the printer port, joining some lines to an external socket for wiring a joystick, nothing major. I was chuffed to see that it was fully expanded memory and support-chip wise. 12K RAM & 12K ROM plus a utility kit that I'm still having trouble finding any info about. I'll have to resort to a disassembler I think. No colour board but then I wouldn't ever have even contemplated that. Too rare.

The mother and key boards were in a real state. I think it had been stored on top of kitchen cupboards at some point in its life and it had received a good layering of yellow grease and fibres. Some keys were sticking and this was the main reason. I've since scrubbed it with a toothbrush and plenty of warm soapy water and now you'd never know! I was shy of putting it in the dishwasher as some people recommend, this may be ok for generic PC keyboards but rare vintage '80s hardware...

At some point in its history the three keys in the lower left had been replaced. They were replaced with high quality keyswitches, which - well I never - made the keycaps sit higher. What had happened was that the previous owner had raised the original keycaps with superglue to match the height of the replacements. The original mechanism is a cheap spring-based creation which I've only ever seen in Atoms ... and a keypad that I was given recently! That was handy, wasn't it! So I hacked the keypad and replaced the switches with something contemporary and far more suitable. And the same height! The caps all came off and were cleaned and repaired where necessary.

The machine is now clean and tidy, ready for work.

One tough trick was finding a plug to fit the odd power socket dimensions. It's some olde fashioned imperial jack size. Eventually located with the assistance of an Atom owning colleague I was able to juice the old girl. To my immense surprise it eventually reset and presented me with the very comforting words:

ACORN ATOM
>

Joy!

A computer's no good without software, right? Like all good '80s machines this one needs some square-wave goodness. I couldn't locate any software to generate the required signals that worked on a modern PC, so being handy with a compiler I got to work. I'm pleased I did all the work on ACE - the techniques transferred instantly and I had a program to translate raw program dumps into WAVS. And text files into raw dumps. And vice versa.

I'll be happy to pass these on to anyone that wants them. The source can be found here. It's vanilla C++. I'll be happy to help with porting, compiling or whatever.

There looks to be a new site emerging that could eventually be a must-see for any Atom fans. I'm speaking in the future tense as it's not there yet but I've spoken with the masters and they're promising big things. Watch this space.

If you're an Atom fan, get in touch.

IO, IO it's off to work we go!

This is my 1st attempt at getting a PCB made. It was rushed so please excuse any glaring stupidity :)

A colleague was having some PCBs manufactured and had a little space left on the plate so I knocked this together during a lunch hour. I'm pretty pleased so far - the boards have come back and, despite being less than optimally packed, they're quite neat.



This is a ZX81 IO board. As I'm sure you've already worked out, esteemed reader! The 8255 is a general purpose IO controller and it's mapped to the Z80 IO space at ports 0x07 and 0x17. I'll be attaching the MMC card to it later :)

I've included links to the schematic and board files in case you want to fire up Eagle and have a play. This is a package with a very steep learning curve. I've been playing with it for about a year and I can only now say that I'm beginning to become confident. Version 5 has just been released which adds some polish to the mix. It wouldn't be uncharitable to say that the package weas a little rough around the edges before..! I haven't pushed V5 much - and it's likely I never will :) - but you can just tell that there's been some love and attention lavished. Hurrah for CadSoft!

Click the schematic for a very high res version. Or get the binaries from HERE.

I'll let you know how it works out when the SMT 74s arrive!