Retromaster’s Electronics Projects

…related to old computers and other assorted stuff…

Posts Tagged ‘board’

CPC6128 Keyboard Daughterboard for UFE

Posted by retromaster on January 17, 2011

This time, a post unrelated to Cumulus (a Cumulus-related update will follow shortly).

At the request of my friend Alcofribas, I’ve been designing and building (in parallel with my work on Cumulus) a CPC6128 keyboard daughterboard for UFE. It allows one to control the UFE floppy emulator using the host keyboard on a CPC6128, just like one does on an Amiga 500 or 1200.

This daughterboard ended up being a bit complicated to setup (at least moreso than the other UFE keyboard daughterboard designs). The existing ribbon connectors on the CPC mainboard are desoldered and female headers are installed in their place. The ribbon connectors are then soldered to the daughterboard. The daughterboard has male headers that match the female headers installed on the CPC mainboard. So, the daughterboard sits upside down on top of the CPC mainboard, and the case cover can be closed without any problems. The rest of the installation is the same as any UFE installation.

The board contains a PIC16F877 along with three 74LS245 chips, four headers/connectors and lots of pull-ups. The 245 octal buffers control the connections between the CPC mainboard and the CPC keyboard, so that UFE can prevent the CPC host from receiving keypresses when its user interface is active. The PIC controls everthing, scans the key matrix and transmits keypresses to the UFE mainboard over I2C.

It seems to work well, but at the moment there is one complication: For the daughterboard to be able to detect the activation keypress, the CPC host must be actively scanning the keyboard. I am told that this may become a problem with some demos and games. So, I’ll need to devise a way to make the daughterboard scan the keyboard at times for the activation keystroke without interfering with the host scan of the keyboard. The most likely solution is that the daughterboard will determine when (or if) it needs to scan the keyboard, by dynamically adapting to the host’s scanning patterns. I am sure that with some more experimentation and practical thinking, a satisfactory solution may be obtained here.

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Cumulus New PCBs Ordered

Posted by retromaster on December 21, 2010

A small update this time. I’ve finished the new Cumulus PCB design (revision B) and just yesterday ordered a few units from a manufacturer in China. Usually I manufacture my own prototypes before ordering from a manufacturer, but this time I did not do that, mainly to save some time. Hopefully there are no (major) problems/mistakes with the design :).

Here is a quick summary of what’s new: Integrated LCD and buttons, 40-pin connector (for use with CumuluBus), better routing on critical lines (to further avoid crosstalk), elimination of 74221-based circuitry and crystal oscillator for driving the PIC and the CPLD.

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A2601 Board Populated

Posted by retromaster on August 23, 2010

I’ve (mostly) finished soldering the new A2601 board. The Xilinx Impact software recognizes the on-board FPGA and Configuration PROM through the parallel port JTAG cable, so I am able to download designs into the FPGA. Just for testing, I’ve uploaded a small design that outputs some NTSC color bars through the composite video port, and it seems to work fine.

It seems some more work needs to be done to get the A2601 FPGA design to work with the new board, though. It has been quite a while since I last worked on this project (judging from the dates on the files, almost three years), and during this time FPGA software (i.e. synthesis, simulation, etc.) has moved on. So I need to remember all the little bits and pieces and fix a few things here and there to make sure that the design works fine in the simulator again (it previously did). Once the design works in the simulator, it should be fine in the FPGA.

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An Older Project Revisited: A2601

Posted by retromaster on August 16, 2010

Having made quite a bit of progress with UFE, I’ve decided it’s time to take a little break from it while I wait and see how recent developments in the manufacturing front pan out (sorry, cannot give details on that yet). So, I am now revisiting a never-before-released project of mine that is almost five years old now.

The project is an ATARI 2600 clone on FPGA, including 6502 and TIA cores in VHDL and dedicated PCB layout, all developed by me. The design fits in a 100K gate Xilinx Spartan 3E, and works quite well in the simulator. The PCB design contains the FPGA, resistor-ladder DACs for video and audio, custom joystick input ports and flash memory for storing ROMs. Few years ago I even built a prototype. It worked well, but unfortunately it did not survive, since my PCB-making techniques were not as advanced back then…

So, what’s new this time around? Basically, a reworking of the PCB layout that fits in a smaller case and contains a single input port for a Genesis/MegaDrive gamepad instead of custom-wired pads. I’ve ended up choosing MegaDrive pads because they are commonly available, very easy to drive and they contain the necessary auxiliary buttons (start/select etc.). I’ve already started manufacturing the board and the photo above shows the work in progress. Hopefully, all will go well and next time I’ll be posting working photos of the board…

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UFE A1200 Keyboard Daughterboard Progress

Posted by retromaster on August 4, 2010

I’ve made some good progress with the A1200 keyboard daughterboard for UFE.

I painfully found out that the PCB design I’ve mentioned in my previous post had a physical issue: It was a little too large! After I made the board, I tried to fit it in an A1200, but it wouldn’t since the corner of the board hit a capacitor. Fortunately, I had not soldered any components yet.

So, back to the drawing board… The solution I came up with was to move the components in the lower left quadrant of the old design to under the PIC chip. Of course, a precision socket had to be fitted in order to raise the PIC to allow components under it. This way, I was able to cut the lower left section altogether, which meant that the new design would fit even with the metal shield in place.

Once I manufactured the PCB, I was glad to see it fit very well. This board is definitely NOT one of my better manufacturing jobs, but it seems it will do the job. The copperclad I used is thinner than the usual stuff, so it fits directly in the flex cable socket once the white plastic lock is removed.

The board has a PIC16F877 installed (I had some handy), although I think that a PIC16F884 would be a better, cheaper alternative, and could be used without modifying the board. During my first tests last night, a funny thing happened. I had a PicKit 2 attached to the daughterboard for testing and it was powering the daughterboard. The PIC was recognized by the PicKit and it was able to program and verify it. But as soon as I plugged the board in the A1200, the PicKit would give VDD Voltage level errors… I was puzzled. I checked for shorts all around the board and I was wondering if the edge connector footprint was incorrect. Then, I realized: The A1200 was off and the poor PicKit was trying to power the whole Amiga through the VCC pin on the flex cable connector! Once I turned the A1200 on, the errors went away 🙂

Last night I managed to write some code for the PIC, too. Now I am able to scan the Alt and Amiga keys so that I can detect the activation keycombo. Once the activation keycombo is detected, the board prevents the A1200 from receiving any keypresses and it looks like it can scan the keyboard without any interference. So, the basic functionality of the board is in place, although I need to do more tests to confirm problem-free operation. Eventually, I’ll implement the I2C communication with the UFE mainboard and I am sure there will be some quirks to sort out.

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UFE A1200 Keyboard Daughterboard Design

Posted by retromaster on July 28, 2010

After the initial release of information on UFE, quite a few people asked about A1200 support. So, here is the early result of my design efforts for an UFE A1200 keyboard daughterboard.

The board is designed to plug into the keyboard flex cable socket on the A1200 motherboard (see the edge connector on the bottom of the PCB). The flex cable from the keyboard goes into the connector on the top left side of the board. So, this board acts as a kind of pass-through.

A 40-pin PIC MCU sits on the right side and is responsible for scanning the keyboard and translating keycodes into the I2C protocol for UFE communication. A couple of 74ls245 chips are placed in between the keyboard and motherboard connectors. These are used to disable the column drivers on the Amiga side when UFE takes over control of the keyboard. This way UFE can scan the keyboard without interference from the Amiga keyboard MCU.

This should all work in theory, based on my understanding of the A1200 schematics and the reverse-engineered A1200 keyboard schematics. Of course, there is a decent chance it could fail, so that’s why there are no schematics accompanying this post. I’ll build the PCB soon and write the firmware for the PIC and see if it actually works :). I haven’t checked the A600 schematics yet, but there is a good chance that this will also work with the A600 without modification.

Part of the challenge here was to make everything fit in the rather limited space. That’s why the board ended up being double-sided. It does not require plated-through holes, however, and there are only a few vias that can easily be handled by soldering wires on both sides. These measures should help keep the cost to a minimum. The current board layout also requires the A1200 metal shield to be removed, but this may perhaps be avoided, by elongating the board even further.

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UFE Prototype Assembled

Posted by retromaster on April 26, 2010

Here is a photo of the UFE prototype, fully-assembled. I am able to program the on-board PIC32 using the PicKit2 programmer. The clock line is connected to the Amiga. Actually, while I was taking this photo, the board was running a tiny test application that alternately blinked the two on-board LEDs. The PIC32 is running at ~75.6Mhz here. The system clock is derived from the 28.375Mhz Amiga system clock, by first dividing the input clock by 6 (to get a ~4.73Mhz clock suitable for the PIC32 PLL) and multiplying it by 16. I am hoping that using the PLL will reduce or eliminate the clock jitter as this was a problem in the earlier TFE+ design, partly due to the relatively long cable over which the clock signal is transmitted. Being able to run the PIC32 at a speed so close to the maximum is very good news, but of course there is no guarantee that problems will not occur with the much more complex setup that will ultimately arise during firmware development.

It is not all good, however. It turns out that my PCB making techniques are still in need of some finetuning. During the soldering process and the cleaning afterwards, the soldermask crackled and popped in several places. I thought it was due to the soldermask not being cured well enough, so it did some tests with a spare, new PCB. It turns out that a thicker layer of paint, plus a higher curing temperature created a much stronger soldermask. During mask removal, the soldermask seems to lose some of its strength, but it works well to cure the board a second time after this step. This step seems to really strengthen the soldermask and that way it is able to withstand soldering and cleaning. I’ll do some more tests to confirm these first findings and finetune the process. Nevertheless, it seems that even in its weak state, a soldermask like this is better than no soldermask at all, since it greatly protects the PCB, especially from the usual trace lifting problems that I frequently experienced previously.

Another issue I encountered with the prototype board was that some of the vias lost connection after the soldermask curing. I am guessing this is due to thermal expansion. I solved the issue by scraping the soldermask above the vias and soldering them on both sides (For hidden vias, I removed the excess solder using a solder wick). This way, a very reliable connection was formed. I’ll soon be updating the page on PCB making with my latest findings.

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New Page on Making PCBs

Posted by retromaster on April 20, 2010

I’ve just put up a new page here on the processes I use for making PCBs. The page also contains photos of the new UFE prototype PCB and more or less documents the making of it.

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New UFE Design Ready

Posted by retromaster on April 19, 2010

As I’ve indicated in my previous post, I’ve been working on a new floppy emulator design, and it’s ready.

The main idea is to implement (hopefully) fool-proof write support by including enough memory to store at least one MFM-decoded floppy image on board. Obviously, your average 40-pin MCU will not be able to access that much external memory, so it was necessary to switch to a more powerful MCU with more pins (in this case, a PIC32). Although static RAM would be simplest to use, they either do not come in the size required or when they do, they are too expensive. So I decided to use SDRAM instead, as there is still a lot of them out there available cheaply in the form of memory modules.

The PIC32 does not have a built-in SDRAM controller, which makes things a little complicated. Luckily, in this project, the external RAM is used like a fast storage area rather than being randomly accessed. So, the plan is to emulate the SDRAM controller in software. It will be accessed quite a bit slower than the usual 100/133Mhz which makes this somewhat easier. In theory, the PIC32 should be able to handle this task, as in comes with a lot of extra computing power, including a built-in DMA controller.

Besides the addition of memory, this new design is a hybrid between the TFE+ design and the older UFE design in terms of features. It features video output for the user interface, just like in the TFE+. The host system’s keyboard will be used to control the emulator, but the keyboard interface is moved to a daughterboard to make the base design host independent. The basic design elements in the original UFE design are in place (PIC32, SDRAM), but SD card is the disk image storage medium instead of a USB flash drive, mainly to reduce complexity of the firmware and simplify PCB layout a bit.

I’ve already started building the PCB for this design and it’s nearing completion. If all goes well, I may soon have another post with the pictures of the actual board.

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Built TFE+ Prototype Board

Posted by retromaster on March 20, 2010

I finally managed to build the TFE+ Amiga version prototype board. I fully assembled the board but did not install the ICs in their sockets yet.

Apparently, I did a rather sloppy job here. The toner transfer and etching went OK (done using glossy paper instead of the usual PnP Blue). I wanted to apply a soldermask afterwards and that’s where things went wrong. It seems I overcooked the board after applying the glass paint (that’s the reason for the discoloration of parts of the board) and I was not able to remove the masked areas after curing. So I removed the paint layer altogether, and just applied the protective flux layer instead. Well, it’s just a prototype, anyway.

Now, it’s time to get the firmware working on this board…

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