Atom 2k18 ?

discussion of games, software, hardware & emulators relating to the Acorn Atom and Acorn System machines.
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1024MAK
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Re: Atom 2k18 ?

Post by 1024MAK » Fri Mar 15, 2019 9:43 pm

roland wrote:
Fri Mar 15, 2019 8:30 pm
I also added a bunch of capacitors that go the the + and - of the IC's (I don't know the exact English words for it but the description will do, at least I hope so).
Do you mean decoupling capacitors?

Mark

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Re: Atom 2k18 ?

Post by hoglet » Fri Mar 15, 2019 10:38 pm

A couple of possible errors in the audio section.

- C6 can be deleted (it was part of the low pass filter for the SID audio which is now gone).

- The order of the J14/J15 and C13/14 should be swapped so the capacitors are directly connected to the DAC. Then the connections to J11 should come from after the capacitor, but before the jumper. Otherwise there is a big DC offset on the audio signals on J11.

- I'm not sure that C5/C13/C14 need to be so large. The capacitor forms a high pass filter to block DC. The impedance it is driving is ~10Kohms. Which gives a cut-off frequency 0.7Hz. It doesn't need to be this low. I would use something like 0.47uF polyester film type. Electrolytic capacitors are best avoided this type of application .

- I'm not sure that C8 is needed. What's it doing?

(large values like 22uF make more sense when driving an 8R speaker directly - is there where the value came from?)

Dave
Last edited by hoglet on Fri Mar 15, 2019 10:46 pm, edited 3 times in total.

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Re: Atom 2k18 ?

Post by roland » Sat Mar 16, 2019 8:31 am

hoglet wrote:
Fri Mar 15, 2019 10:38 pm
- C6 can be deleted (it was part of the low pass filter for the SID audio which is now gone).
It is deleted.
hoglet wrote:
Fri Mar 15, 2019 10:38 pm
- The order of the J14/J15 and C13/14 should be swapped so the capacitors are directly connected to the DAC. Then the connections to J11 should come from after the capacitor, but before the jumper. Otherwise there is a big DC offset on the audio signals on J11.
I intended it this way. Both the DAC audio channels (stereo?) are available at J9. But if somebody wants to use the DAC for something else than audio then he can disconnect the audio connections and use J11. In that case a change of the voltage level does not appear on the speaker.
hoglet wrote:
Fri Mar 15, 2019 10:38 pm
- I'm not sure that C5/C13/C14 need to be so large. The capacitor forms a high pass filter to block DC. The impedance it is driving is ~10Kohms. Which gives a cut-off frequency 0.7Hz. It doesn't need to be this low. I would use something like 0.47uF polyester film type. Electrolytic capacitors are best avoided this type of application . (large values like 22uF make more sense when driving an 8R speaker directly - is there where the value came from?)
Yes, they come from the original Atom diagram. And I just copied and pasted them. I have changed them.
hoglet wrote:
Fri Mar 15, 2019 10:38 pm
- I'm not sure that C8 is needed. What's it doing?
I don't know where it came from. I think I had looked at the wrong place when I putted it there. In the cassette-in there is a similar capacitor. That might have confused me. In the Atom 2k14 diagram there is no C8 equivalent, so I removed it.

I still have to add a few extra pins near the PiTube interface. That's something for later this weekend, I'll have to go now...

Thanks for your feedback. It saves us a lot of troubleshooting afterwards :)
Last edited by roland on Sat Mar 16, 2019 8:32 am, edited 1 time in total.
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Re: Atom 2k18 ?

Post by hoglet » Sat Mar 16, 2019 9:11 am

roland wrote:
Sat Mar 16, 2019 8:31 am
Thanks for your feedback. It saves us a lot of troubleshooting afterwards :)
You are welcome.

One thing to pay attention to when routing the PCB is the ground wiring. The following paths are most critical:
- the FPGA moduile to the level shifters, and especially U8 and U9
- the FPGA moduile to the SRAM

On a board this large, with fast/modern technology, and only two layers, good ground routing is will make a bit difference. I would say this is much more important than with Atom 2014, as FPGAa are very sensitive to noise spikes.

It might take some time, but you can change the connections to the FPGA to simplify the routing, to allow better grounding.

Dave

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Re: Atom 2k18 ?

Post by roland » Mon Mar 18, 2019 4:56 pm

hoglet wrote:
Fri Mar 15, 2019 8:52 pm
You should add "No Connects" to pins you really intend to be unconnected. Then the ERC will not flag them as errors.
Is there a way to do this only for the current schema? I don't want to mark pins as unconnected in the symbol because that will give other issues, like nc pins connected (in case of a 74*244)....
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Re: Atom 2k18 ?

Post by hoglet » Mon Mar 18, 2019 5:14 pm

roland wrote:
Mon Mar 18, 2019 4:56 pm
Is there a way to do this only for the current schema? I don't want to mark pins as unconnected in the symbol because that will give other issues, like nc pins connected (in case of a 74*244)....
Yes, there is.

I don't have access to Kicad at the moment, but I think it's in the Place menu, and it's called the No Connect flag.

Place one on each pin; the result looks like an X.

Dave
Last edited by hoglet on Mon Mar 18, 2019 5:14 pm, edited 1 time in total.

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Re: Atom 2k18 ?

Post by roland » Mon Mar 18, 2019 10:02 pm

Thanks again, I have found it and reduced the ERC to 4 items. After that I changed the pins of the power connector (J4) to Power Outputs and that solved a recurring error that pins are connected but no pin to drive them. And I had to change the pins of the schottky diodes to Power Input and Power Output to get rid of the markers at the +5V of the FPGA board. Result: 0 warnings and 0 errors in CRC check \:D/

I did find an additional error which was not detected by ERC: the break key (J8 pin 20) was not connected to the RST signal on the bus.

Regarding the debugging, Hoglet asked to add a small header with RDY and SYNC, is that really necessary because those signals are already available at the I/O connectors. I don't mind in adding them, but there has to be a good reason. The board is complicated enough :wink:
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Re: Atom 2k18 ?

Post by hoglet » Mon Mar 18, 2019 10:19 pm

roland wrote:
Mon Mar 18, 2019 10:02 pm
Regarding the debugging, Hoglet asked to add a small header with RDY and SYNC, is that really necessary because those signals are already available at the I/O connectors. I don't mind in adding them, but there has to be a good reason. The board is complicated enough :wink:
It just makes it easier to connect the logic analyzer with the usual 15cm fake "Dupont" cables if all the connections are in close proximity to each other. But if these signals are difficult to route, the forget about it. Getting good ground routing with relatively thick traces is far far more important!

Dave

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Re: Atom 2k18 ?

Post by roland » Tue Mar 19, 2019 5:42 pm

A little while ago we discussed the type of RAM for the new Atom.
hoglet wrote:
Wed Mar 13, 2019 8:54 am
Could you make a small carrier PCB to adapt it to a 32-pin DIP footprint, then socket it?
Edit: There is a design here:
https://www.retrobrewcomputers.org/foru ... &#msg_4823
I had a look at the design and it is quite complicated.
RAM-adapter.png
In theory the adapter can be much simpler just by swapping address and data lines, for example:
A0 connects to A1 (both pin 1)
A1 connects to A16 (both pin 2)
and also some "easy" connections between the data lines.

In theory it doesn't matter how the address lines and data lines are connected as long as you write and read in the same order.

But are there any problems to expect in real life, especially when fast read and writes are used in combination with a FPGA?
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Re: Atom 2k18 ?

Post by hoglet » Tue Mar 19, 2019 6:20 pm

roland wrote:
Tue Mar 19, 2019 5:42 pm
In theory it doesn't matter how the address lines and data lines are connected as long as you write and read in the same order.

But are there any problems to expect in real life, especially when fast read and writes are used in combination with a FPGA?
I don't believe there are any issues with swapping address and/or data lines on fast SRAMs. Use whatever order makes the routing easier.

Dave

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