Hi there, this is my first post, so bear with me

I'm in the process of designing a PCB for a design using 65xx chips and had a question about the pullup resistors for the interrupt pins. I'm using the free version of Eagle so board space is pretty limited. In order to save some room and components, I was planning on using a single pullup resistor for IRQ, NMI, and BE (i'm using a WDC 65c02). Would that be acceptable? Alternatively, could I just tie them all to 5V, since I don't plan on using them?

Thanks!

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Since you are going to the trouble of making a PCB, I would tie NMI and BE to VDD and PU IRQ with resistor.

_________________
Michael A.

Thanks for the reply! Also, your 6502 primer has been great help thus far!

My project is an extension of the Atari 2600 video game system. If you can believe it, the original design uses a 6507 which does not have any interrupt pins other then reset! I agree, it may be silly to rule out the possibility that I would like to use the interrupts in the future. I am literally almost out of space though, and a few resistors may put me over the limit. It is just good to know that if I really need to, I could tie them to 5v.



In case anyone is interested in my project, here's some info. Originally, I just wanted to replace the 6507 with a full 6502 for the extra address space (64K over 8K). In doing some research, I came across the WDC 65C02 and couldn't resist trying to overclock the processor. The Atari 2600 has a very primitive video chip with no video RAM whatsoever. There are just a few registers for colors and sprites that need to be changed on the fly as the electron beam of a television draws each pixel. In other words, about 70% of all the CPU time is spent drawing the screen with very precisely timed code. Overclocking the CPU can mean a tremendous improvement in the graphics you can pull off (and also incompatibility with all existing software ). I've also expanded the original 128 bytes or RAM to 16 KB. Here's a thread about the project:

http://atariage.com/forums/topic/201304 ... -wdc65c02/

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!

Thanks

Actually, I am using RDY, it is required to interface with the video chip. Thanks for that info though, definitely something I could run into in the future!

Probably every application on the OP's target, the Atari 2600.. It's very special in that the CPU is almost literally drawing the screen pixel by pixel, changing intensity etc. in real time. Very timing critical, obviously. The leftover time (flyback) is used for actual application processing. I don't know much about the 2600, but there was a link to a youtube video of a talk by one of the more famous game programmers and he explained it in detail. From how he explained it I imagine throwing an interrupt into the mix would be a total disaster.

-Tor

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

That's an interesting video. The one I watched was this one though (managed to find it): https://www.youtube.com/watch?v=MBT1OK6VAIU
David Crane explaining how he wrote the game 'Pitfall' for the Atari 2600, and explaining a lot more at the same time.

-Tor

Processing an interrupt for the start of every line would take up quite a bit of extra CPU time. A lot of games just use the entire time for drawing code. There is a zero 'strobe' register you can hit to wait for the start of the next line, which uses the RDY pin to halt the processor until the line starts.

Now that I think of it, the NMI can be hooked up to a development PC for code upload. There could be a boot ROM with the NMI handler that jumps to code download routines, so you could upload new code with a command line at any point in the testing process, rather than rebooting the system.

There is also a book called "Racing the Beam The Atari Video Computer System (Platform Studies)"

http://www.amazon.com/Racing-Beam-Compu ... 26201257X/

_________________
In theory, there is no difference between theory and practice. In practice, there is. ...Jan van de Snepscheut

Some of the more advanced games (e.g. millipede) included paged RAM and ROM banks in the cartridge to overcome this. The CPU R/W' pin was not connected to the cartridge slot so the cartridge determined whether you were reading or writing RAM based on what address you accessed - you couldn't read RAM at the same address as you wrote to RAM!

_________________
Shift to the left,
Shift to the right,
Mask in, Mask Out,
BYTE! BYTE! BYTE!

Indeed - See the exhaustive document by Kevin Horton at http://blog.kevtris.org/blogfiles/Atari ... appers.txt

Which of course means that you can't do any instructions like INC on them.

Someone came up with a method a few years ago that allows you to read and write RAM from the cartridge port with a single address without any additional signals but I have no idea how it works.