Hi Mike! The ATmega1284P halts the CPU anyway for video memory accesses, so I'm using the same mechanism for updating the I/O memory. So, yes, it's stopping the CPU during keyboard reads.

I forgot to mention that I'm using the address lines as selectors for I/O devices. A0 selects keyboard ...

It's getting bigger! Keyboard input works and just finished wiring logic for 2 atari joystick ports. Keyboard input is processed by ATtiny2313A and send to ATmega1284P through data bus. The ATmega1284P writes the keyboard input as ascii to a SRAM location. 6502 simply reads the data and writes back ...

Thanks Jac! Glad to get some coverage, although the project is still far from finished. I'm implementing a keyboard controller using ATtiny2313 at the moment. Should be ready in a couple of days. I'll probably add two joystick ports too, so I can implement a two player game at some point.

L-Star ...

Hi! Good stuff! But I think your math is a bit off. A 400x256 resolution image (the resolution of ERIC-1) takes 12800 bytes so it can fit into ATmega1284P which has 16K of SRAM. At 320 bytes/scanline it would take 12800/320 = 40 scanlines to copy the image. The entire screen buffer could be thus ...

Interesting approach to reading the video memory! :)

I can see only two problems with that approach: when the AVR is accessing video memory, the 6502 would be massively slowed down, especially with the 400 pixel horizontal resolution of ERIC-1. Also the clock frequency of the 6502 would have be ...

A new blog post is up! It's about making the 6502 and the ATmega communicate with each other.
http://petenpaja.blogspot.fi/2014/12/eric-1-video-memory-interface.html

A short video showing the 6502 updating the screen:
https://www.youtube.com/watch?v=eMqzmynKRAk

Hope you like it! I'll probably add ...

Hi all! I haven't yet had time to debug the BE problem further. However I've written a new blog post with some info about the progress. There is also an updated schematic.

http://petenpaja.blogspot.fi/2014/12/eric-1-cpu-upgrade.html

I also got interfacing between the 6502 and ATmega working. A ...

Unfortunately I don't have control over the timing. The programmer resets the ATmega, which makes the ATmega I/O pin connected to 6502's BE to go high-Z. The pull down resistor I have on the pin ensures that BE goes low but that's all I can do. The programmer will then start to upload code through ...

However, I've now got a new problem...for some reason this is now preventing reprogramming the ATmega… I would have expected this to work because when the microcontroller is programmed, 6502's BE is pulled down and thus the address bus should go to high-Z state. I believe this is a timing issue: I ...

Ok, I replaced the R65C02 with a W65C02S and removed the two 74HC541 buffers on the address lines. However, I've now got a new problem. I have a ICSP programmer connected to 3 pins of the ATmega microcontroller and those same pins are also connected to three bits on the 6502 address bus. Usually ...

Progress on ERIC-1 will be probably slow for a while until the Saleae logic analyzer I ordered today arrives. I'm experimenting with running the CPU clock lockstep with PAL video generation, so that the ATmega can access SRAM during phi2 low. Timing needs to be exactly right, and it's almost ...

4.7K resistor to Vcc to avoid the "RDY is bidirectional on WDC" gotcha.

I'm using method 4 because stopping the clock does not need any extra pins (RDY can be hardwired). The ATmega has "only" 32 I/O pins, and address and data busses already use 24 pins. The remaining pins are used for outputting video signal, 6502 clock, reset, bus enable, SRAM control pins CE and WE ...

What does "the low power Standby Mode" refer to? I could not find any other mention of it in the datasheet. I thought the clock could be stopped any time? The Standby mode is entered, when a WAI or STP instruction is executed. In case of the WAI instruction the µc can be reactivated by IRQ, NMI or ...

Thanks Jac! I'm familiar with regulators. I think a 7805 with a heat sink will be adequate for the time being but I could replace it with a switching regulator if needed later.

I'm replacing the R65C02 with a W65C02S today to get rid of the 74HC541 buffers. While reading the datasheet I spotted ...