6502.org

To celebrate the Synertek Sym's 35th anniversary this year, I decided to play with the concept of a Retro SYM project.

In thinking what the designers may have done if the product line had continued past the SYM-2, while taking into account later technology beyond the 80's, I came up with the following features that such a project should have..

USB interface
RS232 interface, utilising a Max232 style driver and 6551 ACIA chip
MicroSD card slot for storage
GAL address decoding
DIP switches for the address decoding selection
0.1" Headers for the E, A and AA connector and various jumpers
Tact Switches for the Keypad
TTL Oscillator in place of a Crystal for the clock.
Ability to use large capacity EPROMs, Flash memory and SRAM

Many weekends later, this is what I came up with.

What's the advantage of using a TTL oscillator over using a crystal?

Yes, I am intending to build one of these. Still finalising the PCB design at this stage.

The reason for going down the TTL Oscillator route, is because 1MHz crystals are becoming difficult to obtain.
For that matter, so are 1 MHz Oscillators, but at least for the moment RS Components and Element14 (Farnell) still show them in their catalog.

I thought circuit based oscillators were not as stable or regular as crystal oscillators. Am I mistaken? Or was that a "it used to be true but not any more" kind of thing?

Be sure to check out our sticky topic "Techniques for reliable high-speed digital circuits." I know you're not doing high speed in terms of clock rate here, but some of the parts will have much, much faster slew rates (rise times) than those of yesteryear, and the board is still pretty big.
Jan Crystals (http://jancrystals.com/) can make any crystal you could want. I got some from them for amateur radio many years ago and they were inexpensive. I agree that the cans are easier though.
Good oscillator design is not trivial, and involves some rather heavy math. The commercially published circuits should be pretty reliable, but because of differences in construction methods from one individual to another, it would be good to test the oscillator across the ranges of voltage, temperature, and load if you really want to make sure it won't fail in any way. If you buy a pre-made oscillator in a can, that should all be taken care of for you. I have not shopped for one in years, but I remember they used to be current hogs, and just the oscillator might take close to 100mA! I expect that problem has been taken care of by now, but it would be good to check the specifications.

If you (or somebody else) still wanted to consider crystals, I'd recommend getting a 4 MHz type. These are very common (Digi-Key has 76 different types in stock right now), and can be easily divided down to 1 MHz with 2 flip-flops (available in single package). As an added bonus, this will get you a nice 50% duty cycle, and you may be able to use the higher clock rates for some other device on the board.

Boards received from Fab.... ready to build!

Wow! -- the SYM-3 board looks terrific, andysa. Are you going to stuff it with 74LS series glue chips (per the silk-screen legends)? I know I personally would use CMOS wherever possible (74HC or 74HCT series), but maybe you feel that would dilute the "retro" feel.

cheers,
Jeff

Well done.

Where can we order a PC Board?

"Me three!"

I'm however thinking that the supply of 6532's will be sticking point...

Good point. But you only need one.

If you come across an Atari 2600 you can desolder the 6532 out of that. And they do show up on eBay once in a while.

+++

Are the WDC W65C22 chips drop-in replacements for the 6522 chips required by this board?
http://www.westerndesigncenter.com/wdc/w65c22-chip.cfm

1pcs R6532P R6532 8-BIT Microprocessor From adeleparts2010 on eBay $3.80 plus $3.00 shipping.