experiments with a SN76489 sound chip

[dimitri lafaye frazao]

Hi everyone.

First time posting here. Just finished building my rc2014 and tried making some sounds with a SN76489 sound chip.

https://www.youtube.com/watch?v=UgWNzCyP7hc

I've pretty much recreated the digital I/O card, but instead of driving the LEDs, it's driving the SN76489 data bus.

I've also used the Apple II ALF MC1 sound card schematic as a reference.

http://www.applelogic.org/files/ALF%20MC1%20MC16%20Manual%201983.pdf

I'm using a 74HC138 to decode, 74LS374 to latch the data bus and a 74HC08 to drive the CE pin, like in the MC1 schematic.

I used a 3.5795 MHz crystal that I had laying around, which happened to be the sound chip min clock frequency. 

4 being the max. Although the MC1 uses the Apple II Q3, which is 2MHz... I guess it works with lower frequencies.

The output pin is connected to a 2.5w amp and speaker.

It turned on and it works! I entered the digital I/O example code and you can hear one of the square channels changing up and down :)

As you turn on the rc2014, the sound chip makes some default sounds based on whatever value the 74LS374 is outputting.

Not sure how to start the machine with sounds turned off.

Anyhow, hope you guys enjoyed! That was fun, but I need to sit down and learn more about Z80 and microsoft BASIC first.

Then will read more about the SN76489 and how to make some cool music.

Cheers!

[Spencer Owen]

Excellent work Dimitri!

Sounds great, and I love the look of any breadboard full of wires too :-)

Looking forward to seeing... I mean *hearing* where this goes from here!

Cheers

Spencer

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[Robert Wicksall]

Hi Dimitri,

Any chance you are going to release a schematic?  I'm trying the same thing although I'm following the Colecovision schematic.  Unfortunately I'm either missing something or the RC2014 is clocked too fast.  It's a simpler circuit with Just a 74HCT138 for address decoding.  I think it may work on the Coleco because of the lower clock speed and additional wait state to deal with slow memory.  Unfortunately I don't have an oscilloscope so I'm kind of stuck.

http://www.atarihq.com/danb/files/colecovision.pdf

I haven't actually done anything like this since college back in 1992 so it's a steep learning process.

Bob

[dimitri lafaye frazao]

Hi Bob. 

You can't use the rc2014 clock because 7.3728MHz it's too fast. The sn7468an has a max clock of 4MHz. 

You have to either slow down the rc2014 clock with a flip flip or use a second oscillator. I used a 3.579545 MHz oscillator can and it worked fine.

I will need a 74HCT138 and 74LS374 just like in the I/O board schematic, plus a 74HC08.

Check out the ALF card link that I posted above. At the end there is a schematic for the MC1 card and you can see how the sn76489 is being interfaced. I used the same exact schematic, but instead of 2 x 74LS75 I used the 74LS374  to latch the databus. It's pretty straight forward.

But I will draw a schematic and post here asap :)

I'm having slow progress because my wife hates square waves :D

Cheers

[Robert Wicksall]

Thanks for the info!  I am clocking my sound chip at 4MHz and things are working great with an Arduino.  I think my issue may be with driving the CE pin.  My guess is the Coleco design relies on the slower speed of the Z80.   Either that or I have a bad 74HCT138.

Thanks!