[sdiy] SN76477 important info!

[epeasant at telusplanet.net]

Hello, Paul and others,

> IIRC, the "mixer" section of the 76477 was not an audio
> mixer, but a digital 
> one.  That is, the signals are combined using a logical
> AND function.  This 
> is the reason for the external multiplexing circuitry
> used in the Bullet kit, 
> and for the external opamp tapping scheme used by Thomas
> Henry in his 
> SuperController circuit.
> 
> In short, I don't see that there's any real difference
> between the 76477 and 
> the 76477N with regards to the "mixer" section.
> 
> BTW, all the docs I've ever seen on the mixer make note
> of this logical AND 
> technique, with appropriate caveats as to what it really
> means.
 
Apparently I did not make myself sufficiently clear in my 
previous post. 

Most of what you say is correct. Yes the mixer is not 
a "true" analog mixer, it is digital, and that is the 
reason for Mr. Henry's external tapping scheme. It is NOT 
the reason for the multiplexing circuitry in the Bullet 
kit, however.

The short answer is that the mixer logic circuitry itself 
is *different* between the two versions of this chip, they 
are NOT directly compatible. A SN76477 will NOT work 
correctly in a darkstar chaos pcb or the supercontroller 
without circuit modifications.

Here is the long answer:

Pins 25, 26, and 27 are the mixer select pins for this 
chip. Below is a chart showing the differences. The mixer 
input column shows the logic levels sent to the above pins, 
respectively.

MIXER
INPUT
SELECT     76477      76477N

 000     No output   VCO only
 001     LFO only    LFO & Noise
 010     VCO only    LFO only
 100     Noise only  Noise only
 011     No output   VCO, LFO, & Noise
 101     No output   LFO & VCO
 110     No output   VCO & Noise
 111     No output   No output

As you can see, there are some important differences. The 
most important difference is you can only get one signal at 
a time sent to the output when using the 76477. THIS is the 
reason for the Bullet multiplexer circuit, to overcome this 
limitation and enable, for example, both noise and VCO at 
the output at once by switching between them at high 
frequency. See the Bullet documents page 17 on my website 
(http://www.angelfire.com/electronic/epeasant/circuits/bulle
t17.jpg) for their explanation of this technique. This is 
unnecessary with a 76477N.

If you are only routing the noise signal, for example, to 
the output, then either chip will work fine. But if you are 
using the Darkstar or Supercontroller, all mixer select 
pins are left unconnected (logic 000), and so will give 
you "no output" on a 76477, instead of "VCO routed to 
output" as with a 76477N.

I have not seen a data sheet for the SN76477, only the 
SN76477N, so I have had to figure this out by comparing the 
two versions on a breadboard. In some cases, simply 
connecting 5V to the appropriate mixer select line will fix 
the problem. However, there may be other differences 
between these parts as yet undiscovered, and I am not 100% 
sure if I have pins 27 and 28 confused for the 76477. If 
anyone does have a SN76477 data sheet, I'd love to see it 
to confirm this, and to check for other differences that 
might creep up and bite you in the @ss when you don't 
expect it.

Take care,
Doug
______________________
The Electronic Peasant

www.electronicpeasant.com