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John Abshier
Registered Member

Date Joined Jul 2004
Total Posts : 346

Posted 11/6/2009 4:02 PM (GMT -8)

This chip, A6812 from Allegro looked like a good match for the Propeller. But I cannot get it to work. It works fine for any single output or pair of outputs on. If I try to turn on 3 outputs, all outputs come on and the chip stays in that state. Any command to turn specific outputs off is ignored. Output enable still works. Using ViewPort, the waveforms look like what I want; perhaps not what I need. Output enable is low, data is clocked out with strobe low. Then the strobe is pulsed high. Pure Spin so no problem going to fast.

Test bed is Propeller Professional Development Board. Clock, serial data in, strobe and output enable are connected to Prop pins 0-3. Logic ground to ground. Logic supply to 3.3 volts. Power ground to ground. Outputs are connected to LEDs that are connected to 5 volts.

Here are a couple of quotes from the documentation:

3.3 to 5 V logic supply range

CMOS, TTL compatible

The CMOS inputs are compatible with standard CMOS logic levels. TTL circuits may require the use of appropriate pull-up resistors.

If I need pull-up resistors, to what voltage would I pull it up? Anyone have any experience?

John Abshier

File Attachment :
A6821ShiftDriver - Archive [Date 2009.11.06 Time 17.38].zip 16KB (application/x-zip-compressed)

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6821.pdf 296KB (application/pdf)

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Peter Jakacki
thinker tinker

Date Joined Sep 2006
Total Posts : 930

Posted 11/6/2009 4:45 PM (GMT -8)

John,

I have written drivers before for this chip and I haven't had any problems. It might be that you are running your chip off +5V and trying to drive it with 3.3V which is the minimum that is accepted as a logic high. There is no reason why you can't run your chip off 3.3V and keep it compatible with the Prop. Pullup resistors won't work as the Prop does not use open-drain drivers. The code looks like it will work as long as the I/O pins are not interfered with by another routine.

BTW, I have a driver in PASM that runs multiple devices, after setting up all you have to do is write to your output buffer.

*Peter*

John Abshier
Registered Member

Date Joined Jul 2004
Total Posts : 346

Posted 11/7/2009 7:17 AM (GMT -8)

Thanks Peter. One thing I like about this chip is all outputs are on one side and inputs on the other. I want to get the Spin version working for low speed applications because it would not use another cog. If you would paste you PASM version I will use it to test my chip/setup.

John Abshier

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