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 Posted: May 07, 2012 - 12:26 PM |
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Joined: Jun 27, 2005
Posts: 3453
Location: St.Petersburg, Russia
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I'm puzzled by something weird. I'm using a TLV5625 DAC from TI. It's a single-supply 8-bit internal reference, voltage output blah blah DAC. Programmed via SPI. So when the programmer is plugged (it's a reprogrammed pickit2) everything works. When I unplug it, nothing works.
Scoping the signals revealed that pickit2 kind of trims down the signals to 3.3V. But the DAC datasheet says that it's good for 2.7-5.5V operation, and it's powered by 5V. What gives? My SPI lines are connected directly, without pullups or inline resistors. I tried adding pullups, they do nothing. Inline.. I guess they could help but I would like to hear some educated opinions first since they would require doing some controlled damage to the board. Any ideas? |
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Posted: May 07, 2012 - 12:45 PM |
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Joined: Nov 02, 2009
Posts: 3239
Location: Zelenograd, Russia
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| The PICkit2 has pulldowns, not pullups. |
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Warning: Grumpy Old Chuff. Reading this post may severely damage your mental health.
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Posted: May 07, 2012 - 09:38 PM |
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Joined: Apr 25, 2006
Posts: 2304
Location: Germany
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I am confused: Does the TLV5625 really have an internal
Vref ? The headlines in the datasheet say so, but I
only find a description for a VREF input in the
following sections.  |
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Posted: May 07, 2012 - 11:35 PM |
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Joined: Jun 27, 2005
Posts: 3453
Location: St.Petersburg, Russia
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Yes, it has internal VREF.
I "solved" the issue by adding 220Ω/50pF network on SCK line. I can't believe I added a RC filter to a clock line, but with TI all bets are off.
I also solved my double-supply problem. Adding 0.5Ω in line with the heater worked a charm: it takes maybe a little bit longer to warm up than with a separate supply, but that's unimportant. There's still Z-modulation to do, but I'm so lazy about this now — looking at a dekatron tube and some egg-shaped cores I happen to have around ;) |
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Posted: May 07, 2012 - 11:58 PM |
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Joined: Nov 01, 2005
Posts: 6354
Location: Hilversum - the Netherlands
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Quote:
I "solved" the issue by adding 220Ω/50pF network on SCK line. I can't believe I added a RC filter to a clock line, but with TI all bets are off.
Transmission line theory. My first contribution to AVRfreaks: the PPPPD. Same issue. It still resides in my signature.
Even digital is kind of analog 
Nard |
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Posted: May 08, 2012 - 12:15 AM |
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Joined: Jun 27, 2005
Posts: 3453
Location: St.Petersburg, Russia
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| Yeah well.. But in PPPPPPPGJXJJ the lines are long. Here they are like 2cm long and have a groundplane. And it's the first SPI device in my practice that's so sensitive to transients. |
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Posted: May 08, 2012 - 07:08 AM |
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Joined: Apr 25, 2006
Posts: 2304
Location: Germany
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Posted: May 08, 2012 - 04:43 PM |
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Joined: Jun 27, 2005
Posts: 3453
Location: St.Petersburg, Russia
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We definitely do. That Anita is sexy with that argon-filled Dekatron!
I liked that silkscreen marking of the transistor. "That expensive thingy in a top hat goes here". |
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