SX to low voltage I/O
geekything
Posts: 53
What's everyone using for SX (specifically SX48) to 3.0/3.3vdc bidirectional I/O?
-marc
-marc
Comments
· A resistor is usually all that's required.
Bean.
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"SX-Video·Module"·available from Parallax for only $28.95 http://www.parallax.com/detail.asp?product_id=30012
"SX-Video OSD module"·available from Parallax for only·$49.95 http://www.parallax.com/detail.asp?product_id=30015
Product web site: www.sxvm.com
Coming soon... Cheap 4-digit LED display with driver IC·www.hc4led.com
"Sometimes it is better to remain silent and be thought a fool, than to speak and remove all doubt."
Post Edited (Bean (Hitt Consulting)) : 3/22/2006 12:18:31 AM GMT
Sometimes the speed at which you want to read may interfere and reading edges can be a bit tricky under any circumstance.
Another alternative is to modify the SX-48 demo board to your lower voltage {Change the voltage regulator to a 3.0 or a 3.3volt) and change the FUSE setting in the SX-48 to operate at the lower voltage. Parallax has provided a Programming Adapter [noparse][[/noparse]about $10USD] for the SX-Key to assure it gets 5.0volts while your board and whole system can operate at the lower voltage.
I suppose modifying the board will affect warrenty, but at $10 each it seems affordable and much more convientient than a hydrid set up.
·Maybe someday soon, Parallax will provide a 3.3volt board.· But not right now.
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"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)
······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
Post Edited (Kramer) : 3/22/2006 6:11:32 AM GMT
Some 3.3V devices can handle 5V on their I/O pins.
It usually says something like "TTL compatible".
Check the data sheet; you might not have to do anything at all.
Bean, (and Tiger who suggested this elsewhere):
It seems like sometimes the "use a resistor" trick won't work.
Below is an excerpt fom the Xilinx Spartan3 FPGA data sheet.
The device runs on 1.8V, but can be configured for 3.3V I/O
If I'm reading this correctly, it would not be possible to communicate with it using 5V and a resistor.
"ESD Protection
Clamp diodes protect all device pads against damage from
Electro-Static Discharge (ESD) as well as excessive voltage
transients. Each I/O has two clamp diodes: One diode
extends P-to-N from the pad to VCCO and a second diode
extends N-to-P from the pad to GND. During operation,
these diodes are normally biased in the off state. These
clamp diodes are always connected to the pad, regardless
of the signal standard selected. The presence of diodes lim-
its the ability of Spartan-3 I/Os to tolerate high signal volt-
ages. The VIN absolute maximum rating in Table1 in
Module 3: DC and Switching Characteristics specifies the
voltage range that I/Os can tolerate."
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I wonder if this wire is hot...
Now if you tried it WITHOUT the resistor you would surely blow the input, but the clamping diodes limit the voltage on the inputs to 3.3volts and the resistor limits the current through the clamping diode to something reasonable.
If there WASN'T a clamping diode, then you might get into trouble as the input may not "like" seeing 5 volts.
Bean.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"SX-Video·Module"·available from Parallax for only $28.95 http://www.parallax.com/detail.asp?product_id=30012
"SX-Video OSD module"·available from Parallax for only·$49.95 http://www.parallax.com/detail.asp?product_id=30015
Product web site: www.sxvm.com
Coming soon... Cheap 4-digit LED display with driver IC·www.hc4led.com
"Sometimes it is better to remain silent and be thought a fool, than to speak and remove all doubt."
·
The resistor trick likely won't work for me, for the reasons alluded to above.
I'd never seen the "programming adapter"! That actually really makes life a lot easier for me. And nor did I know I could run the SX at 3.0vdc <= 50Mhz (20Mhz would be fine for me), so that actually completely solves my problem! [noparse]:)[/noparse]
-marc
After further investigation I found out that the "clamped" inputs can tolerate 5V if it comes through a 270 Ohm resistor.
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I wonder if this wire is hot...