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Jonte
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| OT: LED connected to UART pins |
 Posted: Fri Nov 11, 2005 9:08 am |
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Hi!
Sorry for this off-topic one, but I'm not sure where else to ask...
If I want two LEDs connected to the UART RX/TX pins for indication of trafic. Is it OK to connect with a resistor to the TTL line (before a 485 chip) or do I need a buffer like AC05 or simular? Runing at 3.3 volt if it makes any difference...
GreatfullApreciating any help.
/Jonte |
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MikeValencia
Joined: 04 Aug 2004
Posts: 238
Location: Chicago
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| Posted: Fri Nov 11, 2005 9:21 am |
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I have seen similiar applications where there are LEDs to indicate two motor encoder lines.
They definitely do not run it right through an LED as this would cause a significant voltage drop that the microcontroller might mis-interpret the logic levels.
So i think the answer to your question is: Don't distort the logic levels by running it thru an LED.
I don't know what buffer to use though. |
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asmallri
Joined: 12 Aug 2004
Posts: 1630
Location: Perth, Australia
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| Posted: Fri Nov 11, 2005 10:22 am |
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| Quote: | | If I want two LEDs connected to the UART RX/TX pins for indication of trafic. Is it OK to connect with a resistor to the TTL line (before a 485 chip) or do I need a buffer like AC05 or simular? Runing at 3.3 volt if it makes any difference... |
Yes you can connect the LEDs directly to the TTL lines but keep the current consumption low (<5mA). This means your series resistor should be >= 300R
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Fri Nov 11, 2005 11:41 am |
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| Quote: |
but keep the current consumption low (<5mA). This means your series resistor should be >= 300R |
3.3v / 300 ohms = 11 ma |
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Humberto
Joined: 08 Sep 2003
Posts: 1215
Location: Buenos Aires, La Reina del Plata
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| Posted: Fri Nov 11, 2005 12:00 pm |
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| (3.3V - Vled) / 300 <= 7mA |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Fri Nov 11, 2005 1:03 pm |
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| You're right. |
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arunb
Joined: 08 Sep 2003
Posts: 492
Location: India
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| RE |
| Posted: Mon Nov 14, 2005 12:44 am |
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Hi,
I have done something similar, but I used a 74123 for this purpose. With this IC you can actually see the LEDs flashing, this is otherwise not possible by just simply connecting the LED to the pins.
thanks
arunb |
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