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 Posted: Sep 21, 2011 - 06:49 PM |
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Joined: Jun 22, 2004
Posts: 3849
Location: South West Utah, USA
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I was looking at the UC3C-EK schematics VCC5:
http://www.atmel.com/dyn/resources/prod ... s_rev2.pdf
First there is an incorrect notice:
Quote:
All on-board components are powered by 3,3V
(Only LCD back light is powered by VCC5)
Among other things both CAN transceiver ATA6660 chips are fed by VCC5 (just search the schematics for "VCC5"). The ATA6660 RXD pin is specified for a minimum of 4 volts (.8 * 5) to 5 volts maximum on the Rx_CAN_0 output that is connected to the UC3C PB4 input port pin. The 3.3 volt UC3C-EK is specified for an absolute maximum of 3.6 volts on its PB4 port pin. This is .4 to 1.4 volts over the UC3C absolute maximum pin limit.
The ATA6660 is specified for a TXD minimum of 2 volts high input and the 3.3 volt UC3C Tx_CAN_0 PB5 high output is 2.5 volts minimum. This works, but is only a 1/2 volt design noise margin.
The Rx_CAN_1/PC11 connection also has this same over the absolute maximum violation problem.
There is no voltage divider or even current limiting resistor shown for the Rx_CAN_0/PB4 or Rx_CAN_1/PC11 connections.
Am I crazy or has ATMEL actually designed a board that violates the UC3C absolute maximum pin input voltage for port pins PB4 and PC11? I did not see any special UC3C data sheet electrical characteristics that made any specification exception for the alternate use CAN pins. Did I miss something? |
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Posted: Oct 05, 2011 - 06:01 AM |
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Joined: Jun 22, 2004
Posts: 3849
Location: South West Utah, USA
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Bump!
Does anyone have any comments at all? Did I get it wrong or did I get it right?
Considering this is an ATMEL product, if I'm correct about exceeding the absolute maximum voltages on two pins, then what does this UC3C data sheet notice really tell us:
Quote:
*NOTICE:
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress rating only and functional operation of the device at these or other conditions beyond those indicated in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
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Posted: Oct 05, 2011 - 07:54 AM |
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Joined: Aug 19, 2003
Posts: 404
Location: Australia
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The UC3C datasheet is very quiet on the topic of clamping diodes on the GPIO pins.
The short-circuit current of the ATA6660 is about 6mA max, so I'd expect the current in the RxCAN line to be lower than that.
Not good practice, but ....
Perhaps the schematic is wrong ? ie, the AT6660s are on VCC3 instead of VCC5 ?. (I don't have a UC3C-EK board to check this) |
_________________
Datasheets are your best friends. (but always check the errata)
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Posted: Oct 07, 2011 - 03:03 AM |
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Joined: Feb 15, 2005
Posts: 3
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I think you got it right, Mike.
The ATA6660 is a 5V CAN transceiver, so its RXD output is a 5V output.
The UC3C-EK is setup with VDDIO at 3.3V, so the maximum input on RXLINE should be 3.3V. The absolute maximum input is 3.6V. This appears to violate the absolute maximum ratings.
If they wanted to run VDDIO at 3.3V for the external memories, they probably should have used a CAN transceiver with a built-in 3.3V I/O level shifter. But I don't think Atmel makes one. So they could have used a separate level shifter. |
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Posted: Oct 08, 2011 - 12:48 PM |
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Joined: Dec 29, 2010
Posts: 466
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It seems like another boo boo.
Where does Atmel find its engineers, high school? |
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Posted: Mar 16, 2012 - 07:16 AM |
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Joined: Jul 21, 2005
Posts: 119
Location: Australia
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| Your thread: "Atmel misses the boat yet again ?" has been removed |
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