MCP2551 RS PIN to ground?

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ranc80
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2012/03/01 08:33:13 (permalink)
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MCP2551 RS PIN to ground?

Hi,

i was wondering if in general i can predict problems when placing ground directly at the RS pin at mcp2551
the reason i am asking is that i would like to save the need for a 4.7kohm resistor and i would like to enable standby mode by connecting the pin the microcontroler (i can still do that with two pins but i would rather use one and no resistor)

ps: the CAN speeds can be from 83.33 to 500 (it is a general board)
if removing the resistor is not advised would it be a problem using 10khm resistor instead of 4.7khm? (i am asking mainly from your experiance point of view)

Thx & BR

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    tom_usenet
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    Re:MCP2551 RS PIN to ground? 2012/03/10 07:15:43 (permalink)
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    > i can predict problems when placing ground directly at the RS pin at mcp2551?

    If you read the Data Sheet it states that "High-Speed mode is selected by connecting the RS pin to VSS." So connecting it to ground is a valid mode if you want to disable the slew rate limiting.

    > the reason i am asking is that i would like to save the
    > need for a 4.7kohm resistor and i would like to enable
    > standby mode by connecting the pin the microcontroler
    > (i can still do that with two pins but i would rather
    > use one and no resistor)

    Two pins? From what I'm reading you shouldn't connect the pin through a resistor to ground AND drive it from a CPU as the CPU will then be driving current into that resistor when it drives the pin high. That would be wasting current when you are trying to be in a low power mode. You should connect the resistor from the MCP2551 to the CPU pin. Drive high for shutdown and drive low to enable (and set the slew rate).

    > if removing the resistor is not advised would it
    > be a problem using 10khm resistor instead of 4.7khm?

    Why not read the Data Sheet? Figure 1-1 (SLEW RATE VS. SLOPE-CONTROL RESISTANCE VALUE) shows how the slew rate changes with different resistor values. I don't know where you get 4k7 from as the minimum value shown there is 10k. The maximum current given in the DC Characteristics Table is 200uA and at 2.5 (Vdd/2) that corresponds to a minimum resistor value of 12k5.

    Tom

    post edited by tom_usenet - 2012/03/15 15:27:02
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    ranc80
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    Re:MCP2551 RS PIN to ground? 2012/03/15 06:11:09 (permalink)
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    thank you for your reply, you have been a great help. i read the datasheet and i thought of the things you said but i wanted knolege from experiance, i been through alot in CAN and there are always things i can't predict or things the datasheet is not that clear about so i rather ask but your thoughts match mine and i already made the board i just wanted to be sure before i change those things Thx again & BR
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    tom_usenet
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    Re:MCP2551 RS PIN to ground? 2012/03/15 15:32:08 (permalink)
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    ranc80
    i read the datasheet and i thought of the things you said


    Sorry, but the way you wrote your question it read like you hadn't read the data sheet at all. If something in the data sheet isn't clear, then it helps us to help you if you explicitly refer to sections in the data sheet, so we know what you've know and what you've done already.

    Tom


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    Blue_Key
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    Re:MCP2551 RS PIN to ground? 2012/03/16 07:15:29 (permalink)
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    As i remember you can directly out the pin to the ground. It's even required for 1Mbit/s
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