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prixsecco
Joined: 18 Dec 2011
Posts: 26
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| I²C / SMBUS |
 Posted: Tue May 29, 2012 1:15 pm |
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Hello,
I've a short question:
I was looking for differences of the I²C and SMBUS Specification and there I was confused with the fixed and dependent I²C-Bus levels.
(like I²C fixed: Vih: 3.0V; I²C dependent: Vih: 0.7 * Vdd).
1)
I know what this mean but when do I have a I²C bus fixed and when is it dependent on VDD ? When I connect it through Pullups to another voltage level than VDD?
2)
In the I²C Specifications is read the Reverse Voltage characteristics for Low and High. For what are they needed?
Thanks |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Tue May 29, 2012 1:33 pm |
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| If you have questions about a document, post a link to the document(s). |
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prixsecco
Joined: 18 Dec 2011
Posts: 26
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Ttelmah
Joined: 11 Mar 2010
Posts: 19329
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| Posted: Wed May 30, 2012 2:08 am |
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Most I2C devices, have the Vih, and Vil levels changing with the supply voltage. The exception to this, is a few special devices, generally using higher supply voltages (for instance, some bus drivers which run the bus at 12v, use 'fixed' levels). Descriptions of the levels involved for both are in the I2C specification.
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a) The fixed levels are intended for non-CMOS devices and/or devices with higher supply voltages than 5 Volt,
e.g. 12 Volt. The I/O levels for fixed level devices are:
LOW level input voltage VIL min. -0.5V max. 1.5V
HIGH level input voltage VIH min. 3.0V max. VDDmax+0.5V
LOW level output voltage VOL1 min. 0V max. 0.4V
HIGH level output voltage VOH open drain output, determined by VDD via an external pull-up resistor.
b) The supply voltage related levels are intended for CMOS devices and/or devices with supply voltages of 5V
or lower. Their I/O levels are:
LOW level input voltage VIL min. -0.5V max. 0.3VDD
HIGH level input voltage VIH min. 0.7VDD max. VDDmax+0.5V
LOW level output voltage VOL1 min. 0V max. 0.4V
HIGH level output voltage VOH open drain output, determined by VDD via an external pull-up resistor.
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Everything you are likely to meet, _unless_ you start using long distance bus drivers (greyhound...... ), will probably use variable levels.
The I2C spec, is the 'answer' to this.
Best Wishes |
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prixsecco
Joined: 18 Dec 2011
Posts: 26
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| Posted: Tue Jun 12, 2012 3:47 am |
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Thank you  |
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