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Our application ( Fast-mode : 400Khz) the measured fall time is only about 4ns - but I2C spec's the minimum allowed fall time according the spec would be 20 x (Vdd/5.5) = 20 x (3.3V / 5.5V) = 12ns .
1. Could it cause any problems(side effect) ?
2. How to increase the fall time to meet spec ?
訊息由此人編輯:ivan lee
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Hi,
> 1. Could it cause any problems(side effect) ?
Some I2C device connected to the bus may not work correctly.
> 2. How to increase the fall time to meet spec ?
Off my head, we usually apply 2.2K ~ 10K resistor for pull-up.
Depending on the value of this resistor the fall/raise time will be changed.
Lager resistor makes the transition slower and smaller resistor makes the transition faster.
The calculation described in the I2C specification may be a good reference.
17 Application Information
17.1 Slope-controlled output stages of Fast-mode I2C-bus device
(I'm referencing The I2C-BUS specification Version 2.1 Jan-2000 from Philips Semiconductors)
moto
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Hi,
> 1. Could it cause any problems(side effect) ?
Some I2C device connected to the bus may not work correctly.
> 2. How to increase the fall time to meet spec ?
Off my head, we usually apply 2.2K ~ 10K resistor for pull-up.
Depending on the value of this resistor the fall/raise time will be changed.
Lager resistor makes the transition slower and smaller resistor makes the transition faster.
The calculation described in the I2C specification may be a good reference.
17 Application Information
17.1 Slope-controlled output stages of Fast-mode I2C-bus device
(I'm referencing The I2C-BUS specification Version 2.1 Jan-2000 from Philips Semiconductors)
moto
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Hi, Moto,
We try to modify resistor for pull-up before , It can improve rising time, but it is not useful for fall time.
Please give us some suggestion to increase the fall time.
BR
Ivan
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Dear Ivan-san,
Then how about adding some resistor serial to the SDL/SCL pin,
so that current drain time will be increased?
Best Regards,
12-Nov-2020
Motoo Tanaka
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Dear Ivan-san,
Although it must be obvious, please keep the value of series resistor
Vdd * Rseries / (Rpull-up + Rseries + (internal resistance of I2C pin)) < VL (Low level voltage).
Best Regards,
12-Nov-2020
Motoo Tanaka