I2C - Fall-time

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Ivan_Lee
Level 5
Level 5
Distributor - Zenitron(GC)
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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 ?

I2C timing.png

訊息由此人編輯:ivan lee

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1 Solution
MotooTanaka
Level 9
Level 9
Distributor - Marubun (Japan)
First comment on blog Beta tester First comment on KBA

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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4 Replies
MotooTanaka
Level 9
Level 9
Distributor - Marubun (Japan)
First comment on blog Beta tester First comment on KBA

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

Ivan_Lee
Level 5
Level 5
Distributor - Zenitron(GC)
100 replies posted 5 likes given 100 sign-ins

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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MotooTanaka
Level 9
Level 9
Distributor - Marubun (Japan)
First comment on blog Beta tester First comment on KBA

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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MotooTanaka
Level 9
Level 9
Distributor - Marubun (Japan)
First comment on blog Beta tester First comment on KBA

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).

001-schematic.JPG

Best Regards,

12-Nov-2020

Motoo Tanaka