Re: [beagleboard] Re: I2C SCL Voltage level too low?

2018-04-09 Thread John Syne 
Yeah, that is much better. Not sure you have to use a 1K pullup. See if a 3K3 
or a 4K7 does gives you clean signals. BTW, your earth pin on your scope is too 
long and that is why you are seeing noisy signals. Search google for "scope 
probe short ground” and look at the images on how to do this. 

Regards,
John





> On Apr 9, 2018, at 7:15 PM, yassyass  wrote:
> 
> Thank you John,
> 
> It seems like my scope was loading it quite a bit and is now giving much 
> better readings using x10 probes. The signals bellow are set to 400 kHz using 
> 1kOhm pull-ups
> 
>  
> 
>  
> 
> 
> 
> 
> 
> On Friday, April 6, 2018 at 9:49:48 AM UTC+12, john3909 wrote:
> Yeah, I agree there is something else going on here. with a 1K resistor, the 
> signals should not have a slow rising time. The rise time doesn’t look like a 
> capacitor, but I agree, that is about the only explanation that would cause 
> the rise time to slow like this. Maybe the I2C part is faulty. Try plugging 
> in another I2C part to see if the problem persists. 
> 
> Regards,
> John
> 
> 
> 
> 
> 
>> On Apr 5, 2018, at 8:02 AM, Graham  
>> wrote:
>> 
>> 
>>  It looks like you have some extra capacitance on the bus.  There should not 
>> be any capacitors bridging the I2C data and clock lines. Some of the 
>> third-party universal interface cards have extra capacitance, so take those 
>> off.
>> 
>> I have never heard of an I2C part with built in pull-up resistors.
>> 
>> Do to the multi-drop nature of the I2C bus, pull up resistors are almost 
>> always external. There are some "weak pull up" resistors you could turn on 
>> in the BBB, but are too high in value for most applications.
>> 
>> I suggest you read up on how to select pull up resistors for an I2C bus.  
>> Phillips (now NXP) initially developed the bus and has good documentation.
>> Google: NXP I2C bus documentation
>> 
>> But the short answer is that for a 3.3V bus, resistors in the range of 1.2K 
>> to 3.3K should work fine. The value is not critical. You want to pull 1 to 3 
>> mA through the resistor when the bus is low.
>> 
>> --- Graham
>> 
>> ==
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> -- 
>> For more options, visit http://beagleboard.org/discuss 
>> 
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>>  
>> .
>> For more options, visit https://groups.google.com/d/optout 
>> .
> 
> 
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> 
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Re: [beagleboard] Re: I2C SCL Voltage level too low?

2018-04-09 Thread yassyass 
Thank you John,

It seems like my scope was loading it quite a bit and is now giving much 
better readings using x10 probes. The signals bellow are set to 400 kHz 
using 1kOhm pull-ups









On Friday, April 6, 2018 at 9:49:48 AM UTC+12, john3909 wrote:
>
> Yeah, I agree there is something else going on here. with a 1K resistor, 
> the signals should not have a slow rising time. The rise time doesn’t look 
> like a capacitor, but I agree, that is about the only explanation that 
> would cause the rise time to slow like this. Maybe the I2C part is faulty. 
> Try plugging in another I2C part to see if the problem persists. 
>
> Regards,
> John
>
>
>
>
>
> On Apr 5, 2018, at 8:02 AM, Graham  
> wrote:
>
>
>  It looks like you have some extra capacitance on the bus.  There should 
> not be any capacitors bridging the I2C data and clock lines. Some of the 
> third-party universal interface cards have extra capacitance, so take those 
> off.
>
> I have never heard of an I2C part with built in pull-up resistors.
>
> Do to the multi-drop nature of the I2C bus, pull up resistors are almost 
> always external. There are some "weak pull up" resistors you could turn on 
> in the BBB, but are too high in value for most applications.
>
> I suggest you read up on how to select pull up resistors for an I2C bus.  
> Phillips (now NXP) initially developed the bus and has good documentation.
> Google: NXP I2C bus documentation
>
> But the short answer is that for a 3.3V bus, resistors in the range of 
> 1.2K to 3.3K should work fine. The value is not critical. You want to pull 
> 1 to 3 mA through the resistor when the bus is low.
>
> --- Graham
>
> ==
>
>
>
>
>
>
>
> -- 
> For more options, visit http://beagleboard.org/discuss
> --- 
> You received this message because you are subscribed to the Google Groups 
> "BeagleBoard" group.
> To unsubscribe from this group and stop receiving emails from it, send an 
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> To view this discussion on the web visit 
> https://groups.google.com/d/msgid/beagleboard/a8ed3ca6-d286-4d29-8dc1-8851aa384db3%40googlegroups.com
>  
> 
> .
> For more options, visit https://groups.google.com/d/optout.
>
>
>

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Re: [beagleboard] Re: I2C SCL Voltage level too low?

2018-04-05 Thread John Syne 
Yeah, I agree there is something else going on here. with a 1K resistor, the 
signals should not have a slow rising time. The rise time doesn’t look like a 
capacitor, but I agree, that is about the only explanation that would cause the 
rise time to slow like this. Maybe the I2C part is faulty. Try plugging in 
another I2C part to see if the problem persists. 

Regards,
John





> On Apr 5, 2018, at 8:02 AM, Graham  wrote:
> 
> 
>  It looks like you have some extra capacitance on the bus.  There should not 
> be any capacitors bridging the I2C data and clock lines. Some of the 
> third-party universal interface cards have extra capacitance, so take those 
> off.
> 
> I have never heard of an I2C part with built in pull-up resistors.
> 
> Do to the multi-drop nature of the I2C bus, pull up resistors are almost 
> always external. There are some "weak pull up" resistors you could turn on in 
> the BBB, but are too high in value for most applications.
> 
> I suggest you read up on how to select pull up resistors for an I2C bus.  
> Phillips (now NXP) initially developed the bus and has good documentation.
> Google: NXP I2C bus documentation
> 
> But the short answer is that for a 3.3V bus, resistors in the range of 1.2K 
> to 3.3K should work fine. The value is not critical. You want to pull 1 to 3 
> mA through the resistor when the bus is low.
> 
> --- Graham
> 
> ==
> 
> 
> 
> 
> 
> 
> 
> -- 
> For more options, visit http://beagleboard.org/discuss 
> 
> --- 
> You received this message because you are subscribed to the Google Groups 
> "BeagleBoard" group.
> To unsubscribe from this group and stop receiving emails from it, send an 
> email to beagleboard+unsubscr...@googlegroups.com 
> .
> To view this discussion on the web visit 
> https://groups.google.com/d/msgid/beagleboard/a8ed3ca6-d286-4d29-8dc1-8851aa384db3%40googlegroups.com
>  
> .
> For more options, visit https://groups.google.com/d/optout 
> .

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