[clug] HWR question

Eyal Lebedinsky eyal at eyal.emu.id.au
Sun Mar 1 01:57:29 MST 2015

On 01/03/15 19:45, David Austin wrote:
> On 1 March 2015 at 19:28, Eyal Lebedinsky <eyal at eyal.emu.id.au> wrote:
>> On 01/03/15 17:02, Alex Satrapa wrote:
>>> On 1 Mar 2015, at 15:18, Eyal Lebedinsky <eyal at eyal.emu.id.au> wrote:
>>>> So I am looking at interfacing a 3.3v mcu to a 5v device (over i2c).
>>> Here’s one option:
>>> http://letsmakerobots.com/node/39234
>>> I have not tested this, so I make no claims as to the performance of this
>>> solution.
>>> I have one of those level converters, so if you want to test it out let’s
>>> make a date to meet at MHV, any night next week but Monday is good for me,
>>> and I think we are trying to get “Electronics Night” up and running again
>>> for Thursdays.
>> I agree, and these devices are what I will use. I actually bought one from
>> AX a few months ago,
>> but when I unwrapped it this week I found it to be an RS232 device, where
>> the lines are probably
>> not bidirectional.
>> The device I highlighted in the question was different, it did not have
>> active signal conversion
>> and I could not see how it is safe al all.
> Most often you will not need active level conversion.  That is a function
> of the devices
> being open-drain/pull-down.  It also depends on the different voltage
> levels - 5V and
> 3.3V are close.
> The easiest option is to have the pullups connected to the lowest supply
> voltage (3.3V).
> You will have to read the datasheet of the MCU and I2C device in question.
> For example, the DS1307 you mention, requires a Logic 1 input V_IH
> of at least 2.2V - therefore, it should work just fine on an I2C bus where
> the
> pullups are connected to 3.3V.
> I2C devices are generally (intentionally) designed to be fairly tolerant of
> the actual bus voltages.

I understand this. Yet, the mcu says it is not 5v tolerant, and the ds1307
will drive 5v to the SDA. I wonder what the risk is. Should I blow a full
$3 on testing this?

BTW, the da1307 did not work with this mcu, but the ds3231 is happy (as
it promised).


> David

Eyal Lebedinsky (eyal at eyal.emu.id.au)

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