[clug] HWR question

[Eyal Lebedinsky]

On 01/03/15 15:41, Bob Edwards wrote:
> On 01/03/15 15:18, Eyal Lebedinsky wrote:
>> So I am looking at interfacing a 3.3v mcu to a 5v device (over i2c). I
>> look on aliexpress for a small
>> converter and see a few. One 8 channel device is
>>          http://www.aliexpress.com/item/-/32238089139.html
>> looks interesting, but when I examine the provided images I notice this one
>>
>> http://i01.i.aliimg.com/wsphoto/v0/32238089139_6/8CH-IIC-I2C-Logic-Level-Converter-Bi-Directional-Module-DC-DC-5V-to-3-3V-Setp.jpg
>>
>>
>> Am I misreading it or is there no conversion on the 8 data lines (just
>> resistors)?
>>
>> TIA
>>
>
> Yep - I2C is "open-drain" (or pull-down only), so the drivers only need
> to pull the SCL and SDA lines to GND and don't really care how high they
> go when not being pulled (within reason). So a simple low-value inline
> resistor on each line is all that you should need - you would probably
> get by without them.

You say that the 5v i2c device will accept the 3.3v signals on SDA/SCL? I know
for a fact that I have a 5v device and it does not work with the 3.3v mcu. For
example a DS1307 RTC.

And when SDA is driven by the 5v device, the 3.3v mcu will be rather unhappy
as it is not 5v tolerant AFAIK. What am I missing here?

Eyal

> cheers,
>
> Bob Edwards.

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