So confused about the usage of this connector - Page 1
Forum section: electronics / beginners

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 05, 2021, 10:00:35 pm
content: ThePortenta H7has aJST-SH connectorfor I2C in a 5-pin configuration. However, the connector outputs 5 volts. On the other hand, the Portenta's operating voltage is 3.3V. Thus, from that, I believe that I2C resistors should have one end be connected to 3.3V in order to function correctly with the processor.Seeing all this makes me wonder: if the processor requires 3.3V to properly work, why does the JST connector use 5-volt for the operating voltage? Looking around, I saw some quick I2C-based connectors such asSTEMMAandQWIIC, but they operate at different voltages. Well, actually, QWIIC operates at 3.3V. However, STEMMA says they operate at 3-5 volts, with the caveat that I2C devices must accept 3-5V DC and must use a regulator if the device requires 3V or less power voltage. Plus, I2C controllers may provide 3 or 5V. Adafruit's various I2C modules that use matching connectors all have level shifters that convert 5 volts down to 3.3V. Is that why I'm able to use a JST cable to connect to these sensors even though the Portenta operates at 3.3V? I was able to use an appropriate JST cable (5-pin to 4-pin) to connect from my Portenta to the Adafruit sensor successfully and read from it, but I want to understand why that is and why it doesn't damage the Portenta.

author: langwadt
author_rank: Super Contributor
timestamp: October 05, 2021, 10:46:24 pm
content: afaict the Portenta uses an STM32, almost all the pins are 5V compatible so it should have no issue running I2C at 5V

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 05, 2021, 11:01:12 pm
content: Thanks for replying. The Portenta runs theSTM32H747 processor, which states the standard operating voltage is from 1.62 to 3.6 volts. It also stays that the I/O input voltage in table 22 can range from -0.3 volts to the minimum voltage (among VDD, VDDA, or VDD33USB) plus 3.6 volts, up to a max of 5.5 volts. Is that what you mean? I made a board recently that assumed +5 volts from the Portenta for I2C, and I was kind of worried seeing the 3.3V operating voltage of the portenta.

author: langwadt
author_rank: Super Contributor
timestamp: October 05, 2021, 11:52:06 pm
content: Quote from: LoveLaika on October 05, 2021, 11:01:12 pmyes, check that I2C is on FT pins (which it almost certainly is) and make sure to not turn on the pull-ups, then it can handle 5.5V

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 06, 2021, 02:57:44 am
content: Thanks. What do you mean by FT?Also, the sensor has 10k ohm resistors on both the 5 volt side and 3.3 volt side. I had some extra 4.7k resistors alongside that on the 5 volt side, but i found it better to remove them.

author: langwadt
author_rank: Super Contributor
timestamp: October 06, 2021, 07:51:51 am
content: Quote from: LoveLaika on October 06, 2021, 02:57:44 amFT pins are the pins that are 5V compatible

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 06, 2021, 05:22:41 pm
content: I see. According to the datasheet, it says that the I2C3_SCL and I2C3_SDA pins (the ones I'm using on the Portenta if the board schematic holds true) are structured as FT_fa and FT_fha, respectively.Following what you said earlier, I found thisapp noteon the STM32H747 product page, stating how the I/O structure for FT makes it a Five-Volt tolerant I/O pin. Figure 8 seems to enforce this.