UART interference with USI port acting as I2C master on attiny1634

Hi all,

The title sort of says it all - I'm using an attiny1634 MCU, and have got a 6 pin ICSP header connected so that I can program the chip using SPI. However, I'm also using the USART that's on the same pins as SPI - to communicate with a FTDI USB - UART converter. The ICSP header is connected directly to these pins, and the USB - UART connected through 10K resistors - so there shouldn't be any sort of electrical interference when programming the chip over SPI.

Also, USB will never be connected whilst programming with the SPI port anyway, so that chip shouldn't be driving its pins. (do I need to check if the converter goes into high impedance here when disconnected, or does the fact I've got 10K series R make the programmer see the chip as high impedance anyway?)

However, I also want to use the USI interface (universal serial interface, that can act as a I2C master amongst other things) which is of course located on the same pins as said USART, to communicate with a FRAM memory over I2C.

The USB - UART converter will never be connected to a USB host whilst the I2C bus is active, as there is no way a user could plug in a USB cable during the condition that would cause a write to the FRAM device, as they cannot physically get to the device during operation.

So, my main question is - will the pull ups required by the I2C SDA line interfere with the UARTs RX communication from the USB converter?, when the I2C mode is inactive, and the normal UART mode is active?

(that leads me on to another question, I wonder if the UART mode can be deactivated, and the USI mode activated during code execution, and not as a start up setup)

Many thanks in advance!

do I need to check if the converter goes into high impedance here when disconnected,

No.

You do need to check the maximum current rating of the converter’s RX and TX pins. It has to be less than or equal to 5 volts # / 10K ohms = 0.5 mA.

# I assume your device is running at five volts.

So, my main question is - will the pull ups required by the I2C SDA line interfere with the UARTs RX communication from the USB converter?, when the I2C mode is inactive, and the normal UART mode is active?

No.

However, you will have problems if the FRAM decides to pull either line low while in “UART mode”. I assume that’s a possibility if the FRAM confuses serial communications with I2C bus activity that requires a response.