Allows a 5V ATmega328p to interface over SPI with one at 3.3V (or vice verse). The 74LVC07A has IOFF circuitry which disables the driver when the device is powered down. It is powered by the target so removing power from the target activates the IOFF feature.
Note: connectors are not soldered when shipped.
I use an Arduino Uno with the ArduinoISP sketch from examples to program some of the AVR based boards I'm working with (RPUftdi, RPUadpt). They are at 3.3V while the Uno is at 5V so I need a way to level shift the SPI interface. After figuring out how IOFF works I wanted to use it since that results in an SPI interface that is happy when the target power is off.
IOFF is one form of magic that allows a circuit to be powered on while interfaced with a circuit that is powered off. I have been gaining a greater appreciation for this subject as I have been trying to do the RPUpi board that interfaces an RPUno over RS-422 and SPI to a Pi Zero. The RPUno can turn off its Vin shield pin, which is used to power the Pi Zero. Unfortunately doing so causes the SIP, I2C, and UART of the Pi to have zero volts and pull down anything interfaced to them.
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United States Postal Service: Priority Mail
Priority Mail Small Flat Rate Box will hold two RPUno, or two shields (RPUftdi or RPUadpt) and an RPUno.
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I am working on what may be thought of as a garden SCADA system, it is based on securely connecting a host computer through RS-422 to multiple microcontrollers. I have strayed from RS-485 in favor of retaining existing software (e.g. avrdude, picocom, PySerial...) and have an interface that can act like an interactive terminal session.