I have an atmega328p (running on it's internal 8mhz clock) receiving data over rf. I need the data received to be communicated to the raspberry pi 2 using its gpio pins, I cannot use USB. I don't have the budget for a gertduino. Is there any way to do this directly? I only need to be able to receive serial output on the pi, no uploading sketches.
Both the Arduino and the Raspberry PI have interfaces like SPI, I2C and UART. These interfaces will make it easy for you to communicate between the two. I suggest you search for UART unless you need a very high data throughput, then you need to go for SPI. But unless you want to send over a thousand messages per second UART will work just fine.