USART is a device (or peripheral). SPI is a standard method of connecting things.
USART stands for Universal Synchronous/Asynchronous Receiver/Transmitter, and is the basic thing you need if you want to transmit using RS-232.422/485/etc. The Synchronous part of a USART is not used very often, and is sometimes that functionality is left out of the device-- and then it's called a UART (pronounced You-Art).
USARTs (with an appropriate RS-232/etc driver/receiver) are mainly used to talk with devices over a cable. Sometimes they are used to talk between devices on the same PCB, or within the same box, but it is much more common to talk with another device over a cable.
SPI, the Serial Peripheral Interface Bus, is a completely different thing than a USART. SPI is mainly used to talk with devices on the same PCB or in the same box. For example, an MCU talking with a digital temperature sensor. It is almost never used to talk over a cable, from box to box.
The nice thing about SPI is that it is super simple, and the devices using SPI do not have to be MCUs. USARTs almost always require that MCUs of some sort are on both ends of the communication link. But USARTs can be connected using less wires over longer distances.
You should check the settings of the TRISB register. Read section 4.2 of this document (http://ww1.microchip.com/downloads/en/DeviceDoc/39582C.pdf). It explains how the TRIS (tristate) registers control the input/output behavior of the PIC16F family. If TRISB[0] is set to 0 then the RB0/INT pin will be outputting low and overriding the interrupt mechanism.
I've lost many hours to that little register in the past.
Best Answer
Here are the details of how normal SPI and framed SPI are implemented on a PIC32.
"In Normal mode operation, the SPI Master controls the generation of the serial clock. The number of output clock pulses corresponds to the transfer data width: 8, 16, or 32 bits."
"In Framed mode operation, the Frame Master controls the generation of the frame synchronization pulse. The SPI clock is still generated by the SPI Master and is continuously running."