You can think of an FPGA as a bunch of "programmable transistors". You've got a whole bunch of basic logic building blocks, and you program the wires between them to build a logic circuit. This means it is great for building relatively simple but high-speed logic (grab sample from sensor, do some additions and multiplications, store result in external DRAM chip, repeat at 5GHz). However, they are really inflexible: getting them to do different operations depending on some condition is extremely costly.
The PIO, on the other hand, is essentially a really basic CPU core. It reads and executes a stream of instructions, and it can do (very simple) math and conditional logic. This means it is great for building some kind of state machine, which dynamically adjusts it behaviour based on some kind of external condition. The unique selling point of the PIO is that the instruction set is completely designed around super-fast IO, so reading or writing two dozen pins can be done in one or even zero(!) instructions. And because every instruction executes in exactly one cycle, you've got really good control over the exact timing. This makes it ideal for implementing hardware-level protocols in software.
You can think of an FPGA as a bunch of "programmable transistors". You've got a whole bunch of basic logic building blocks, and you program the wires between them to build a logic circuit. This means it is great for building relatively simple but high-speed logic (grab sample from sensor, do some additions and multiplications, store result in external DRAM chip, repeat at 5GHz). However, they are really inflexible: getting them to do different operations depending on some condition is extremely costly.
The PIO, on the other hand, is essentially a really basic CPU core. It reads and executes a stream of instructions, and it can do (very simple) math and conditional logic. This means it is great for building some kind of state machine, which dynamically adjusts it behaviour based on some kind of external condition. The unique selling point of the PIO is that the instruction set is completely designed around super-fast IO, so reading or writing two dozen pins can be done in one or even zero(!) instructions. And because every instruction executes in exactly one cycle, you've got really good control over the exact timing. This makes it ideal for implementing hardware-level protocols in software.