Fsm ps2

From HDLBits


The PS/2 mouse protocol sends messages that are three bytes long. However, within a continuous byte stream, it's not obvious where messages start and end. The only indication is that the first byte of each three byte message always has bit[3]=1 (but bit[3] of the other two bytes may be 1 or 0 depending on data).

We want a finite state machine that will search for message boundaries when given an input byte stream. The algorithm we'll use is to discard bytes until we see one with bit[3]=1. We then assume that this is byte 1 of a message, and signal the receipt of a message once all 3 bytes have been received (done).

The FSM should signal done in the cycle immediately after the third byte of each message was successfully received.

Some timing diagrams to explain the desired behaviour

Under error-free conditions, every three bytes form a message:

When an error occurs, search for byte 1:

Note that this is not the same as a 1xx sequence recognizer. Overlapping sequences are not allowed here:

Module Declaration

module top_module(
    input clk,
    input [7:0] in,
    input reset,    // Synchronous reset
    output done); 

    // State transition logic (combinational)

    // State flip-flops (sequential)
    // Output logic

  • Although in[7:0] is a byte, the FSM only has one input: in[3].
  • You need ~4 states. Three states likely wouldn't work because one of them needs to assert done, and done is asserted for only one cycle for each received message.

Fsm ps2.png

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