Hardware interview practice
Check a single-outstanding request/grant protocol
Implement a clocked checker for one-cycle request pulses. Only one request may be pending, and its grant must arrive from 1 through 10 clocks after the request cycle.
Starting point
Question code
input logic clk, rst_n, req, grant;
output logic error;Reviewed example
Work through one case
Input
req pulses at cycle 4; grant pulses at cycle 14Expected output
pass, with error remaining 0Cycle 14 is exactly ten rising edges after the request cycle, so it is the last legal grant cycle under the stated convention.
What to cover
Requirements
- Start counting only when a request is accepted while no request is pending.
- Flag a second request before the outstanding request completes.
- Accept a grant only 1 through 10 clocks after its request and treat an unsolicited grant as an error.
- Make error sticky until reset, and clear pending state, age, and error on reset.