719 metastability experiment
719 : metastability experiment
- Author: yubex
- Description: The design purpose is to evaluate, if metastability can be used as a true random number generator source in an ASIC design.
- GitHub repository
- Clock: 0 Hz
How it works
Generally a Flip Flop can enter a metastable state if the setup or hold time is violated. In this design I try to reach this usually unwanted behaviour.
A toggle Flip Flop is used to create edges on the data inputs of the other 2 Flip Flops. There are 2 modes: Manual and Auto. Manual mode uses the switch on the PCB as trigger for one edge.
Auto mode generates a cyclic trigger within the ASIC. The data output of the toggle Flip Flop is connected to the next Flip Flop by a programmable delay line.
The delay line is created by pairs of inverters. The verilog keep attribute is necessary here, to avoid optimization during synthesis.
The delay_ctrl input selects the number of inverters which are used as delay. The number of inverters used is the delay_ctrl input value times 2. If you set delay_ctrl to 1, 2 inverters are used.
The maximum number of inverters is 128. After each state change of the toggle Flip Flop (exactly 3 clock cycles after that) the output of all Flip Flops are compared.
In case the states are different, metastability has occured.
How to test
For testing select the mode you want to try out. Set the mode to 0 for auto mode and to 1 for manual mode. You can experiment with delay_ctrl input an try to produce metastability. Also try different clock frequency's. In case of metastability the dot of the 7 segment display should change its state.
External hardware
none
IO
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | trigger | segment a, mode | |
| 1 | mode | segment b, toggle_dff_en | |
| 2 | delay_ctrl[0] | segment c, toggle_dff | |
| 3 | delay_ctrl[1] | segment d, delayed_toggle_dff | |
| 4 | delay_ctrl[2] | segment e, meta_dff_0 | |
| 5 | delay_ctrl[3] | segment f, meta_dff_1 | |
| 6 | delay_ctrl[4] | segment g, toggle_dff_en_3t | |
| 7 | delay_ctrl[5] | dot, meta_err_detected |