.. _trng:

Introduction
------------------------
The TRNG is a true random number generator that provides full entropy outputs to the application as 32-bit samples. It is composed of a live entropy source (digital) and an internal conditioning component.


The TRNG has been tested under NIST-Random Test.

Features
----------------
- The TRNG delivers 32-bit true random numbers, produced by a digital entropy source.

- The TRNG is embedded with a health test unit and an error management unit.

- Two independent FIFOs, the one with low priority is for non-secure world, while the other with high priority for secure world

- The throughput of the TRNG is up to about 5Mbps.

Block Diagram
--------------------------
The block diagram of TRNG is shown in below.

.. figure:: ../figures/trng_block_diagram.svg
   :scale: 100%
   :align: center

   TRNG block diagram

The TRNG includes the following sub-modules:

- Clock

   - TRNG bus clock is 40MHz.

- Noise Source

   - The noise source is digital OSC, as a random number source, it is internally composed of ring oscillator.

- TRNG control

   - A bit is added to control whether the control register can be accessed from non-secure world.

   - Ensure that the default setting for OSC can work. ROM will use it only without configuring ROSC.

   - This area is the real control register, and the Control_S is the access window in the secure world, Control_NS is the access window in the non-secure world.

- Debias and LFSR and Extractor

   - A serial post-processing circuit

- RCT and APT

   - Two health tests of NIST specification

- Control_S

   - This area is the access window in the secure world; the real address is "Control".

- Status_S

   - Indicates the available data in FIFO_S.

   - Indicates whether an error has happened.

- FIFO_S

   - Only have one window register instead of all the registers.

   - Read and return all zero when FIFO is empty.

   - FIFO size is 256 bits.

   - When the available data is less than 128 bits, hardware will fill the FIFO_S to full in a high priority.

- Control_NS

   - This area is the access window in the non-secure world; the real address is "Control".

   - Only can be accessed when S bit in Control is 0.

- Status_NS

   - Indicates the available data in FIFO_NS

   - Indicates whether an error has happened.

- FIFO_NS

   - Only have one window register instead of all the registers.

   - Read and returns all zero when FIFO is empty.

   - FIFO size is 128 bits.

   - This FIFO has a lower priority than FIFO_S. If available data is less than 128 bits in FIFO_S, hardware will not feed any data to this FIFO.

Usage
----------
- If you need to run the system with security attributes, it is suggested to configure TRNG as secure so that the Control Register can only be accessed from secure world.

- When a large amount of random data is required both by secure world and non-secure world simultaneously, request from secure world will be satisfied first for the former has a higher priority. After the request from secure world ends, random data will be generated to satisfy non-secure world.

- It is suggested to call ``_rand()`` function to get a 32-bit random data.