Rework TRNG module

CHANGELOG.md

@@ -26,6 +26,7 @@ defined by the `hw_version_c` constant in the main VHDL package file [`rtl/core/
 
 | Date (*dd.mm.yyyy*) | Version | Comment |
 |:----------:|:-------:|:--------|
+| 22.11.2021 | 1.6.3.10 | reworked **TRNG** (less hardware requirements, improved quality), see [PR #212](https://github.com/stnolting/neorv32/pull/212) and [stnolting/neoTRNG](https://github.com/stnolting/neoTRNG) |
 | 21.11.2021 | 1.6.3.9 | minor rtl edits: configuring an IMEM or DMEM size (`MEM_INT_IMEM_SIZE` / `MEM_INT_DMEM_SIZE` generic) of 0 will now exclude the according memory from synthesis (and also clears the according `NEORV32_SYSINFO.SOC` flags) |
 | 18.11.2021 | 1.6.3.8 | TWI: removed TWI_CTRL_CKSTEN flag (enable clock stretching) from control registers, clock-stretching is now _always_ enabled |
 | 14.11.2021 | 1.6.3.7 | major control unit and ALU logic optimizations, reduced hardware footprint; :lock: closed further illegal instruction encoding holes (system environment instructions, ALU and ALU-immediate instructions, FENCE instructions); [PR #204](https://github.com/stnolting/neorv32/pull/204) |

docs/datasheet/soc_trng.adoc

@@ -13,72 +13,43 @@
 | CPU interrupts:          | none | 
 |=======================
 
+
 **Theory of Operation**
 
-The NEORV32 true random number generator provides _physical true random numbers_ for your application.
-Instead of using a pseudo RNG like a LFSR, the TRNG of the processor uses a simple, straight-forward ring
-oscillator as physical entropy source. Hence, voltage and thermal fluctuations are used to provide true
-physical random data.
+The NEORV32 true random number generator provides _physical_ true random numbers.
+Instead of using a pseudo RNG like a LFSR, the TRNG uses a simple, straight-forward ring
+oscillator concept as physical entropy source. Hence, voltage, thermal and also semiconductor manufacturing
+fluctuations are used to provide a true physical entropy source.
+
+The TRNG is based on the neoTRNG[https://github.com/stnolting/neoTRNG], which is a "spin-off project" of the
+NEORV32 processor. The TRNG uses the default neoTRNG configuration, which showed very good results in the
+`dieharder` battery of random number tests. More detailed information about the neoTRNG, it's architecture and a
+detailed evaluation of the random number quality can be found it it's repository: https://github.com/stnolting/neoTRNG
 
 [NOTE]
 The TRNG features a platform independent architecture without FPGA-specific primitives, macros or
-attributes.
-
-**Architecture**
-
-The NEORV32 TRNG is based on simple ring oscillators, which are implemented as an inverter chain with
-an odd number of inverters. A **latch** is used to decouple each individual inverter. Basically, this architecture
-is some king of asynchronous LFSR.
-
-The output of several ring oscillators are synchronized using two registers and are XORed together. The
-resulting output is de-biased using a von-Neumann randomness extractor. This de-biased output is further
-processed by a simple 8-bit Fibonacci LFSR to improve whitening. After at least 8 clock cycles the state of
-the LFSR is sampled and provided as final data output.
+attributes so it can be synthesized for _any_ FPGA.
 
-To prevent the synthesis tool from doing logic optimization and thus, removing all but one inverter, the
-TRNG uses simple latches to decouple an inverter and its actual output. The latches are reset when the
-TRNG is disabled and are enabled one by one by a "real" shift register when the TRNG is activated. This
-construct can be synthesized for any FPGA platform. Thus, the NEORV32 TRNG provides a platform
-independent architecture.
-
-**TRNG Configuration**
-
-The TRNG uses several ring-oscillators, where the next oscillator provides a slightly longer chain (more
-inverters) than the one before. This increment is constant for all implemented oscillators. This setup can be
-customized by modifying the "Advanced Configuration" constants in the TRNG's VHDL file:
-
-* The `num_roscs_c` constant defines the total number of ring oscillators in the system. num_inv_start_c
-defines the number of inverters used by the first ring oscillators (has to be an odd number). Each additional
-ring oscillator provides `num_inv_inc_c` more inverters that the one before (has to be an even number).
-* The LFSR-based post-processing can be deactivated using the `lfsr_en_c` constant. The polynomial tap
-mask of the LFSR can be customized using `lfsr_taps_c`.
 
 **Using the TRNG**
 
 The TRNG features a single register for status and data access. When the _TRNG_CTRL_EN_ control register (`CTRL`)
 bit is set, the TRNG is enabled and starts operation. As soon as the _TRNG_CTRL_VALID_ bit is set, the currently
 sampled 8-bit random data byte can be obtained from the lowest 8 bits of the `CTRL` register
-(_TRNG_CTRL_DATA_MSB_ : _TRNG_CTRL_DATA_LSB_). The _TRNG_CTRL_VALID_ bit is automatically cleared
-when reading the control register.
+(_TRNG_CTRL_DATA_MSB_ : _TRNG_CTRL_DATA_LSB_). These bits always keep the latest valid data obtained from the TRNG
+entropy source. The _TRNG_CTRL_VALID_ bit is automatically cleared when reading the control register.
 
-[IMPORTANT]
-The TRNG needs at least 8 clock cycles to generate a new random byte. During this sampling time
-the current output random data is kept stable in the output register until a valid sampling of the new byte has
-completed.
+[NOTE]
+The TRNG core does not provide a dedicated reset. In order to ensure correct operations, the TRNG should be
+disabled (=reset) by clearing the _TRNG_CTRL_EN_ and waiting some milliseconds before re-enabling it.
 
-Randomness "Quality"
-I have not verified the quality of the generated random numbers (for example using NIST test suites). The
-quality is highly effected by the actual configuration of the TRNG and the resulting FPGA mapping/routing.
-However, generating larger histograms of the generated random number shows an equal distribution (binary
-average of the random numbers = 127). A simple evaluation test/demo program can be found in
-`sw/example/demo_trng`.
 
 .TRNG register map (`struct NEORV32_TRNG`)
 [cols="<2,<2,<4,^1,<7"]
 [options="header",grid="all"]
 |=======================
 | Address | Name [C] | Bit(s), Name [C] | R/W | Function
-.3+<| `0xffffffb8` .3+<| `NEORV32_TRNG.CTRL` <|`7:0` _TRNG_CTRL_DATA_MSB_ : _TRNG_CTRL_DATA_MSB_ ^| r/- <| 8-bit random data output
-                                             <|`30` _TRNG_CTRL_EN_                             ^| r/w <| TRNG enable
-                                             <|`31` _TRNG_CTRL_VALID_                          ^| r/- <| random data output is valid when set
+.3+<| `0xffffffb8` .3+<| `NEORV32_TRNG.CTRL` <|`7:0` _TRNG_CTRL_DATA_MSB_ : _TRNG_CTRL_DATA_MSB_ ^| r/- <| 8-bit random data
+                                             <|`30` _TRNG_CTRL_EN_                               ^| r/w <| TRNG enable
+                                             <|`31` _TRNG_CTRL_VALID_                            ^| r/- <| random data is valid when set
 |=======================

rtl/core/neorv32_package.vhd

@@ -64,7 +64,7 @@ package neorv32_package is
   -- Architecture Constants (do not modify!) ------------------------------------------------
   -- -------------------------------------------------------------------------------------------
   constant data_width_c : natural := 32; -- native data path width - do not change!
-  constant hw_version_c : std_ulogic_vector(31 downto 0) := x"01060309"; -- no touchy!
+  constant hw_version_c : std_ulogic_vector(31 downto 0) := x"01060310"; -- no touchy!
   constant archid_c     : natural := 19; -- official NEORV32 architecture ID - hands off!
 
   -- External Interface Types ---------------------------------------------------------------