Update readme with info on API, status, usage and performance

Signed-off-by: Joachim Strömbergson <joachim@assured.se>

hw/application_fpga/core/trng/README.md

@@ -1,57 +1,90 @@
 # trng
+
 Implementation of the Tillitis True Random Number Generator (TRNG).
 
 ## Introduction
-Applications running on the Tillitis TKey device may have a need of random numbers.
-As unpredictable initial vectors, as challnges, random tokens etc.
 
-The Tillitis TRNG supports these applications by providing a hardware based
-source of entropy (digital noise) with a uniform distribution.
+Applications running on the Tillitis TKey device may have a need of
+random numbers.  For unpredictable initial vectors, challnges, random
+tokens etc.
 
-Note that the data provided by the TRNG is entropy, not processed random numbers.
-The data should NOT be used directly, but used as seed for a (cryptographically safe)
-random number generator algorithm.
+The Tillitis TRNG supports these applications by providing a hardware
+based source of entropy (digital noise) with a uniform distribution.
+
+Note that the data provided by the TRNG is entropy, not processed
+random numbers.  The data should NOT be used directly, but used as
+seed for a (cryptographically safe) random number generator (CSPRNG)
+algorithm.
 
 
 ## Status
-First version completed. In testing. Use with caution.
 
-## How to use
-The ready bit in the status register indicates that there is a new word of
-entropy available to read out. Applications requiring multiple words of
-entropy MUST wait for the ready bit to be set before reading ut
-subseqent words. Not waiting for the ready bit to be set will lead to reading out
-the same entropy data more than once.
+First version of TKey completed. The TRNG has been tested and provides
+good entropy suitable as seed for a CSPRNG.
 
-Applications that need cryptographically safe random number should use the output
-from the TRNG as seed to a Digital Random Bit Generator (DRBG), for example a Hash_DRBG.
+
+## API
+
+The TRNG API provides to readable addresses:
+
+```
+	ADDR_STATUS:      0x09
+	STATUS_READY_BIT: 0
+	ADDR_ENTROPY:     0x20
+```
+
+The STATUS_READY_BIT in the status register indicates that there is a
+new word of entropy available to read out. When the word is read from
+ADDR_ENTROPY register, the ready bit is cleared.
+
+Applications requiring multiple words of entropy MUST wait for the
+ready bit to be set again before reading ut subseqent words. Not
+waiting for the ready bit to be set will lead to reading out (at least
+parts of) the same entropy data more than once.
+
+Applications that need cryptographically safe random number should use
+the output from the TRNG as seed to a Digital Random Bit Generator
+(DRBG), for example a Hash_DRBG.
 
 
 ## Implementation details
-The implementation is based on free running digital oscillators. The implementation creates
-two sets of oscillators by instantiating a number if LCs configured as one bit inverter gates,
-where the output of the inverter is connected to its own input. The oscillators will have a toggle
-rate based on the given internal gate delay and the wire delay through given by the feedback
-circuit.
 
-After a given number of clock cycles the outputs from the oscillators in each group are
-XOR combined and sampled into two separate registers. This process is repeated a second time,
-producing two more bits, one for each group. These two sets of two bits are then XOR combined
-to produce a single entropy bit. This means that an entropy bit is the XOR combined result
-from two oscillator groups over two sampling events.
+The implementation is based on free running digital oscillators. The
+implementation creates two sets of oscillators by instantiating a
+number if LCs configured as one bit inverter gates, where the output
+of the inverter is connected to its own input. The oscillators will
+have a toggle rate based on the given internal gate delay and the wire
+delay through given by the feedback circuit.
 
-Entropy bits are collected into an entropy word. When at least 32 bits have been collected,
-the ready bit is set, indicating to SW that a new entropy word is available.
+After a given number of clock cycles (4096) the outputs from the
+oscillators in each group are XOR combined and sampled into two
+separate registers. This process is repeated a second time, producing
+two more bits, one for each group. These two sets of two bits are then
+XOR combined to produce a single entropy bit. This means that an
+entropy bit is the XOR combined result from two oscillator groups over
+two sampling events.
 
-Note that the entropy word is not held for the SW to read out. Sampling and collection is running
-continuosly, and the word read by SW will contain the latest 32 bits collected. Entropy bits
-not read by SW will be discarded at the same rate as new bits are collected.
+Entropy bits are collected into an entropy word. When at least 32 bits
+have been collected, the ready bit is set, indicating to SW that a new
+entropy word is available.
 
-Currently the following build time parameters are used to configure the implementation:
+Note that the entropy word is not held for the SW to read
+out. Sampling and collection is running continuosly, and the word read
+by SW will contain the latest 32 bits collected. Entropy bits not read
+by SW will be discarded at the same rate as new bits are collected.
+
+Currently the following build time parameters are used to configure
+the implementation:
 
 - 4096 cycles between sampling
 - 16 oscillators in each group
 - 64 bits collected before setting the ready flag
 
+With the TKey device running at 18 MHz this means that we sample bits
+at 4.3 kbps. Since we sample twice to produce a single bit, the
+effective raw bitrate is 2.1 kbps.
+
+The 64 bits collected means that there is a separation of at least 32
+collected entropy bits between bits in the words read out.
 
 ---