Enable dual USART output functionality.

In other words, enable the USART connected to the serial port on
the Raspberry Pi GPIO header.

Sending a newline to either USART directs the generated entropy
to that USART.

1 files changed, 19 insertions, 2 deletions

diff --git a/README.md b/README.md
index 059b363..bb38fdf 100644
--- a/README.md
+++ b/README.md
@@ -67,13 +67,30 @@ statically at the beginning of main(): MODE_DELTAS and MODE_ENTROPY.
 MODE_ENTROPY is the default, and means the microcontroller will send
 entropy as binary data as fast as it can get it, which is about 24 kB/s
 in the current version of hardware and software. To get some entropy
-and perform rudimentary analysis of it, and assuming the device was
-enumerated as ttyUSB0, do
+and perform rudimentary analysis of it, and assuming USB is used and
+the device was enumerated as ttyUSB0, do
 
   ldattach -8 -n -1 -s 460800 tty /dev/ttyUSB0
+  echo > /dev/ttyUSB0
   cat /dev/ttyUSB0 | rngtest -c 10
   cat /dev/ttyUSB0 | head -c 100000 | ent
 
+For Raspberry-Pi, follow any of the guides on the internet for how to
+enable the serial port on the GPIO pin header and then try
+
+  ldattach -s 115200 -8 -n -1 tty /dev/ttyAMA0
+  echo > /dev/ttyAMA0
+  cat /dev/ttyAMA0 | rngtest -c 10
+  cat /dev/ttyAMA0 | head -c 100000 | ent
+
+(the baud rate used with the R-Pi could probably be increased with a
+little hardware debugging effort).
+
+Which UART on the board that will receive the entropy is controlled
+by the sending of a newline to the UART ('echo > /dev/ttyUSB0' and
+'echo > /dev/ttyAMA0' in the examples above). The power on default is
+the USB UART.
+
 
 MODE_DELTAS is a quality assurance mode, and outputs the raw Timer IC
 values captured for analysis. The stand alone program in src/delta16/