add README.md

Signed-off-by: Jaroslav Kysela <perex@perex.cz>

README.md

@@ -0,0 +1,52 @@
+Picoprobe with the logic analyzer implementation based on the SUMP protocol
+===========================================================================
+
+Supported features:
+    
+- up to 16 probes (gpio 6-21 by default)
+- 200kB RAM for samples
+- RLE encoding
+- test mode (external pattern)
+  * probe 0 and 1 - 10Mhz PWM
+  * probe 1 and 2 - 1Mhz PWM
+  * probe 3 and 4 - 1kHz PWM
+  * probe 8 and 9 - 1kHz PWM (swapped levels)
+- test pin (gpio 22) - 5Mhz 50%/50% PWM for probe tests (activated only when sampling)
+    
+Limits:
+    
+- 50Mhz sampling rate when compiled with the TURBO_200MHZ define (otherwise 31.25Mhz)
+- basic triggers are implemented (functional up to 10Mhz - TODO: PIO support)
+    
+This protocol is supported in sigrok as openbench-logic-sniffer:
+    
+```
+  pulseview --driver=ols:conn=/dev/ttyACM1
+    
+  sigrok-cli --driver=ols:conn=/dev/ttyACM1 --config samplerate=50Mhz \
+             --config pattern=External --samples 256 --channels 0-1
+```
+
+Compile:
+
+```
+  mkdir build
+  cd build
+  TURBO_200MHZ=1 PICO_SDK_PATH=/opt/pico-sdk cmake ..
+```
+
+Compile RAM code (debug):
+
+```
+  mkdir build
+  cd build
+  TURBO_200MHZ=1 PICO_SDK_PATH=/opt/pico-sdk cmake -DPICO_NO_FLASH=1 -DCMAKE_BUILD_TYPE=Debug ..
+```
+    
+Misc:
+    
+- picoprobe reset pin is on gpio 28 (instead 6) now
+
+
+[Link to protocol](https://www.sump.org/projects/analyzer/protocol) | 
+[Link to libsigrok](https://github.com/sigrokproject/libsigrok/tree/master/src/hardware/openbench-logic-sniffer)