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+
+Walkera WK-0701 transmitter is supplied with a ready to fly Walkera
+helicopters such as HM36, HM37, HM60. The walkera0701 module enables to use
+this transmitter as joystick
+
+Devel homepage and download:
+http://zub.fei.tuke.sk/walkera-wk0701/
+
+or use cogito:
+cg-clone http://zub.fei.tuke.sk/GIT/walkera0701-joystick
+
+
+Connecting to PC:
+
+At back side of transmitter S-video connector can be found. Modulation
+pulses from processor to HF part can be found at pin 2 of this connector,
+pin 3 is GND. Between pin 3 and CPU 5k6 resistor can be found. To get
+modulation pulses to PC, signal pulses must be amplified.
+
+Cable: (walkera TX to parport)
+
+Walkera WK-0701 TX S-VIDEO connector:
+ (back side of TX)
+     __   __              S-video:                                  canon25
+    /  |_|  \             pin 2 (signal)              NPN           parport
+   / O 4 3 O \            pin 3 (GND)        LED        ________________  10 ACK
+  ( O 2   1 O )                                         | C
+   \   ___   /      2 ________________________|\|_____|/
+    | [___] |                                 |/|   B |\
+     -------        3 __________________________________|________________ 25 GND
+                                                          E
+
+
+I use green LED and BC109 NPN transistor.
+
+Software:
+
+Build kernel with walkera0701 module. Module walkera0701 need exclusive
+access to parport, modules like lp must be unloaded before loading
+walkera0701 module, check dmesg for error messages. Connect TX to PC by
+cable and run jstest /dev/input/js0 to see values from TX. If no value can
+be changed by TX "joystick", check output from /proc/interrupts. Value for
+(usually irq7) parport must increase if TX is on.
+
+
+
+Technical details:
+
+Driver use interrupt from parport ACK input bit to measure pulse length
+using hrtimers.
+
+Frame format:
+Based on walkera WK-0701 PCM Format description by Shaul Eizikovich.
+(downloaded from http://www.smartpropoplus.com/Docs/Walkera_Wk-0701_PCM.pdf)
+
+Signal pulses:
+                   (ANALOG)
+    SYNC      BIN   OCT
+  +---------+      +------+
+  |         |      |      |
+--+         +------+      +---
+
+Frame:
+ SYNC , BIN1, OCT1, BIN2, OCT2 ... BIN24, OCT24, BIN25, next frame SYNC ..
+
+pulse length:
+   Binary values:		Analog octal values:
+
+   288 uS Binary 0		318 uS       000
+   438 uS Binary 1		398 uS       001
+				478 uS       010
+				558 uS       011
+				638 uS       100
+  1306 uS SYNC			718 uS       101
+				798 uS       110
+				878 uS       111
+
+24 bin+oct values + 1 bin value = 24*4+1 bits  = 97 bits
+
+(Warning, pulses on ACK ar inverted by transistor, irq is rised up on sync
+to bin change or octal value to bin change).
+
+Binary data representations:
+
+One binary and octal value can be grouped to nibble. 24 nibbles + one binary
+values can be sampled between sync pulses.
+
+Values for first four channels (analog joystick values) can be found in
+first 10 nibbles. Analog value is represented by one sign bit and 9 bit
+absolute binary value. (10 bits per channel). Next nibble is checksum for
+first ten nibbles.
+
+Next nibbles 12 .. 21 represents four channels (not all channels can be
+directly controlled from TX). Binary representations ar the same as in first
+four channels. In nibbles 22 and 23 is a special magic number. Nibble 24 is
+checksum for nibbles 12..23.
+
+After last octal value for nibble 24 and next sync pulse one additional
+binary value can be sampled. This bit and magic number is not used in
+software driver. Some details about this magic numbers can be found in
+Walkera_Wk-0701_PCM.pdf.
+
+Checksum calculation:
+
+Summary of octal values in nibbles must be same as octal value in checksum
+nibble (only first 3 bits are used). Binary value for checksum nibble is
+calculated by sum of binary values in checked nibbles + sum of octal values
+in checked nibbles divided by 8. Only bit 0 of this sum is used.
+