fix #7 initialization bug (#8)

* fix #7 initialization bug
* remove position from begin()
* extract X9C base class
* update documentation

Author: RobTillaart

README.md

@@ -13,10 +13,12 @@ Arduino Library for X9C10X series digital potentiometer.
 
 ## Description
 
-This experimental library provides a X9C10X base class and four derived classes for specific digital potentiometer.
+This **experimental** library provides a X9C base class, a X9C10X class and 
+four derived classes for specific digital potentiometer.
 
-| type   | resistance | tested  |  notes       |
+| class  | resistance | tested  |  notes       |
 |:------:|:----------:|:-------:|:-------------|
+| X9C    | no support |    Y    |  minimalistic base class |
 | X9C10X |    generic |    Y    |  base class  |
 | X9C102 |    1 KΩ    |    N    |  10 \* 10^2  |
 | X9C103 |   10 KΩ    |    Y    |  10 \* 10^3  |
@@ -26,7 +28,23 @@ This experimental library provides a X9C10X base class and four derived classes
 
 _Note: Ω Ohm sign = ALT-234_
 
-The library keeps cache of the position. 
+The X9C10X object keeps track of the position of the potentiometer,
+but the user should set it with **setPosition(pos, true);**
+Otherwise the library and device will probably not be in sync.
+
+Since 0.2.0 the library has a minimal X9C class. See below.
+
+
+### Multiple devices
+
+Multiple devices can be controlled by assigning them an unique selectPin (CS).
+This behaviour is similar to the SPI select pin.
+
+It should be possible to share the U/D and INC lines (not tested) when controlling  multiple X9C devices.
+
+Note: one should select one device at a time.
+Sharing a CS pin or sending pulses to multiple devices at the same time will
+cause the library and devices get oout of sync.
 
 
 ### PINOUT
@@ -42,58 +60,92 @@ The library keeps cache of the position.
 //  GND  | o    o |  Rwiper
 //       +--------+
 //
-//  INC   pulses
-//  U/D   UP = 1 DOWN = 0
-//  VCC   +5V
-//
+//  INC     pulses
+//  U/D     UP = 1   DOWN = 0
+//  VCC     +5V
+//  GND     ground
+//  RH      resistor high end
+//  RL      resistor low end
+//  Rwiper  resistor wiper
+//  CS      chip select
 ```
 
+It is advised to use pull up resistors - e.g. 10 KΩ - on the CS, UD and INC line. 
+This will help the lines to start in a defined state and will
+improve the signal quality. 
+The pulses esp. INC can be quite short, so especially with longer lines the
+quality can become an issue. (not investigated further)
+
 
 ## Interface
 
 
-## X9C10X base class
+## X9C base class
 
-Use **\#include "X9C10X.h"**
+This is the most minimalistic base class. 
+It does not provide position information but that is sometimes enough.
 
-- **X9C10X(uint32_t Ohm = 10000)** Constructor, default initializes the resistance to 10000 Ω. 
-To calibrate one can fill in any other value e.g. 9950 Ω.
-- **void begin(uint8_t pulsePin, uint8_t directionPin, uint8_t selectPin, uint8_t position = 0)** 
-sets the pins used by the device, and resets the position (default to 0).
-The position parameter allows to start the device with a previous stored position. 
-Use this position with care.
+Use **\#include "X9C10X.h"**
 
+- **X9C()** Constructor.
+- **void begin(uint8_t pulsePin, uint8_t directionPin, uint8_t selectPin)** 
+sets the INC, UD and CS pins used by the device.  
 Note: **begin()** has a hard coded 500uS delay so the device can wake up.
+- **void incr()** moves one position up (if possible). 
+- **void decr()** moves one position down (if possible).
+- **void store()** stores the current position in NV-RAM to be used at the next restart. 
+Does not return a value as the position cannot be read from the device.
+So the user should keep track of the position if needed.
 
-Note: multiple devices can be controlled, just by giving them an unique selectPin.
-This behaviour is similar to the SPI select pin.
-
-- **void setPosition(uint8_t position, bool forced = false)** sets the wiper to a position between 0 and 99. The movement is relative to the current (cached) position.
-If forced is set to true, the cached position is ignored and the new position will be cached. 
-- **uint8_t getPosition()** returns the current position.
-- **bool incr()** moves one position up (if possible). 
-Returns true if moved and false if already at end position.
-- **bool decr()** moves one position down (if possible).
-Returns true if moved and false if already at end position.
-- **uint32_t getOhm()** returns the position expressed in Ohm.
-The returned value does depend on the value passed in the constructor.
-- **uint32_t getMaxOhm()** returns the maximum value ( =  parameter from constructor). 
 
+## X9C10X base class
 
-#### Store 
+This class is derived from the X9C class but adds position, Ohm and type information.
 
-Warning: use with care.
+Use **\#include "X9C10X.h"**
 
+- **X9C10X(uint32_t Ohm = 10000)** Constructor, default initializes the resistance to 10000 Ω. 
+To calibrate one can fill in any other (measured) value e.g. 9950 Ω.
+This can be useful e.g. if one sets a fixed resistor parallel over the X9C one.
+- **void begin(uint8_t pulsePin, uint8_t directionPin, uint8_t selectPin)** 
+sets the INC, UD and CS pins used by the device.  
+Note: **begin()** has a hard coded 500uS delay so the device can wake up.
+- **void setPosition(uint8_t position, bool forced = false)** sets the wiper 
+to a position between 0 and 99. 
+The movement is relative to the current (internal) position.
+If forced is set to true, the wiper will be moved to the closest "end" position 
+and from there moved to the requested position. 
+The internal position is replaced by the new position. 
+- **uint8_t getPosition()** returns the current (internal) position. 0..99
+- **bool incr()** moves one position up (if possible). 
+Returns true if moved and false if already at end position
+according to internal position math.
+- **bool decr()** moves one position down (if possible).
+Returns true if moved and false if already at begin position
+according to internal position math.
 - **uint8_t store()** stores the current position in the NVRAM of the device, 
-and returns the current position so it can later be used as position parameter for **begin()**. 
+and returns the current position so it can later be used as position parameter for **setPosition()**.
+Warning: use with care (not tested).
+Note: **store()** blocks for 20 milliseconds.
+
+Note: **begin()** changed in 0.2.0 as the implicit parameter position
+was removed for the explicit function call to **setPosition()**.
+If **setPosition()** is not called, the device uses the last stored
+value as position. Unfortunately the position cannot be read from the device.
+This will result in a mismatch between the internal position and the 
+external one.
 
-If one uses an incorrect parameter position in **begin()** the internal state and the device 
-will probably be out of sync. One way to sync is call **begin()** with the right parameters. 
-The other way is to call **setPosition(0)** followed by **setPosition(99)** (or vice versa) 
-to get a defined internal state.
 
+#### Ohm
 
-#### Calibration
+- **uint32_t getOhm()** returns the position expressed in Ohm.
+The returned value does depend on the value passed in the constructor
+and the current position. 
+Note: value returned might differ a bit from the actual value, see below.
+- **uint32_t getMaxOhm()** returns the maximum value ( =  parameter from constructor). Convenience function.
+- **uint32_t Ohm2Position(uint32_t value, bool invert = false)**
+Calculates (with rounding) the position nearest to the requested value. 
+If **invert == true** it uses the other wiper end as reference.
 
 One can measure the resistance between RH and RL and use this value to set 
 in the constructor. Although the value will not differ much from the datasheet
@@ -109,8 +161,8 @@ There are 4 derived classes, each with a other (appropriate) default value for t
 - **X9C104(uint32_t Ohm = 100000)** idem.
 - **X9C503(uint32_t Ohm = 50000)** idem.
 
-These classes have the same interface as the base class.
-The only difference is that the type is set.
+These classes have the same interface as the X9C10X base class.
+The only difference is that the type is set to a non zero value.
 
 
 #### Performance
@@ -151,21 +203,27 @@ The digital potentiometer (esp 10 KΩ and up) can be used as a voltage divider.
 Connect RL to GND and RH to +5V and you can do 5V in 100 steps of ~0.05V
 A voltage of **3V3** would be **setPosition(66)**. 
 
-Note: check datasheet for the range of the max voltage allowed.
+Note: check datasheet for the range of the max voltage and current allowed.
 
 
 ## Future
 
+- update documentation
 - test different platforms
-- improve the hardcoded 500us delay in **begin()**
 - add error codes ?
-- test **store()**
+- add examples
+- investigate and test **store()**
+- test multiple devices configuration
+
+
+#### won't
+
+- voltage divider example
 - in the constructor rename **Ohm** parameter to value? 
   - The potentiometer can be used as a voltage divider (see above)
     so a better parameter name could be the anonymous **value**.
   - **getOhm()** ==> **getValue()**
   - **getMaxOhm()** ==> **getMaxValue()**
   - think milliVolt, ohm, lux, speed, etc. 
-    User can do this too with **getPosition()**
-
+    User can do this too with **getPosition() * factor**