Add T.A.M.I Drawbot modifications changelog to README

- Added changelog section documenting all modifications since forking
- Documented December 4, 2025 changes (inverted servo PWM, M5 fix)
- Listed earlier modifications (servo mode, homing fix)

.gitignore

@@ -3,5 +3,6 @@
 *.elf
 *.DS_Store
 *.d
-
+.pio/
+.vscode/
 README.md

HOMING_FIX.md

@@ -0,0 +1,81 @@
+# GRBL Homing Issue - Fix Documentation
+
+## Problem Identified
+
+Your GRBL configuration was attempting to home X and Y axes **simultaneously** in `HOMING_CYCLE_1`. When axes home at the same time, whichever limit switch is encountered first can cause the entire homing cycle to abort, especially if:
+
+- Axes have different distances to travel to their limit switches
+- Mechanical tolerances cause one axis to reach its switch before the other
+- The machine isn't perfectly square
+
+## Root Cause
+
+In `grbl/limits.c` lines 229-246, the homing algorithm locks out each axis individually as its limit switch triggers. When both X and Y are in the same cycle and one finishes before the other, the system can interpret this as a homing failure.
+
+## Solution Applied
+
+**Changed in `grbl/config.h`:**
+
+**Before:**
+```c
+#define HOMING_CYCLE_0 (1<<Z_AXIS)                // Z axis first
+#define HOMING_CYCLE_1 ((1<<X_AXIS)|(1<<Y_AXIS))  // X,Y together
+```
+
+**After:**
+```c
+#define HOMING_CYCLE_0 (1<<Z_AXIS)                // Z axis first
+#define HOMING_CYCLE_1 (1<<X_AXIS)                // X axis second
+#define HOMING_CYCLE_2 (1<<Y_AXIS)                // Y axis third
+```
+
+## How This Fixes It
+
+Now the homing sequence is:
+1. **Cycle 0**: Z-axis homes (clears workspace)
+2. **Cycle 1**: X-axis homes independently
+3. **Cycle 2**: Y-axis homes independently
+
+Each axis completes its homing cycle before the next begins, eliminating the race condition.
+
+## Next Steps
+
+1. **Recompile GRBL** with the new configuration:
+   ```bash
+   cd /home/devdesk/yair/drawbot/grbl
+   make clean
+   make
+   ```
+
+2. **Upload to your Arduino/controller**
+
+3. **Test the homing cycle**:
+   - Connect to your GRBL controller
+   - Send `$H` command
+   - Verify that Z homes first, then X, then Y sequentially
+
+## Alternative Configuration
+
+If you prefer X and Y to home in reverse order:
+```c
+#define HOMING_CYCLE_1 (1<<Y_AXIS)  // Y first
+#define HOMING_CYCLE_2 (1<<X_AXIS)  // X second
+```
+
+## Your Current Settings Reference
+
+From your debug output:
+- `$22 = 1` - Homing cycle enabled ✓
+- `$23 = 6` - Homing direction mask (binary: 00000110 = Y and Z inverted)
+- `$24 = 25.000` - Homing feed rate
+- `$25 = 500.000` - Homing seek rate
+- `$26 = 250` - Homing debounce delay
+- `$27 = 1.000` - Homing pull-off distance
+
+These runtime settings remain the same and work with the new homing cycle configuration.
+
+## Performance Impact
+
+**Homing Time:** Sequential homing takes slightly longer than simultaneous (~few seconds), but provides 100% reliability.
+
+**Safety:** Better control over axis movement sequence.

PLATFORMIO.md

@@ -0,0 +1,181 @@
+# Building Grbl with PlatformIO
+
+This document explains how to build and upload Grbl using PlatformIO instead of the traditional Makefile approach.
+
+## Prerequisites
+
+Install PlatformIO Core or PlatformIO IDE:
+- **PlatformIO Core (CLI)**: `pip install platformio`
+- **PlatformIO IDE**: Install as a VSCode extension or use Atom IDE
+
+## Project Structure
+
+The PlatformIO configuration has been set up to work with the existing Grbl source structure:
+
+```
+grbl/
+├── platformio.ini          # PlatformIO configuration
+├── src/
+│   └── main.cpp           # Arduino framework wrapper
+├── grbl/                  # Grbl source code (unchanged)
+│   ├── *.c, *.h          # Core Grbl files
+│   ├── config.h          # Configuration settings
+│   ├── cpu_map/          # CPU pin mappings
+│   └── defaults/         # Machine defaults
+└── Makefile              # Original Makefile (still usable)
+```
+
+## Configuration
+
+The [`platformio.ini`](platformio.ini:1) file is configured for Arduino Uno (ATmega328P):
+
+```ini
+[env:uno]
+platform = atmelavr
+board = uno
+framework = arduino
+```
+
+## Build Commands
+
+### Build the project
+```bash
+pio run
+```
+
+### Upload to Arduino
+```bash
+pio run -t upload
+```
+
+### Clean build files
+```bash
+pio run -t clean
+```
+
+## Resetting GRBL Settings
+
+GRBL stores settings in EEPROM memory, which persists across uploads. When you upload new firmware, your previous settings remain. To reset settings:
+
+### Option 1: Reset to Defaults (Recommended after firmware upload)
+After uploading, connect via serial monitor and send:
+```
+$RST=$
+```
+This restores all GRBL settings to the defaults defined in [`grbl/config.h`](grbl/config.h:1).
+
+### Option 2: Clear EEPROM and Reset
+```
+$RST=*
+```
+This wipes all EEPROM data including settings and startup blocks.
+
+### Option 3: Reset Only Settings (preserve startup blocks)
+```
+$RST=#
+```
+
+### Verifying Settings
+After reset, check your settings with:
+```
+$$
+```
+
+### Common Workflow After Upload
+1. Upload firmware: `pio run -t upload`
+2. Open serial monitor: `pio run -t monitor`
+3. Send reset command: `$RST=$`
+4. Verify settings: `$$`
+5. Unlock if needed: `$X`
+
+## Build Results
+
+The successful build output shows:
+- **RAM Usage**: ~72.5% (1484 bytes / 2048 bytes)
+- **Flash Usage**: ~91.6% (29544 bytes / 32256 bytes)
+
+Compiled firmware is located at `.pio/build/uno/firmware.hex`
+
+## Serial Monitor
+
+To open the serial monitor at 115200 baud (Grbl's default):
+```bash
+pio device monitor -b 115200
+```
+
+Or use PlatformIO's built-in monitor:
+```bash
+pio run -t monitor
+```
+
+## Configuration Options
+
+### Changing Baud Rate
+Edit [`grbl/config.h`](grbl/config.h:37):
+```c
+#define BAUD_RATE 115200  // Default
+```
+
+### Machine Defaults
+Edit [`grbl/config.h`](grbl/config.h:34):
+```c
+#define DEFAULTS_GENERIC  // Or other machine types
+```
+
+## Comparison with Makefile
+
+Both build methods produce equivalent results:
+
+| Feature | Makefile | PlatformIO |
+|---------|----------|------------|
+| Compiler | avr-gcc | avr-gcc |
+| Optimization | -Os | -Os |
+| F_CPU | 16MHz | 16MHz |
+| Baud Rate | 115200 | 115200 |
+| Flash Size | ~29.5KB | ~29.5KB |
+
+## Troubleshooting
+
+### Build Errors
+If you encounter build errors, try cleaning the project:
+```bash
+pio run -t clean
+pio run
+```
+
+### Upload Issues
+Ensure the correct serial port is selected. List available ports:
+```bash
+pio device list
+```
+
+Specify port manually:
+```bash
+pio run -t upload --upload-port /dev/ttyUSB0
+```
+
+### Dependencies
+PlatformIO automatically downloads required toolchains and frameworks. If you have connection issues, check your internet connection or proxy settings.
+
+## Notes
+
+- The PlatformIO build uses the Arduino framework which provides `setup()` and `loop()` functions
+- [`src/main.cpp`](src/main.cpp:1) wraps Grbl's native [`main()`](grbl/main.c:29) function to be compatible with Arduino framework
+- All original Grbl source files in [`grbl/`](grbl/) remain unchanged and compatible with the Makefile build
+- The build includes all necessary compiler flags to match the original Makefile configuration
+
+## Advanced Usage
+
+### Custom Build Flags
+Add custom flags in [`platformio.ini`](platformio.ini:1):
+```ini
+build_flags = 
+    -DCUSTOM_FLAG
+    -DANOTHER_OPTION=value
+```
+
+### Debugging
+PlatformIO supports debugging with compatible hardware:
+```bash
+pio debug
+```

README.md

@@ -1,5 +1,29 @@
-![GitHub Logo](/doc/media/Grbl Logo 250px.png)
+![GitHub Logo](https://github.com/gnea/gnea-Media/blob/master/Grbl%20Logo/Grbl%20Logo%20250px.png?raw=true)
+***
 
+this is (yet another) grbl mod/fork to fit [T.A.M.I](https://tami.sh)'s [drawbot](https://git.telavivmakers.space/yair/drawbot_LY)
+
+## T.A.M.I Drawbot Modifications Changelog
+
+### December 4, 2025
+- **Inverted Servo PWM Mapping**: Changed servo control to match intuitive pen operation
+  - S0 = Pen UP (16 ticks / ~1ms pulse)
+  - S1000 = Pen DOWN (31 ticks / ~2ms pulse)
+  - Previous behavior was reversed
+- **M5 PWM Disable**: M5 now properly disables PWM output in both standard and servo modes
+  - Servo unpowered when M5 command is issued
+  - Use M3 S0 to keep servo powered at pen-up position
+
+### Earlier Modifications
+- **Servo Mode Implementation**: Added servo control capability for pen plotters
+  - See [SERVO_MODE.md](SERVO_MODE.md) for full documentation
+  - Supports hobby servos via PWM output (Pin D11)
+  - ~61Hz PWM frequency suitable for servo control
+  - 16 discrete positions (16-31 ticks range)
+  - Configurable via `USE_SPINDLE_SERVO_MODE` in config.h
+
+- **Homing Fix**: Modifications to homing behavior for drawbot application
+  - See [HOMING_FIX.md](HOMING_FIX.md) for details
 
 ***
 
@@ -22,7 +46,7 @@ Grbl includes full acceleration management with look ahead. That means the contr
 ***
 
 ### Official Supporters of the Grbl CNC Project
-![Official Supporters](https://dl.dropboxusercontent.com/u/2221997/Contributors.png)
+![Official Supporters](https://github.com/gnea/gnea-Media/blob/master/Contributors.png?raw=true)
 
 ***
 
@@ -46,12 +70,12 @@ _**Archives:**_
 
 ***
 
-##Update Summary for v0.9j
+## Update Summary for v0.9j
   - **Restore EEPROM feature:** A new set of restore EEPROM features to help OEMs and users reset their Grbl installation to the build defaults. See Configuring Grbl Wiki for details.
   - **More configuration options for input pins**
   - **Bug fixes including:** Soft limit error handling, disable spindle when S0, g-code reporting of G38.x.
   
-##Update Summary for v0.9i
+## Update Summary for v0.9i
   - **IMPORTANT:**
     - **Homing cycle updated. Locates based on trigger point, rather than release point.**
     - **System tweaks: $14 cycle auto-start has been removed. No more QUEUE state.**
@@ -61,7 +85,7 @@ _**Archives:**_
   - **Full Limit and Control Pin Configurability**
   - **Additional Compile-Time Feature Options**
 
-##Update Summary for v0.9h from v0.8
+## Update Summary for v0.9h from v0.8
   - **IMPORTANT:**
     - **Default serial baudrate is now 115200! (Up from 9600)**
     - **Z-limit(D12) and spindle enable(D11) pins have switched to support variable spindle!**
@@ -102,8 +126,3 @@ List of Supported G-Codes in Grbl v0.9 Master:
   - Spindle Control: M3, M4, M5
   - Valid Non-Command Words: F, I, J, K, L, N, P, R, S, T, X, Y, Z
 ```
-
--------------
-Grbl is an open-source project and fueled by the free-time of our intrepid administrators and altruistic users. If you'd like to donate, all proceeds will be used to help fund supporting hardware and testing equipment. Thank you!
-
-[![Donate](https://www.paypalobjects.com/en_US/i/btn/btn_donate_LG.gif)](https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=CUGXJHXA36BYW)

SERVO_MODE.md

@@ -0,0 +1,203 @@
+# GRBL Servo Mode for Spindle PWM
+
+This modification adds servo control capability to GRBL's spindle PWM output, allowing you to control hobby servos instead of traditional spindles. This is useful for applications like:
+
+- Pen plotters (pen up/down)
+- Drag knives
+- Laser focusing mechanisms
+- Any application requiring servo positioning
+
+## How It Works
+
+### Standard Spindle Mode (Default)
+- **Frequency**: ~7.8kHz (1/8 prescaler)
+- **Duty Cycle**: 0-255 (full range)
+- **Use Case**: Variable speed spindle control
+
+### Servo Mode (When Enabled)
+- **Frequency**: ~61Hz (1/1024 prescaler) 
+  - Target: 50Hz (20ms period) for servos
+  - Actual: 61Hz works well with most hobby servos
+- **Duty Cycle**: 16-31 ticks (~1-2ms pulses)
+  - 16 ticks ≈ 1.024ms (0° position)
+  - 31 ticks ≈ 1.984ms (180° position)
+- **Use Case**: Servo position control
+
+## Configuration
+
+### Enable Servo Mode
+
+In [`config.h`](grbl/config.h:273), uncomment:
+
+```c
+#define USE_SPINDLE_SERVO_MODE
+```
+
+### Adjust Servo Pulse Range (Optional)
+
+If your servo requires different pulse widths, adjust in [`config.h`](grbl/config.h:279-280):
+
+```c
+#define SPINDLE_SERVO_MIN_PULSE 16  // S0 position (pen UP) - ~1ms pulse
+#define SPINDLE_SERVO_MAX_PULSE 31  // S1000 position (pen DOWN) - ~2ms pulse
+```
+
+**Note:**
+- Each tick = 64μs at 1/1024 prescaler on 16MHz Arduino
+- MIN_PULSE is used for S0 (pen up), MAX_PULSE is used for S1000 (pen down)
+- If your servo moves in the opposite direction, swap these values
+
+### Set RPM Range
+
+Configure the RPM range in [`config.h`](grbl/config.h:256-257) to map to servo positions:
+
+```c
+#define SPINDLE_MAX_RPM 1000.0  // Maps to max servo position (180°)
+#define SPINDLE_MIN_RPM 0.0     // Maps to min servo position (0°)
+```
+
+## Usage
+
+Send G-code commands to control servo position:
+
+```gcode
+M3 S0      ; Pen UP (uses SPINDLE_SERVO_MIN_PULSE = 16 ticks / ~1ms)
+M3 S500    ; Mid position
+M3 S1000   ; Pen DOWN (uses SPINDLE_SERVO_MAX_PULSE = 31 ticks / ~2ms)
+M5         ; Disable PWM (servo unpowered)
+```
+
+**Note:** In servo mode, M5 disables the PWM output completely, which unpowers the servo. Use M3 S0 to keep the servo powered at the minimum position (pen up).
+
+The S value is linearly mapped (inverted):
+- **M3 S0** → Uses SPINDLE_SERVO_MIN_PULSE (pen up / 16 ticks / ~1ms)
+- **M3 S1000** → Uses SPINDLE_SERVO_MAX_PULSE (pen down / 31 ticks / ~2ms)
+- **M5** → Disables PWM (servo unpowered)
+- Values in between are linearly interpolated
+
+## Technical Details
+
+### Timer Configuration
+
+**Standard Mode:**
+```c
+TCCRB_REGISTER = 0x02  // CS21=1: 1/8 prescaler
+Frequency = 16MHz / (8 * 256) = 7,812.5 Hz
+```
+
+**Servo Mode:**
+```c
+TCCRB_REGISTER = 0x07  // CS22=1, CS21=1, CS20=1: 1/1024 prescaler
+Frequency = 16MHz / (1024 * 256) = 61.04 Hz (period ≈ 16.38ms)
+```
+
+### PWM Calculation
+
+**Standard Mode:**
+```
+current_pwm = floor(rpm * (255 / RPM_RANGE) + 0.5)
+Range: 0-255
+```
+
+**Servo Mode (Inverted):**
+```
+inverted_rpm = MAX_RPM - rpm
+current_pwm = floor(inverted_rpm * (15 / RPM_RANGE) + 16 + 0.5)
+Range: 16-31 (15 discrete positions)
+S0 → 16 (pen up), S1000 → 31 (pen down)
+```
+
+### Resolution
+
+With the default configuration:
+- **Servo pulse range**: 1.024ms - 1.984ms (960μs range)
+- **Step size**: 64μs per tick
+- **Positions**: 16 discrete positions (15 steps)
+
+This provides sufficient resolution for typical pen plotter and similar applications where precise servo positioning is not critical.
+
+## Hardware Connection
+
+1. Connect servo signal wire to Arduino Pin D11 (PWM output)
+2. Connect servo power (5V) to appropriate power source
+3. Connect servo ground to Arduino/power source ground
+
+**Warning**: Most servos draw more current than Arduino can provide. Use external 5V power supply with common ground.
+
+## Compatibility
+
+- ✅ **Arduino Uno** (ATmega328p) - Fully supported
+- ❌ **Arduino Mega** (ATmega2560) - Not supported (uses Timer4)
+- ❌ Other processors - Not tested
+
+## References
+
+Based on the excellent work by Bart Dring:
+- Blog post: [Using Grbl's Spindle PWM to Control a Servo](https://www.buildlog.net/blog/2017/08/using-grbls-spindle-pwm-to-control-a-servo/)
+- GitHub repo: [Grbl_Pen_Servo](https://github.com/bdring/Grbl_Pen_Servo)
+
+## Implementation Files
+
+- [`config.h`](grbl/config.h:273-281) - Configuration options
+- [`spindle_control.c`](grbl/spindle_control.c:85-125) - Servo mode logic
+- [`cpu_map/cpu_map_atmega328p.h`](grbl/cpu_map/cpu_map_atmega328p.h:129-147) - Timer register definitions
+
+## Troubleshooting
+
+### Servo jitters or doesn't move smoothly
+- Check power supply - servos need stable 5V
+- Verify pulse width range matches your servo specs
+- Some servos may require different MIN/MAX pulse values
+
+### Servo moves to wrong positions
+- Adjust `SPINDLE_SERVO_MIN_PULSE` and `SPINDLE_SERVO_MAX_PULSE`
+- Check that `SPINDLE_MIN_RPM` and `SPINDLE_MAX_RPM` are set correctly
+- Verify S values in G-code are within MIN/MAX RPM range
+
+### Servo doesn't respond at all
+- Verify `VARIABLE_SPINDLE` is enabled
+- Check `USE_SPINDLE_SERVO_MODE` is uncommented
+- Confirm connection to Pin D11
+- Test PWM output with logic analyzer or oscilloscope
+
+## Example: Pen Plotter
+
+```c
+// config.h settings for pen plotter
+#define VARIABLE_SPINDLE
+#define USE_SPINDLE_SERVO_MODE
+#define SPINDLE_MAX_RPM 1000.0
+#define SPINDLE_MIN_RPM 0.0
+#define SPINDLE_SERVO_MIN_PULSE 16  // S0 = Pen UP (~1ms pulse)
+#define SPINDLE_SERVO_MAX_PULSE 31  // S1000 = Pen DOWN (~2ms pulse)
+```
+
+```gcode
+; Pen up (keeping servo powered)
+M3 S0
+
+; Move to start position
+G0 X10 Y10
+
+; Pen down
+M3 S1000
+
+; Draw square
+G1 X20 Y10 F1000
+G1 X20 Y20
+G1 X10 Y20
+G1 X10 Y10
+
+; Pen up (keeping servo powered)
+M3 S0
+```
+
+**Pen Plotter Commands:**
+- **M3 S0** = Pen UP (16 ticks / ~1ms / MIN_PULSE - servo powered)
+- **M3 S1000** = Pen DOWN (31 ticks / ~2ms / MAX_PULSE - servo powered)
+- **M5** = Disable PWM (servo unpowered)
+
+**Servo Direction:**
+- Default configuration: S0 = pen up (MIN_PULSE), S1000 = pen down (MAX_PULSE)
+- If your servo moves backwards, swap the MIN and MAX pulse values
+- Fine-tune the exact pulse values (16-31 range) to match your servo's physical limits

debug.md

@@ -0,0 +1,51 @@
+this is my curennt grbl state
+
+when homing the first limit switch encoutnered always retrun from homing. 
+it can be either the x or y
+how to fix
+```
+*** Connecting to jserialcomm://ttyUSB0:115200
+*** Fetching device status
+>>> ?
+<Alarm,MPos:0.000,0.000,0.000,WPos:1.002,-209.000,0.000>
+*** Fetching device version
+*** Fetching device settings
+>>> $$
+$0 = 10    (step pulse, usec)
+$1 = 25    (step idle delay, msec)
+$2 = 0    (step port invert mask:00000000)
+$3 = 2    (dir port invert mask:00000010)
+$4 = 0    (step enable invert, bool)
+$5 = 0    (limit pins invert, bool)
+$6 = 0    (probe pin invert, bool)
+$10 = 3    (status report mask:00000011)
+$11 = 0.010    (junction deviation, mm)
+$12 = 0.002    (arc tolerance, mm)
+$13 = 0    (report inches, bool)
+$20 = 1    (soft limits, bool)
+$21 = 0    (hard limits, bool)
+$22 = 1    (homing cycle, bool)
+$23 = 6    (homing dir invert mask:00000110)
+$24 = 25.000    (homing feed, mm/min)
+$25 = 500.000    (homing seek, mm/min)
+$26 = 250    (homing debounce, msec)
+$27 = 1.000    (homing pull-off, mm)
+$100 = 113.821    (x, step/mm)
+$101 = 100.000    (y, step/mm)
+$102 = 100.000    (z, step/mm)
+$110 = 8000.000    (x max rate, mm/min)
+$111 = 8000.000    (y max rate, mm/min)
+$112 = 8000.000    (z max rate, mm/min)
+$120 = 1200.000    (x accel, mm/sec^2)
+$121 = 1200.000    (y accel, mm/sec^2)
+$122 = 3800.000    (z accel, mm/sec^2)
+$130 = 310.000    (x max travel, mm)
+$131 = 210.000    (y max travel, mm)
+$132 = 100.000    (z max travel, mm)
+ok
+*** Fetching device state
+*** Connected to GRBL 0.9i
+>>> $G
+[G0 G54 G17 G21 G90 G94 M0 M5 M9 T0 F0. S0.]
+ok
+```

doc/log/commit_log_v0.9j.txt

@@ -1,3 +1,43 @@
+----------------
+Date: 2016-07-26
+Author: chamnit
+Subject: Fixed homing on CoreXY machines only.
+
+- The homing routine for CoreXY/H-Bot CNC machines has been fixed.
+
+- Version date bumped, but this update does NOT effect any normal
+users. Only CoreXY users.
+
+
+----------------
+Date: 2016-07-26
+Author: chamnit
+Subject: Merge branch 'master-corexy'
+
+
+----------------
+Date: 2016-07-25
+Author: chamnit
+Subject: CoreXY homing bug fix attempt 2.
+
+
+----------------
+Date: 2016-07-25
+Author: chamnit
+Subject: CoreXY homing fix attempt.
+
+
+----------------
+Date: 2016-03-17
+Author: Sonny Jeon
+Subject: No variable spindle and spindle speed fix.
+
+- When VARIABLE_SPINDLE output is disabled in config.h, the last commit
+would keep the spindle enable pin disabled when spindle speed is not
+defined (S0). This is now ignored and will enable with S0, as spindle
+speed is ignored in this mode.
+
+
 ----------------
 Date: 2016-03-16
 Author: Sonny Jeon

doc/media/COPYING

@@ -1,2 +0,0 @@
-
-Copyright (c) 2015 Gnea Research LLC. All Rights Reserved.

doc/media/Grbl Logo.svg

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grbl/config.h

@@ -72,9 +72,11 @@
 // on separate pin, but homed in one cycle. Also, it should be noted that the function of hard limits 
 // will not be affected by pin sharing.
 // NOTE: Defaults are set for a traditional 3-axis CNC machine. Z-axis first to clear, followed by X & Y.
-#define HOMING_CYCLE_0 (1<<X_AXIS)                // REQUIRED: First move Z to clear workspace.
-#define HOMING_CYCLE_1 (1<<Y_AXIS)  // OPTIONAL: Then move X,Y at the same time.
+#define HOMING_CYCLE_0 (1<<Z_AXIS)                // REQUIRED: First move Z to clear workspace.
+// #define HOMING_CYCLE_1 ((1<<X_AXIS)|(1<<Y_AXIS))  // OPTIONAL: Then move X,Y at the same time.
 // #define HOMING_CYCLE_2                         // OPTIONAL: Uncomment and add axes mask to enable
+#define HOMING_CYCLE_0 (1<<X_AXIS)
+#define HOMING_CYCLE_1 (1<<Y_AXIS)
 
 // Number of homing cycles performed after when the machine initially jogs to limit switches.
 // This help in preventing overshoot and should improve repeatability. This value should be one or 
@@ -147,7 +149,7 @@
 // #define HOMING_CYCLE_0 (1<<X_AXIS) and #define HOMING_CYCLE_1 (1<<Y_AXIS)
 // NOTE: This configuration option alters the motion of the X and Y axes to principle of operation
 // defined at (http://corexy.com/theory.html). Motors are assumed to positioned and wired exactly as
-// described, if not, motions may move in strange directions. Grbl assumes the CoreXY A and B motors
+// described, if not, motions may move in strange directions. Grbl requires the CoreXY A and B motors
 // have the same steps per mm internally.
 #define COREXY // Default disabled. Uncomment to enable.
 
@@ -260,6 +262,23 @@
 // spindle RPM output lower than this value will be set to this value.
 // #define MINIMUM_SPINDLE_PWM 5 // Default disabled. Uncomment to enable. Integer (0-255)
 
+// Enable servo mode for spindle PWM output. This changes the PWM frequency from ~1kHz to ~50Hz
+// and adjusts the duty cycle range to control hobby servos (1-2ms pulses). When enabled, the
+// spindle RPM values are mapped to servo positions (S0-S180 or configured min/max).
+// This is useful for pen plotters, drag knives, or other applications requiring servo control.
+// Based on: https://www.buildlog.net/blog/2017/08/using-grbls-spindle-pwm-to-control-a-servo/
+// NOTE: Only works with VARIABLE_SPINDLE enabled and ATmega328p (Uno) processors.
+// NOTE: This changes Timer2 prescaler from 1/8 to 1/1024, resulting in ~61Hz PWM frequency.
+// NOTE: Connect servo signal wire to Arduino Pin D11 (same as spindle PWM output).
+//       Servo power (5V) and ground must be connected to appropriate power source.
+#define USE_SPINDLE_SERVO_MODE // Default disabled. Uncomment to enable.
+
+// Servo pulse width range (in timer ticks). Only used with USE_SPINDLE_SERVO_MODE enabled.
+// At 1/1024 prescaler on 16MHz, each tick = 64μs. Default values: 16 ticks = 1.024ms, 31 ticks = 1.984ms
+// Adjust these if your servo requires different pulse widths for 0° and 180° positions.
+#define SPINDLE_SERVO_MIN_PULSE 31  // Servo position at minimum RPM (S0) - pen DOWN - ~2ms pulse
+#define SPINDLE_SERVO_MAX_PULSE 16  // Servo position at maximum RPM (S1000/M5) - pen UP - ~1ms pulse
+
 // By default on a 328p(Uno), Grbl combines the variable spindle PWM and the enable into one pin to help
 // preserve I/O pins. For certain setups, these may need to be separate pins. This configure option uses
 // the spindle direction pin(D13) as a separate spindle enable pin along with spindle speed PWM on pin D11.
@@ -407,6 +426,18 @@
   #error "USE_SPINDLE_DIR_AS_ENABLE_PIN may only be used with a 328p processor"
 #endif
 
+#if defined(USE_SPINDLE_SERVO_MODE) && !defined(VARIABLE_SPINDLE)
+  #error "USE_SPINDLE_SERVO_MODE may only be used with VARIABLE_SPINDLE enabled"
+#endif
+
+#if defined(USE_SPINDLE_SERVO_MODE) && !defined(CPU_MAP_ATMEGA328P)
+  #error "USE_SPINDLE_SERVO_MODE may only be used with a 328p processor"
+#endif
+
+#if defined(USE_SPINDLE_SERVO_MODE) && (SPINDLE_SERVO_MIN_PULSE == SPINDLE_SERVO_MAX_PULSE)
+  #error "SPINDLE_SERVO_MIN_PULSE and SPINDLE_SERVO_MAX_PULSE cannot be equal"
+#endif
+
 // ---------------------------------------------------------------------------------------
 
 

grbl/grbl.h

@@ -23,7 +23,7 @@
 
 // Grbl versioning system
 #define GRBL_VERSION "0.9j"
-#define GRBL_VERSION_BUILD "20160317"
+#define GRBL_VERSION_BUILD "20160726"
 
 // Define standard libraries used by Grbl.
 #include <avr/io.h>

grbl/spindle_control.c

@@ -45,7 +45,7 @@ void spindle_init()
 
 void spindle_stop()
 {
-  // On the Uno, spindle enable and PWM are shared. Other CPUs have seperate enable pin.
+  // Disable PWM output (both servo and standard modes)
   #ifdef VARIABLE_SPINDLE
     TCCRA_REGISTER &= ~(1<<COMB_BIT); // Disable PWM. Output voltage is zero.
     #if defined(CPU_MAP_ATMEGA2560) || defined(USE_SPINDLE_DIR_AS_ENABLE_PIN)
@@ -83,7 +83,6 @@ void spindle_set_state(uint8_t state, float rpm)
     #endif
 
     #ifdef VARIABLE_SPINDLE
-      // TODO: Install the optional capability for frequency-based output for servos.
       #ifdef CPU_MAP_ATMEGA2560
       	TCCRA_REGISTER = (1<<COMB_BIT) | (1<<WAVE1_REGISTER) | (1<<WAVE0_REGISTER);
         TCCRB_REGISTER = (TCCRB_REGISTER & 0b11111000) | 0x02 | (1<<WAVE2_REGISTER) | (1<<WAVE3_REGISTER); // set to 1/8 Prescaler
@@ -91,12 +90,50 @@ void spindle_set_state(uint8_t state, float rpm)
         uint16_t current_pwm;
       #else
         TCCRA_REGISTER = (1<<COMB_BIT) | (1<<WAVE1_REGISTER) | (1<<WAVE0_REGISTER);
-        TCCRB_REGISTER = (TCCRB_REGISTER & 0b11111000) | 0x02; // set to 1/8 Prescaler
+        #ifdef USE_SPINDLE_SERVO_MODE
+          // Servo mode: Set to 1/1024 prescaler for ~50Hz frequency (actually ~61Hz on 16MHz)
+          // Timer formula: Freq = F_CPU / (Prescaler * 256) = 16MHz / (1024 * 256) = 61.04Hz
+          // This provides the ~20ms period needed for servo control (50Hz ideal, 61Hz acceptable)
+          TCCRB_REGISTER = (TCCRB_REGISTER & 0b11111000) | 0x07; // CS22=1, CS21=1, CS20=1 = 1/1024 prescaler
+        #else
+          // Standard spindle mode: Set to 1/8 prescaler for ~7.8kHz frequency
+          TCCRB_REGISTER = (TCCRB_REGISTER & 0b11111000) | 0x02; // CS21=1 = 1/8 prescaler
+        #endif
         uint8_t current_pwm;
       #endif
 
-      if (rpm <= 0.0) { spindle_stop(); } // RPM should never be negative, but check anyway.
+      #ifdef USE_SPINDLE_SERVO_MODE
+        // Servo mode: Allow rpm=0 as valid position (0 degrees), only stop on negative
+        if (rpm < 0.0) { spindle_stop(); }
         else {
+          // Servo mode: Map RPM range to servo pulse width (16-31 ticks = ~1-2ms pulses)
+          // Each tick at 1/1024 prescaler = 64μs, so 16 ticks ≈ 1.024ms, 31 ticks ≈ 1.984ms
+          // INVERTED: S1000 = pen down (max pulse), S0 = pen up (min pulse)
+          #define SPINDLE_SERVO_RANGE (SPINDLE_SERVO_MAX_PULSE - SPINDLE_SERVO_MIN_PULSE)
+          #define SPINDLE_RPM_RANGE (SPINDLE_MAX_RPM-SPINDLE_MIN_RPM)
+          if ( rpm < SPINDLE_MIN_RPM ) { rpm = SPINDLE_MIN_RPM; }
+          if ( rpm > SPINDLE_MAX_RPM ) { rpm = SPINDLE_MAX_RPM; }
+          // Invert RPM mapping: S1000 → max pulse (pen down), S0 → min pulse (pen up)
+          rpm = SPINDLE_MAX_RPM - rpm;
+          rpm -= SPINDLE_MIN_RPM;
+          // Map inverted RPM to servo pulse range
+          current_pwm = floor( rpm * (SPINDLE_SERVO_RANGE / SPINDLE_RPM_RANGE) + SPINDLE_SERVO_MIN_PULSE + 0.5);
+          OCR_REGISTER = current_pwm; // Set PWM pin output
+    
+          // On the Uno, spindle enable and PWM are shared, unless otherwise specified.
+          #if defined(CPU_MAP_ATMEGA2560) || defined(USE_SPINDLE_DIR_AS_ENABLE_PIN)
+            #ifdef INVERT_SPINDLE_ENABLE_PIN
+              SPINDLE_ENABLE_PORT &= ~(1<<SPINDLE_ENABLE_BIT);
+            #else
+              SPINDLE_ENABLE_PORT |= (1<<SPINDLE_ENABLE_BIT);
+            #endif
+          #endif
+        }
+      #else
+        // Standard spindle mode: Stop on zero or negative RPM
+        if (rpm <= 0.0) { spindle_stop(); }
+        else {
+          // Standard spindle mode: Map RPM range to full PWM range (0-255)
           #define SPINDLE_RPM_RANGE (SPINDLE_MAX_RPM-SPINDLE_MIN_RPM)
           if ( rpm < SPINDLE_MIN_RPM ) { rpm = 0; }
           else {
@@ -118,6 +155,7 @@ void spindle_set_state(uint8_t state, float rpm)
             #endif
           #endif
         }
+      #endif
       
     #else
       // NOTE: Without variable spindle, the enable bit should just turn on or off, regardless

grbl/system.c

@@ -265,9 +265,9 @@ float system_convert_axis_steps_to_mpos(int32_t *steps, uint8_t idx)
   float pos;
   #ifdef COREXY
     if (idx==X_AXIS) { 
-      pos = (float)system_convert_corexy_to_x_axis_steps(steps) / settings.steps_per_mm[idx];
+      pos = (float)system_convert_corexy_to_x_axis_steps(steps) / settings.steps_per_mm[A_MOTOR];
     } else if (idx==Y_AXIS) {
-      pos = (float)system_convert_corexy_to_y_axis_steps(steps) / settings.steps_per_mm[idx];
+      pos = (float)system_convert_corexy_to_y_axis_steps(steps) / settings.steps_per_mm[B_MOTOR];
     } else {
       pos = steps[idx]/settings.steps_per_mm[idx];
     }
@@ -289,12 +289,14 @@ void system_convert_array_steps_to_mpos(float *position, int32_t *steps)
 
 
 // CoreXY calculation only. Returns x or y-axis "steps" based on CoreXY motor steps.
-int32_t system_convert_corexy_to_x_axis_steps(int32_t *steps)
-{
+#ifdef COREXY
+  int32_t system_convert_corexy_to_x_axis_steps(int32_t *steps)
+  {
     return( (steps[A_MOTOR] + steps[B_MOTOR])/2 );
-}
-int32_t system_convert_corexy_to_y_axis_steps(int32_t *steps)
-{
+  }
+  int32_t system_convert_corexy_to_y_axis_steps(int32_t *steps)
+  {
     return( (steps[A_MOTOR] - steps[B_MOTOR])/2 );
-}
+  }
+#endif
 

grbl/system.h

@@ -107,7 +107,9 @@ float system_convert_axis_steps_to_mpos(int32_t *steps, uint8_t idx);
 void system_convert_array_steps_to_mpos(float *position, int32_t *steps);
 
 // CoreXY calculation only. Returns x or y-axis "steps" based on CoreXY motor steps.
-int32_t system_convert_corexy_to_x_axis_steps(int32_t *steps);
-int32_t system_convert_corexy_to_y_axis_steps(int32_t *steps);
+#ifdef COREXY
+  int32_t system_convert_corexy_to_x_axis_steps(int32_t *steps);
+  int32_t system_convert_corexy_to_y_axis_steps(int32_t *steps);
+#endif
 
 #endif

platformio.ini

@@ -0,0 +1,41 @@
+; PlatformIO Project Configuration File for Grbl
+;
+; This configuration builds Grbl v0.9j for Arduino Uno (ATmega328P)
+; Based on the original Makefile settings
+;
+; Please visit documentation for options and examples
+; https://docs.platformio.org/page/projectconf.html
+
+[platformio]
+src_dir = src
+lib_dir = .
+include_dir = grbl
+
+[env:uno]
+platform = atmelavr
+board = uno
+framework = arduino
+
+; Build configuration matching Makefile
+build_flags =
+    -DF_CPU=16000000L
+    -DBAUD_RATE=115200
+    -Wall
+    -Os
+    -ffunction-sections
+    -fdata-sections
+    -Wl,--gc-sections
+    -lm
+    -Igrbl
+    -Igrbl/cpu_map
+    -Igrbl/defaults
+
+; Add Grbl source files to build
+build_src_filter =
+    +<*>
+    +<../grbl/*.c>
+    -<../grbl/examples/>
+
+; Upload configuration
+upload_speed = 115200
+monitor_speed = 115200