# Maple-Arduino Compatibility Notes

- Overview
- Software Language/Library Changes
- Shield and Device Compatibility
- Library Porting Status
- Header Numbering and Incompatibilities
- Sketch and Library Porting HOWTO

## Overview

The biggest difference between the Maple and most Arduino boards is that the Maple uses a 32-bit ARM Cortex-M3 architecture chip while the Arduinos have 8-bit Atmel AVR chips. The different instruction set means that machine code (executable binary programs) is incompatible between the two, and a different compiler (actually just a different version of gcc) is required. The compiler for the regular Arduino IDE is the popular "avr-gcc" package; the compiler for the Maple version of the IDE is CodeSourcery's edition of gcc for the ARM EABI target (aka "arm-non-eabi-gcc").

The bitwidth of the processor means that the Maple can process 32-bit operations (like adding two 32-bit integers) in a single instruction, while an Arduino processor would have to split up large operations into several smaller ones. In a lot of cases 8-bit operations are plenty (integers 0-255, single characters of text, etc.), but if you're dealing with higher resolution data, the speed up could be significant. A trade-off is that code could be larger as well; program instructions and memory locations can be up to 32-bits each, which in the worst case would quadruple program size. In reality, the removal of extra instructions and fancy packing together of simple instructions means that programs aren't much larger (or are even smaller).

## Header Numbering and Incompatibilities

The numbering of headers is different; on the Maple each GPIO has a unique number: D0, D1, D2, all the way up to D37 (actually, there are a couple more...). On the Arduino, the analog pins are numbered separately (A0-A5) from the digital pins (D0-D13).

The incompatible hardware differences are:

- i2c port: on most Arduinos, the i2c port is Analog Input 4 (SDA) and Analog Input 5 (SCL); on the Maple, i2c port 1 is D5 (SCL) and D9 (SDA), and i2c port 2 is D29 (SCL) and D30 (SDA). It should be possible to skywire, sacrificing signal quality (increased capacitance). Alternatively, i2c can be bit banged reasonably well in software. This peripheral could potentially be rerouted internally, but we haven't looked into it.
- PWM on D10: all the other standard Arduino PWM headers have PWM functionality on the Maple (D2,D3,D6,D9,D11), but not D10. We did our best! It may be possible to reroute this peripheral internally using low level configuration, but we haven't looked in to it.
- No External Voltage Reference: The Arduino has an Aref pin which allows the use of an external ADC voltage reference; the Maple has an extra GPIO pin (14) with PWM capability in this spot, and does not allow an external voltage reference to be configured.
- EEPROM: the Maple does not have any internal EEPROM. This functionality can be emulated with regular persistent flash memory, or with an external EEPROM chip.
- ISP Programming: the Maple does not use an ISP/ICSP bus for debugging; it uses JTAG.

## Software Language/Library Changes

- 32-bit int: many standard functions either expect or return full 32-bit (4 byte) integer values instead of the regular 16-bit (2 byte)Arduino values.
- pinMode types: any GPIO (including analog pins) can be configured into one of the following modes with a single call to pinMode: OUTPUT, OUTPUT_OPEN_DRAIN, INPUT_FLOATING, INPUT_PULLUP, INPUT_PULLDOWN. Additionally the PWM and Analog Input pins can be configured as INPUT_ANALOG and PWM respectively. See GPIO docs for more info.
- Serial1 syntax: like the Arduino Mega, the Maple has multiple USART ports. By default `Serial` is not mapped to any of them, use `Serial1` through `Serial3` instead.
- `tone()` unimplemented: On the TODO list!
- `pulseIn()` unimplemented: On the TODO list!
- 16-bit PWM
- 12-bit ADC

## Shield and Device Compatibility

Shield/Device | Compatible? | Notes
--- | --- | ---
Ethernet Shield | Yes! | Tested; no library yet
Midi Shield | Yes! | Tested?
XBee Shield | Unknown |
Bluetooth Shield | Unknown |
WiFi Shield | Unknown |
Cellular Shield | Unknown |

## Library Porting Status

Library | Ported? | Notes
--- | --- | ---
Wiring | Not yet | In progress
Ethernet | Not yet | Planned
EEPROM | No | The Maple doesn't have EEPROM; use flash instead. This library could be emulated?
Firmata | Not yet | Planned
Matrix | Not yet |
SoftwareSerial | Not yet | Planned
Sprite | Not yet |
LiquidCrystal | Tested | Will be included in v0.0.7 of IDE
Stepper | Not yet | Planned

## Sketch and Library Porting HOWTO

1. First check the hardware and header differences above and see if your project or shield needs to be modified (eg, add 3.3v level converters or reroute PWM to header D10).
2. Check for ported library functionality. We intend to port all of the core and popular libraries (like Wire, Ethernet, and the LCD screen driver), but we may not be finished.
3. Check for peripheral conflicts; changing the configuration of timers and bus speeds for a feature on one header may impact all the features on that hardware "port"; for example changing the timer prescaler to do long PWM pulses could impact i2c communications on nearby headers.
4. Rewrite any low-level code. This could potentially be very difficult, but hopefully you've used the Arduino libraries to abstract away the registers and other hardware-specific details. Your sketch probably doesn't have any low-level code; a library which wraps a particular peripheral very well may.
5. Redeclare variable sizes if necessary: generics like `int` will probably work unless you depend on side-effects like rollover.
6. Check every pinMode: the Maple has more modes for GPIO pins. For example, make sure to set analog pins to `INPUT_ANALOG` before reading and PWM pins to `PWM` before writing.
7. Modify PWM writes: pinMode must be set to `PWM`, the frequency of the PWM pulse configured, and the duty cycle written with up to 16-bit resolution.
8. Modify ADC reads: analogRead takes the full pin number (not 0-5) and returns a full 12-bit reading. The ADC pin must be set to `INPUT_ANALOG` using pinMode first.
9. Possibly convert all Serial-over-USB communications to use HardwareUsb instead of HardwareSerial. The Maple has a dedicated USB port which is not connected to the USART TX/RX pins in any way.
10. Check timing; Maple clock cycles are just 13.9 nanoseconds, though the peripheral bus speeds (which limit GPIO output) are clocked slower.

### About this Document

A more recent version of this document may be available from the LeafLabs website. Our documentation is versioned on github; you can track changes to the master branch at this link.

This documentation is released under a Creative Commons Attribution-Share Alike 3.0 license.

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