Imported from GitHub: h4ctar/retro-gauge · commit a7ec970 · license GPL-3.0

Description

Open Source Motorcycle Tachometer

README

Retro Gauge

Introduction

This is an open source motorcycle tacho project with the following features:

• Analog tachometer
• Digital speedometer
• Odometer
• Trip meter
• Battery voltage
• Standard indicators (turn signals, oil pressure, neutral and high beam)
• Clock

It is housed in a classic round casing, with a retro looking 8 digit starburst LCD.

The toolchain is all open source; the PCB's are designed with KiCad and the case is designed in FreeCAD.

System Design

The system design is simple, the only interesting decision is that the indicators all go via the microcontroller, this lets us do fun things with the indicators (test them when powered on or reuse the oil pressure indicator for other warnings).

Microcontroller

An ATmega328P-PU microcontroller will be used to process all the inputs, and drive the outputs.

Here are all the required inputs and outputs:

Function	Type	Number of pins
Tacho	Digital input	1
Speedo	Digital input	1
Oil pressure	Digital input	1
Turn signals	Digital input	1
Neutral	Digital input	1
High beam	Digital input	1
Mode button	Digital input	1
Battery voltage	Analog input	1
Stepper motor	Digital output	4 (could be 3)
RBG LEDs	Digital output	1
LCD	Digital output	3
ISP	Digital IO	3 (can be shared)
Real time clock	Digital IO	2

For a total of 18 digital IOs and one analog input, the ATmega328P-PU has more than enough at 23 programmable IO lines.

The ATmega328P-PU also has 1024 bytes of EEPROM which can be used to store the configuration, trip and odometer values.

An external 16 MHz crystal will be used to run the ATmega328P-PU at it's fastest clock speed.

The microcontroller will be programmed with an in-circuit serial programmer (ICSP). A bootloader doesnt need to be burned when using ICSP.

The low byte fuses configure the clock:

Bit	Name	Description	Value
7	CKDIV8	When set, divides the clock speed by 8	1
6	CKOUT	When set, the clock pulse is output on PB0	1
5	SUT1	Sets start up delay time	1
4	SUT0		1
3	CKSEL3	Sets the clock source	1
2	CKSEL2		1
1	CKSEL1		1
0	CKSEL0		1

The high byte fuses:

Bit	Name	Description	Value
7	RSTDISBL	External reset disable	1
6	DWEN	Debug wire enable	1
5	SPIEN	Enable serial programming	0
4	WDTON	Watchdog timer always on	1
3	EESAVE	Preserve EEPROM memory through chip erase	0
2	EBOOTSZ1	Sets the bootloder size memory	1
1	EBOOTSZ0		1
0	BOOTRST	Select the reset vector	0

The extended fuses:

Bit	Name	Description	Value
7		Not used	1
6			1
5			1
4			1
3			1
2	BODLEVEL2	Sets the brown-out detector level	1
1	BODLEVEL1		0
0	BODLEVEL0		1

There is a simple Makefile that compiles and links the code and has targets to burn the fuses and the code.

Power Supply

A linear voltage regulator will be used to convert the bikes rough 14V down to a smooth 5V for the rest of the system. A smoothing capacitor is added on the bikes side to help smooth out the input before it gets to the regulator.

Indicator Lights

There are indicator lights for low oil pressure, neutral, high beam and turn signals.

The oil pressure and neutral sensors are simple switches to ground. So these will be directly connected to digital input pins. I'm not sure if the internal pull ups will be good enough so there will be provision on the PCB for external pull up resistors. There will be a diode to protect against connecting this pin to 12V on the bike.

The highbeam and turn signals indicators should light up when they see 12V. We'll use an optocoupler to give some extra protection. The left and right turn signals need to go through diodes to stop them turning each other on.

Diode forward voltage: 0.715V
Photodiode forward voltage: 1.3V
Photodiode forward current: 10mA
R1 = (14V - 0.715V - 1.3V) / 10mA = 1200 ohm ~= 1k ohm

CTR: 50%
Output current: 50% of 10mA = 5mA
Output voltage: 4V
R2 = (5V - 4V) / 5mA = 200 ohm

Tachometer

There are a few choices for the tachometer pickup, an inductive or capacitive pickup on one of the spark plug wires, or a hall effect sensor on a rotating part of the engine. The most robust solution is to go with the hall effect sensor, and the most universal location for it is in replacement of the mechanical drive off the camshaft of the original tachometer.

Speedometer

The speedometer will also be a hall effect pickup, but will be a simpler design with a few magnets on the wheel and an NPN hall effect sensor mounted on an axel spacer.

Real Time Clock

A PCF8563T RTC module will be used to keep the time for the clock. It will have a coin cell to keep the clock ticking when the key is off.

LCD

The VIM-878-DP-FC-S-LV LCD display will be used. It is a transflective LCD so works well in both bright sunlight and at night with a backlight. It is an 8 character 14 (plus 2) segment "starburst" display. It has a total of (14 + 2) * 8 = 128 segments and uses 4 backplanes.

The HT1621B will be used to drive the LCD. There are some existing Arduino libraries to interface with it. It can drive 32 segments by 4 commons which is exactly what is required to drive the selected LCD.

The schematic of the driver to the display follows the application notes in the datasheet and is optimised for the PCB layout. It is controlled with 3 pins that are also used by the ICSP.

This table has the LCD segments from the datasheet and what memory address they are according to the PCB layout:

LCD Pin	COM0	COM1	COM2	COM3	Memory address
1	1D	1E	1F	CA1	31
2	1L	1K	1J	1I	30
3	2D	2E	2F	CA2	29
4	2L	2K	2J	2I	27
5	3D	3E	3F	CA3	28
6	3L	3K	3J	3I	26
7	4D	4E	4F	CA4	25
8	4L	4K	4J	4I	24
9	5D	5E	5F	CA5	23
10	5L	5K	5J	5I	22
11	6D	6E	6F	CA6	21
12	6L	6K	6J	6I	20
13	7D	7E	7F	CA7	19
14	7L	7K	7J	7I	18
15	8D	8E	8F	CA8	17
16	8L	8K	8J	8I	16
17	COM0
18		COM1
19			COM2
20				COM3
21	DP8	8C	8B	8A	15
22	8M	8N	8G	8H	14
23	DP7	7C	7B	7A	13
24	7M	7N	7G	7H	12
25	DP6	6C	6B	6A	11
26	6M	6N	6G	6H	10
27	DP5	5C	5B	5A	9
28	5M	5N	5G	5H	8
29	DP4	4C	4B	4A	7
30	4M	4N	4G	4H	6
31	DP3	3C	3B	3A	5
32	3M	3N	3G	3H	4
33	DP2	2C	2B	2A	3
34	2M	2N	2G	2H	2
35	DP1	1C	1B	1A	1
36	1M	1N	1G	1H	0

From this table we can see that each digit is powered by 4 LCD pins that map to 4 memory addresses. For example digit 1 is LCD pins 35, 1, 2 and 36, which map to memory address' 1, 31, 30 and 0.

The font maps ASCII characters to the segments in order CA,F,E,D,I,J,K,L,A,B,C,DP,H,G,N,M. The segments are in this order so the nibbles match the memory layout of the driver.

The address mapping maps digits to the 4 memory addresses that the font nibbles should be written to.

Digit	Address'
1	1,31,30, 0
2	3,29,27, 2
3	5,28,26, 4
4	7,25,24, 6
5	9,23,22, 8
6	11,21,20,10
7	13,19,18,12
8	15,17,16,14

This font table and address mapping should be enough to display text on the LCD.

Stepper motor

The X27.168 stepper motor will be used, it is designed for gauges and can be directly driven from a microcontroller.

People on the internet say that clamping diodes are required, so they will be added.

The X25 datasheet shows the required pattern to step the motor.

Indicators

PCB Design

There are three PCBs: the main board, the sensor board and the face board. The boards are stacked using standoffs and are connected together with 2.54mm stacking header pins.

The main board has the microcontroller, stepper motor, LCD, real time clock and indicators. The sensor board has the power supply and the signal conditioning. The face board is the dial face and has transparent icons for the indicators.

Case Design

The case is 3D printed out of ASA.

EEPROM

The ODO, trip and configuration will be stored in EEPROM.

EEPROM has like 100,000 writes before it wears out, so in order to be able to store the ODO and trip we will need to be able to rotate to new address'. To do this we will have a table at the start of the EEPROM that stores the address of each value and if the EEPROM gets corrupt we will find a new place to store it and update the table.

Address	Size	Description
0	8	ODO Address
1	8	Trip Address
2	8	Wheel diameter
15	8	Next available address
16	32	ODO value
20	32	Trip value

Firmware

The source code is in the firmware/src directory.

There is a makefile with a default target to compile and link the source into a hex file; a burn target to burn this hex to the micro and a fuses target to write the fuses. It is configured to use avrdude and a usbtiny programmer to talk to the microcontroller, but it should be possible to modify it to use other methods. The code is compiled with clang. There is a compile_flags.txt file that is used to configure a language server so that Vim or Visual Studio Code can give nice completion and code navigation.

On Arch linux I install the following tools and dependencies:

• llvm
• clang
• avrdude
• avr-libc
• lib32-glibc

Datasheets

• ATmega328p
• HT1621B
• WS2812B