Quick Reference Card

A quick reference for the THREE Turntable I2C Protocol.

I2C Configuration

• Slave Address (7-bit): 0x45
• Bus Speed: 10 kHz
• CRC Polynomial: 0x07

I2C Connection

When looking at the back of the THREE, PIN 1 is on the far right side.

Pin	Name	Value
1	PWR	12V Supply from THREE
2	GND	Ground
3	DET	5V Detection (Active pull to GND)
4	SCL	5V Clock
5	SDA	5V Data

Commands Quick Reference

Cmd	Name	Write Format	Read Format
0x00	STOP_ROT	[0x00][CRC]	-
0x02	STATUS_W_POS	[0x02][CRC]	[STATUS][POS_L][POS_H][CRC]
0x03	POSITION	[0x03][POS_L][POS_H][CRC]	-
0x04	ROTATE_ABS	[0x04][POS_L][POS_H][CRC]	-
0x08	RAMP_DIST	[0x08][RAMP][CRC]	-
0x0B	ERROR	[0x0B][CRC]	[ERROR][CRC]

Status Byte Flags

Bit	Mask	Name	Description
0	0x01	STAT_ERR	Error occurred
1	0x02	STAT_BACKLASH	Backlash active (ignored by THREE)
2	0x04	STAT_HALTED	Halted (ignored by THREE)
3	0x08	STAT_RESTING	Resting (ignored by THREE)
6	0x40	STAT_TURN	Currently rotating
7	0x80	STAT_BOOT	Boot complete (required)

Error Code Flags

Bit	Mask	Name	THREE Action
0	0x01	ERR_PARAM_COUNT	Throws exception
1	0x02	ERR_BAD_COM	Throws exception
2	0x04	ERR_UNRECOGNIZED_COM	Throws exception
3	0x08	ERR_ROT_TIME	Attempts recovery (3x)
4	0x10	ERR_ROT_DIR	Throws exception

Position Encoding

• Format: 16-bit little-endian
• Range: 0-359 degrees
• Example: 90° = [0x5A][0x00]

CRC Calculation

Write (Forward)

crc = 0
for each byte in [CMD, DATA...]:
    crc ^= byte
    for 8 bits:
        if (crc & 0x80): crc = (crc << 1) ^ 0x07
        else: crc <<= 1

Read (Reverse)

crc = 0
for each byte in [DATA_N, ..., DATA_1, DATA_0] (reverse):
    crc ^= byte
    for 8 bits:
        if (crc & 0x80): crc = (crc << 1) ^ 0x07
        else: crc <<= 1

Timing Requirements

• Poll Period: 100ms (during rotation)
• Rotation Timeout: 2000ms (no progress)
• Command Response: <1ms (for reads)
• Recovery Retries: 3 attempts (for ERR_ROT_TIME)

Typical Operation Flow

Initialization

1. Read STATUS_W_POS → verify STAT_BOOT set
2. Write POSITION 0° → set home
3. Write RAMP_DIST 15° → set ramp
4. Read STATUS_W_POS → verify position

Rotation

1. Write ROTATE_ABS target
2. Poll STATUS_W_POS every 100ms
3. Wait for STAT_TURN to clear
4. Verify position = target

Error Recovery

1. Read STATUS_W_POS → STAT_ERR set
2. Read ERROR → get code, clear error
3. If ERR_ROT_TIME:
   - Write RAMP_DIST (lower value)
   - Write ROTATE_ABS (retry)
   - Repeat up to 3 times
4. If other errors: stop operation

Implementation Checklist

Required Features

• ✅ I2C slave at address 0x45
• ✅ All 6 commands implemented
• ✅ CRC validation on all messages
• ✅ STAT_BOOT set after initialization
• ✅ STAT_TURN set/cleared during rotation
• ✅ Position tracking with 1° resolution
• ✅ Shortest path rotation (≤180° travel)
• ✅ Ramp distance deceleration
• ✅ Timeout detection (2 seconds)
• ✅ Error reporting via ERROR register
• ✅ Position wraparound at 0°/359°

Optional Features

• ⭕ STAT_BACKLASH flag
• ⭕ STAT_HALTED flag
• ⭕ STAT_RESTING flag
• ⭕ Physical homing routine
• ⭕ EEPROM position storage

Test Vectors

Commands

STOP_ROT:          [0x00][0x07]
POSITION 0°:       [0x03][0x00][0x00][0x5A]
POSITION 90°:      [0x03][0x5A][0x00][0x29]
ROTATE_ABS 180°:   [0x04][0xB4][0x00][0x43]
RAMP_DIST 15:      [0x08][0x0F][0x08]
ERROR read:        [0x0B][0x0C]

Responses

STATUS boot, 0°, not rotating:  [0x80][0x00][0x00][0x87]
STATUS boot, 90°, rotating:     [0xC0][0x5A][0x00][0x5E]
ERROR timeout:                  [0x08][0x0F]

Common Issues

Symptom	Likely Cause	Solution
“Turntable did not boot”	STAT_BOOT not set	Set bit 7 (0x80) in status
Rotation never completes	STAT_TURN not cleared	Clear when position = target
Position drifts	Encoder missed counts	Use interrupts, check wiring
Frequent timeouts	Too slow or stuck	Increase speed, check mechanics
CRC failures	Wrong algorithm	Use 0x07 poly, check forward/reverse
Can’t reach position	Poor accuracy	Tune ramp, reduce backlash

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Version: 1.0
Date: November 2025
Compatible with: Matter and Form THREE Scanner