Detailed specifications for each supported LCD controller. For quick selection, see Supported LCDs.
PCF85176 / PCF8576 (I2C)
Driver: SegDriver_PCx85, SegDriver_PCF85176, SegDriver_PCF8576
Note: PCF8576 is an older variant of PCF85176 with identical functionality. SegLCDLib treats them the same.
Physical Characteristics
| Property | Value |
| Protocol | I2C (2-wire) |
| I2C Address | 0x38 or 0x39 (SA0 pin) |
| Subaddresses | A0-A2 (protocol subaddress, not I2C) |
| RAM | 40 bytes (39 usable) |
| Max Segments | 320 (40 bytes × 8 bits) |
| Power | 3.3V or 5V (controller dependent) |
I2C and Subaddressing
The I2C address is determined by the SA0 pin only:
- SA0 = 0 → I2C address 0x38
- SA0 = 1 → I2C address 0x39
A0, A1, A2 are NOT I2C address pins - they are subaddresses used within the communication protocol to select one of 8 devices on the same I2C address.
I2C Address (SA0 pin):
SA0 = 0 → 0x38
SA0 = 1 → 0x39
Subaddress (A0-A2, in protocol):
A2 A1 A0 → Subaddress
0 0 0 → 0 (default)
0 0 1 → 1
...
1 1 1 → 7
Finding Address: Use I2C scanner or the PCF85176/RawLCD example:
#include "SegLCD_PCF85176_Raw.h"
void setup() {
Wire.begin();
Serial.begin(9600);
for (uint8_t addr : {0x38, 0x39}) {
Wire.beginTransmission(addr);
if (Wire.endTransmission() == 0) {
Serial.print("Found at: 0x");
Serial.println(addr, HEX);
}
}
}
Communication Protocol
Initialization Sequence:
- I2C START condition
- Address byte (0x38 or 0x39 based on SA0)
- Subaddress + control byte (A0-A2 bits + function setting)
- RAM data bytes (0-39)
- I2C STOP condition
Command Structure:
Byte 0: 0x00 - Static mode (no multiplexing)
0x01 - 1/2 duty
0x02 - 1/3 duty
0x04 - 1/4 duty
+ 0x00-0x30 for bias/contrast (varies by device)
Functional Modes
Drive Mode (Display Refresh)
Controls how the LCD multiplexer refreshes segments:
| Mode | Duty | Use Case |
| Static (1/1) | 100% of segments on at once | Single digit displays (rare) |
| 1/2 Duty | 2 digit rows | Multi-digit displays, simpler |
| 1/3 Duty | 3 digit rows | 6-digit displays (common) |
| 1/4 Duty | 4 digit rows | 4-digit displays |
Higher multiplexing requires more RAM but reduces pin count on LCD module.
Bias Mode
Voltage bias between LCD common and segment pins (determines contrast):
| Mode | Voltage Divider | Typical Use |
| 1/2 Bias | VDD/2 | Older LCD displays |
| 1/3 Bias | VDD/3 | Modern LCD displays (typical) |
Note: Only affects display when in multiplexed mode (1/2, 1/3, 1/4 duty). Static mode ignores bias.
RAM Layout
Byte Index Content Segments
─────────────────────────────────────────
0-2 Digit 1 (3 bytes) 0-23
3-5 Digit 2 (3 bytes) 24-47
...
36-38 Digit 13 (3 bytes) 288-311
Byte 39 Unused
Each byte controls 8 segments:
Byte Layout:
Bit 7 6 5 4 3 2 1 0
↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓
Segment a b c d e f g H (7-segment notation)
Wiring (Arduino → LCD Module)
Arduino PCF85176 Module
─────────────────────────────
SDA (A4) → SDA (pin 18)
SCL (A5) → SCL (pin 19)
3.3V/5V → VCC
GND → GND
Note: I2C pull-ups (4.7kΩ) typically already on LCD module.
Code Example
#include <Wire.h>
void setup() {
Wire.begin();
SegLCD_PCF85176_TempHum lcd(Wire);
lcd.init();
}
void loop() {
lcd.setCursor(0, 0);
lcd.print(23);
}
PCF85176 driver for temperature/humidity LCD.
HT1621 (3-Wire Serial)
Driver: SegDriver_HT1621
Physical Characteristics
| Property | Value |
| Protocol | 3-wire serial (bit-bang GPIO) |
| RAM | 16 bytes |
| Max Segments | 128 (16 × 8) |
| Clock Speed | 32 kHz (typical) |
| Power | 3.3V typical, 5V tolerant |
| Integration | Usually integrated into LCD module (COB) |
3-Wire Protocol
Three GPIO pins control communication:
CLK (Clock) - Synchronization signal
DATA (Data) - Serial data line
CS (Select) - Chip select (active low)
Bit Timing:
CLK Cycle:
┌──────┐ ┌──────┐
│ │ │ │
────┘ └────────┘ └────
↑ ↑ ↑ ↑
Data Read Data Read
Setup at Setup at
Rising Rising
Minimum clock period: 100μs (32 kHz max)
Command Structure
Three command types:
- System Control Command
9 bits: 100_0xxxxx
└─ SYS_EN (0=disable, 1=enable)
└─ SYS_DIS (0=display on, 1=off)
└─ LCD_ON (1=LCD enabled)
- Drive/Bias Mode Command
9 bits: 101_xxyzzz
└─ RC (1/2 vs 1/3 bias)
└─ Duty (1/2 to 1/4 multiplexing)
└─ Drive frequency
- Data Write Command
9 bits: 101_AAAAA (address, 32 addresses for 16 bytes)
8 bits: Data byte
RAM Layout
Address 0-3 4-7 8-11 12-15 16-19 20-31
Content Digit1 Digit2 Digit3 Digit4 Digit5 Digit6
Bits 0-31 32-63 64-95 96-127 128-159 160-191
Each address holds 8 segments for one digit.
Write Sequence
CS_LOW();
WriteCommand(0x140);
WriteData(0x00);
WriteData(0xAA);
CS_HIGH();
Wiring (Arduino → HT1621 LCD Module)
Arduino HT1621 Module
────────────────────────
Pin 5 → CLK
Pin 6 → DATA
Pin 7 → CS
3.3V → VCC
GND → GND
Note: Verify module voltage (3.3V or 5V); most are 3.3V.
Initialization Code
const int CLK = 5, DATA = 6, CS = 7;
void setup() {
pinMode(CLK, OUTPUT);
pinMode(DATA, OUTPUT);
pinMode(CS, OUTPUT);
lcd.init();
}
4-digit 7-segment LCD with degree symbol and colon (HT1621).
Definition SegLCD_HT1621_4SegDegree.h:19
HT1622 (3-Wire Serial, Enhanced)
Driver: SegDriver_HT1622
Physical Characteristics
| Property | Value |
| Protocol | 3-wire serial (enhanced timing) |
| RAM | 32 bytes |
| Max Segments | 256 (32 × 8) |
| Clock Speed | 32 kHz (typical) |
| Min Pulse Width | 4μs (vs 3.33μs for HT1621) |
| Power | 3.3V typical |
| Integration | Usually integrated into LCD module |
Differences from HT1621
| Feature | HT1621 | HT1622 |
| RAM | 16 bytes | 32 bytes |
| Max Digits | 6 (8 segments/digit) | 10-16 (depends on layout) |
| Segment Type | 7-segment | 7 or 16-segment |
| Pulse Width | 3.33μs (typical) | 4μs (minimum) |
Timing Requirements
HT1622 requires more precise timing:
Clock Period: ≥ 32μs (31.25 kHz max)
Low Pulse: ≥ 4μs
High Pulse: ≥ 4μs
Bit-banging code must respect 4μs minimum:
void writebit(uint8_t bit) {
digitalWrite(CLK, LOW);
digitalWrite(DATA, bit ? HIGH : LOW);
delayMicroseconds(4);
digitalWrite(CLK, HIGH);
delayMicroseconds(4);
}
RAM Layout
Address 0-7 8-15 16-23 24-31
Content Digit1 Digit2 Digit3 ...
Bits 0-63 64-127 128-191 192-255
Greater RAM supports larger segment displays (16-segment alphanumeric).
Wiring
Same as HT1621 (3 GPIO pins: CLK, DATA, CS).
VK0192 (3-Wire Serial, Advanced)
Driver: SegDriver_VK0192
Physical Characteristics
| Property | Value |
| Protocol | 3-wire serial (irregular addressing) |
| RAM | 24 bytes (48 4-bit addresses) |
| Max Segments | 192 (24 × 8, but addressed as 4-bit) |
| Clock Speed | 250 kHz (faster than HT1621) |
| Min Pulse Width | 4μs (critical timing) |
| Power | 3.3V typical |
| Address Mode | 4-bit addressing (vs 8-bit for HT1621) |
Key Difference: Irregular Addressing
Unlike HT1621 (sequential), VK0192 has non-sequential segment mapping:
Display Layout: VK0192 RAM Addresses
┌─────────────┐
│ Digit 1 2 3 │ Digit 1: Addresses 0, 1, 8, 9
│ Signal ●●●● │ Digit 2: Addresses 2, 3, 10, 11
│ Battery ▓▓ │ Digit 3: Addresses 4, 5, 12, 13
│ Progress ▓▓ │ Symbols: Addresses 14-47 (scattered)
└─────────────┘
Each digit spans non-sequential addresses (splits between lower and upper halves).
4-Bit Addressing
Data Format:
Byte = [ Address High (4 bits) | Data (4 bits) ]
Example:
Address 0x12 = 0x1 << 4 | data
= 0x10 | data
Timing Requirements (Strict)
Clock Period: ≥ 4μs (250 kHz max)
Pulse Width: Exactly 4μs (critical)
Setup/Hold: 1μs minimum
Important: Timing must be precise. Use delayMicroseconds() carefully:
void writeBit(uint8_t bit) {
digitalWrite(CLK, LOW);
digitalWrite(DATA, bit ? HIGH : LOW);
delayMicroseconds(4);
digitalWrite(CLK, HIGH);
delayMicroseconds(4);
}
RAM Layout and Addressing
Byte Address 4-bit Addresses Content
0 0, 1 Digit 1 upper, Digit 2 upper
1 (data bits)
...
12 24, 25 Signal strength segments
...
23 46, 47 Progress indicator
Segment mapping is display-specific due to irregular addressing.
Wiring
Same as HT1621/HT1622 (3 GPIO pins).
Segment Mapping Challenge
VK0192 requires careful address mapping because digits don't occupy sequential RAM:
void _mapSegmentToAddress(uint8_t digit, uint8_t segment,
uint8_t& addr, uint8_t& bit) {
if (digit == 0) {
if (segment < 4) addr = 0;
else addr = 8;
}
}
};
VK0192 LCD segment display driver.
Definition SegDriver_VK0192.h:12
Quick Comparison
| Feature | PCF85176 | HT1621 | HT1622 | VK0192 |
| Protocol | I2C | 3-wire | 3-wire | 3-wire |
| Pins | 2 (+ power) | 3 | 3 | 3 |
| RAM | 40 bytes | 16 bytes | 32 bytes | 24 bytes |
| Max Digits | 13 | 6 | 10+ | 5 |
| Addressing | Sequential | Sequential | Sequential | Irregular |
| Timing Critical | No | Moderate | High | Very High |
| Integration | Separate IC | Integrated | Integrated | Integrated |
| Typical Cost | $$$ | $$ | $$ | $$ |
Selection Guide
Choose PCF85176 if:
- Need many digits (10+)
- Want I2C (only 2 pins needed)
- Have 3.3V/5V available for I2C pull-ups
- Working with pre-made module
Choose HT1621 if:
- Need 4-6 digits
- Have GPIO pins available
- Want simple 3-wire protocol
- Module is integrated (no separate IC)
Choose HT1622 if:
- Need 10+ digits
- Want 16-segment capability
- Can handle stricter timing
- Module is already purchased
Choose VK0192 if:
- Working with specific VK0192 module
- Can handle complex addressing
- Need precise timing control
- Display already purchased/committed
Protocol Comparison: Write Sequence
PCF85176 (I2C)
START → ADDR → CONTROL → [DATA₀-₃₉] → STOP
Single transaction, all data at once.
HT1621 (3-Wire)
CS ↓ → CMD(9b) → ADDR(5b) → DATA(8b) → CS ↑
Bit-banged protocol, address then data.
VK0192 (3-Wire)
CS ↓ → CMD(9b) → NIBBLE_ADDR(4b) | DATA(4b) → CS ↑
4-bit addressing, must repeat for each nibble.
Reference
1.9.8