LCD Controllers

Detailed specifications for each supported LCD controller. For quick selection, see Supported LCDs.

---

Transport Headers

SegTransport.h defines the abstract transport interfaces used by drivers and LCD models.

Arduino transport implementations are declared in SegTransportArduino.h:

• SegTransportI2CArduino
• SegTransport3WireArduino

Arduino code using these transport classes must include SegTransportArduino.h explicitly.

Transport Build Macros

SEGLCD_DISABLE_ARDUINO_TRANSPORT disables Arduino transport declarations and implementation.

Use it in environments that provide their own SegTransportI2C or SegTransport3Wire implementation and do not have Arduino Wire.h available. Arduino builds should normally leave this undefined.

---

PCF85134 / PCF85176 / PCF8576 (I2C)

Driver: SegDriver_PCx85, SegDriver_PCF85134, SegDriver_PCF85176, SegDriver_PCF8576

Note: PCF85134, PCF85176, and PCF8576 share the PCx85 I2C driver model. PCF8576 is an older variant of PCF85176 with identical functionality in this library.

Physical Characteristics

Property	Value
Protocol	I2C (2-wire)
I2C Address	0x38 or 0x39 (SA0 pin)
Subaddresses	A0-A2 (protocol subaddress, not I2C)
RAM	PCF85134: 60 bytes max address range; PCF85176/PCF8576: 40 bytes (39 usable)
Max Segments	Controller/display dependent
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 a PCx85 raw example:

#include "SegLCD_PCF85176_Raw.h"
 
void setup() {
    Wire.begin();
    Serial.begin(9600);
 
    // Try both possible I2C addresses
    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:

1. I2C START condition
2. Address byte (0x38 or 0x39 based on SA0)
3. Subaddress + control byte (A0-A2 bits + function setting)
4. RAM data bytes (0-39)
5. 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 "SegTransportArduino.h"
#include "SegLCD_PCF85176_TempHum.h"
#include <Wire.h>
 
SegTransportI2CArduino transport(Wire);
SegLCD_PCF85176_TempHumidity lcd(transport);  // Default address 0x38
 
void setup() {
    Wire.begin();
    lcd.init();
}
 
void loop() {
    lcd.setCursor(0, 0);
    lcd.print(23);      // Temperature
}

---

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:

1. 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)

2. Drive/Bias Mode Command

   9 bits: 101_xxyzzz
           └─ RC (1/2 vs 1/3 bias)
           └─ Duty (1/2 to 1/4 multiplexing)
           └─ Drive frequency

3. 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

// Example: Write 0xAA to address 0x00
CS_LOW();
WriteCommand(0x140);      // Data write command (9 bits)
WriteData(0x00);          // Address (5 bits)
WriteData(0xAA);          // Data (8 bits)
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

#include "SegTransportArduino.h"
 
const int CLK = 5, DATA = 6, CS = 7;
SegTransport3WireArduino transport(DATA, CLK);
SegLCD_HT1621_4SegDegree lcd(transport, CS);
 
void setup() {
    lcd.init();  // Initialize controller and clear display
}

---

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)

Driver: SegDriver_VK0192

Physical Characteristics

Property	Value
Protocol	3-wire serial, same command/address/data framing family as HT1621/HT1622
RAM	24 bytes (24 × 8 bit RAM)
Max Segments	192 (24 × 8)
Power	2.4V–5.2V
Address Mode	Internal driver uses nibble-style HW write addresses over 24 × 8 bit RAM

Timing is handled by the shared SegTransport3Wire implementation — the library uses the same clock timing for HT1621, HT1622, and VK0192 (see SegTransportArduino.cpp), so there is no VK0192-specific timing requirement to account for.

Segment Mapping Is Display-Specific, Not Controller-Specific

The VK0192 controller itself addresses its 24×8 bit RAM sequentially, the same way HT1621/HT1622 do. The reference display module wired up in SegLCD_VK0192_5DigSigBattProgress happens to have its digits spread across non-adjacent RAM addresses, because of how that particular module's SEG/COM pins are wired to the glass:

Display Layout:      RAM Addresses on this module
┌─────────────┐
│ 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)
└─────────────┘

This is the same kind of module-specific mapping work needed for any segment LCD (HT1621/HT1622 modules can have irregular layouts too) — it is not a property of the VK0192 controller, and other VK0192 modules may map sequentially.

4-Bit Addressing

Data Format:
Byte = [ Address High (4 bits) | Data (4 bits) ]
 
Example:
Address 0x12 = 0x1 << 4 | data
             = 0x10 | data

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

The table above reflects the reference SegLCD_VK0192_5DigSigBattProgress module; other VK0192-based displays may map differently.

Wiring

Same as HT1621/HT1622 (3 GPIO pins).

Segment Mapping Example

Like any segment LCD with a non-trivial physical layout, mapping logical digits/segments to RAM addresses is module-specific work done once per display class:

class SegLCD_VK0192_5DigSigBattProgress : public SegDriver_VK0192 {
    void _mapSegmentToAddress(uint8_t digit, uint8_t segment,
                              uint8_t& addr, uint8_t& bit) {
        // This module: digit 0 spans addresses 0, 1, 8, 9
        if (digit == 0) {
            if (segment < 4) addr = 0;      // Lower nibble
            else            addr = 8;       // Upper nibble
        }
        // ... similar for digits 1, 2, 3, 4
    }
};

---

Quick Comparison

Feature	PCF85134	PCF85176/PCF8576	HT1621	HT1622	VK0192
Protocol	I2C	I2C	3-wire	3-wire	3-wire, same framing family as HT1621/HT1622
Pins	2 (+ power)	2 (+ power)	3	3	3
RAM	60 bytes max address range	40 bytes	16 bytes	32 bytes	24 bytes
Max Digits	display-dependent	13	display-dependent	display-dependent	display-dependent
Controller Addressing	Sequential	Sequential	Sequential	Sequential	Sequential
Integration	Separate IC	Separate IC	Integrated	Integrated	Integrated
Typical Cost	$$$	$$$	$$	$$	$$

Note: whether a specific module's digits/segments land on sequential or scattered RAM addresses depends on how that module's glass is wired to the controller's SEG/COM pins, not on which of these controllers it uses. All three 3-wire controllers use the same clock timing in this library's transport implementation.

---

Selection Guide

Choose PCF85134/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
• Module is already purchased

Choose VK0192 if:

• Working with a specific VK0192 module
• 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) → HW_ADDR → DATA(8b) → CS ↑

Same framing as HT1621, over a 24 × 8 bit RAM. SegLCDLib uses internal HW write addresses that advance in half-byte steps for compatibility with the existing driver model.

---

Reference

• PCF85176 Datasheet: NXP Semiconductors
• HT1621/HT1622 Datasheet: Holtek
• VK0192 Datasheet: VK Inc.
• Getting Started
• Supported LCDs
• Architecture