HT600 Series Trinary Decoder

A lightweight, high-performance, and hardware-agnostic C++ library for decoding trinary RF signals from Holtek HT600, HT680, and HT6207 encoders.

While developed to be cross-platform, it has been specifically tested on Arduino Duemilanove and ATtiny45.

Key Features

• Hardware Agnostic: Decoupled from hardware timers and interrupts. You simply feed it the pin state and a timestamp (ticks/micros).
• Trinary Logic Support: Fully supports 0, 1, and Open (High-Z) states.
• Highly Configurable: Adjustable oscillator frequency, tolerance, and noise filtering.
• Memory Efficient: Optimized to run on chips with very limited RAM (e.g., ATtiny series).

Compatible Hardware

The library supports the standard Holtek 3^18 series encoders. Although the physical pinouts differ, the data structure is identical.

General Specifications:

• Operating Voltage: 2.4V ~ 12V.
• Oscillator: Determined by a single external resistor. Standard is 100kHz with a 330kΩ resistor between OSC1 and OSC2.

Chip Differences

Chip	Address Bits	Data Bits	Transmission Trigger
HT600	9 (A0 A1 A2 A3 A4 - A6 A7 A8 A9 -)	5 (AD11 AD12 AD13 AD14 AD15)	TE Pin (Active HIGH)
HT680	8 (A0 A1 A2 A3 - - A6 A7 A8 A9 -)	4 (AD11 AD12 - AD14 AD15 )	TE Pin (Active HIGH)
HT6207	10 (A0 A1 A2 A3 A4 - A6 A7 A8 A9 - A11)	4 (D12 D13 D14 D15 )	Any Data Pin (Active HIGH)

Note on Packet Size: The physical protocol transmits 18 bits. However, this library decodes and stores only the first 16 bits. The last two bits are ignored as they are "dummy" bits in standard packages and contain no usable information.

Protocol Structure

The signal consists of a Pilot period, a Sync pattern, followed by the Address/Data payload.

    ┆<─────── Pilot ──────>┆   ┆<──────────────────── Sync ────────────────────>|<──────── Address Code ────────>|<───── Data Code ────>|
    ┆       (6 bits)       ┆   ┆                       ┆                        ┆            (11 bits)           ┆        (7 bits)      ┆
    ┆                      ├───┤   ┌───────┐       ┌───┤   ┌───────┐       ┌────┼────────────────────────────────┼──────────────────────┤
    ┆                      │   │   │       │       │   │   │       │       │    │╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳┆╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳╳│              
 ───┴──────────────────────┘   └───┘       └───────┘   └───┘       └───────┘    └────────────────────────────────┴──────────────────────┴────────

The protocol uses PWM-like symbols. A logical bit is composed of two symbols.

            ┆   ┌───────┤ 
Symbol 0': ─┼───┘       ├── (Short Low / Long High)
            ┆           ┆         
            ┆       ┌───┤  
Symbol 1': ─┼───────┘   ├── (Long Low / Short High)

By combining two symbols, we define the logic states:

          ┆←-------- bit --------→┆
          ┌─┐ ┌─┐ ┌─┐ ┌─┐ ┌─┐ ┌─┐ ┌─┐
fosc/33: ─┘ └─┘ └─┘ └─┘ └─┘ └─┘ └─┘ └──
          ┆           ┆           ┆
          ┆       ┌───┐       ┌───┐
1'    :  ─┆───────┘   └───────┘   └────  (Symbol 1 + Symbol 1)
          ┆           ┆           ┆
          ┆   ┌───────┐   ┌───────┐ 
0'    :  ─┆───┘       └───┘       └────  (Symbol 0 + Symbol 0)
          ┆           ┆           ┆
          ┆       ┌───┐   ┌───────┐ 
Z'    :  ─┆───────┘   └───┘       └────  (Symbol 1 + Symbol 0)
          ┆           ┆           ┆
          ┆   ┌───────┐       ┌───┐ 
SYNC' :  ─┆───┘       └───────┘   └────  (Symbol 0 + Symbol 1)
          ┆← SYMBOL  →┆←  SYMBOL →┆

Configuration & Tuning

To ensure reliable decoding, the library must be configured to match the hardware transmitter. The frequency decreases as battery voltage drops, so a generous tolerance is recommended (but do not overdo it to avoid overlapping in the recognition of the short and long periods).

Resistor to Frequency & Timing Table (at 12V)

Use this table to find the correct timing values. The Short Pulse corresponds to the base time unit $(T = 1 / (F_{osc} / 33))$.

Resistor	Macro	Freq ($F_{osc}$)	Short ( $1T$ )	Long ( $2T$ )	Pilot ( $36T$ )
120 kΩ	HT680_120K_FOSC	~265 KHz	125 μs	250 μs	4.50 ms
150 kΩ	HT680_150K_FOSC	~215 KHz	153 μs	306 μs	5.50 ms
180 kΩ	HT680_180K_FOSC	~180 KHz	183 μs	366 μs	6.58 ms
220 kΩ	HT680_220K_FOSC	~150 KHz	220 μs	440 μs	7.92 ms
270 kΩ	HT680_270K_FOSC	~120 KHz	275 μs	550 μs	9.90 ms
330 kΩ	HT680_330K_FOSC	100 KHz	330 μs	660 μs	11.88 ms
390 kΩ	HT680_390K_FOSC	~85 KHz	388 μs	776 μs	13.96 ms
470 kΩ	HT680_470K_FOSC	~70 KHz	471 μs	942 μs	16.95 ms
560 kΩ	HT680_560K_FOSC	~60 KHz	550 μs	1100 μs	19.80 ms
680 kΩ	HT680_680K_FOSC	~50 KHz	660 μs	1320 μs	23.76 ms
820 kΩ	HT680_820K_FOSC	~40 KHz	825 μs	1650 μs	29.70 ms
1.0 MΩ	HT680_1M0_FOSC	~33 KHz	1000 μs	2000 μs	36.00 ms
1.5 MΩ	HT680_1M5_FOSC	~22 KHz	1500 μs	3000 μs	54.00 ms
2.0 MΩ	HT680_2M0_FOSC	~16 KHz	2062 μs	4125 μs	74.23 ms

Constructor Parameters

1. Fosc (kHz): The oscillator frequency (usually determined by the resistor).
2. Tolerance: Percentage (e.g., 0.3 for 30%) to account for voltage drops/component variance.
3. Tick Resolution: The resolution of your timestamp in microseconds (usually 1.0 if using micros()).
4. Noise Filter: Minimum pulse width to accept (in microseconds).

Usage Example (Arduino/PlatformIO)

#include <HT600.h>

#define RF_PIN 2

// 1. Setup the decoder
// Freq: 100kHz, Tolerance: 30%, Tick: 1us, Filter: 50us
HT600 decoder(HT680_330K_FOSC, 0.3f, 1, 50);

// 2. Interrupt Service Routine
void IRAM_ATTR handleInterrupt() {
    // Feed the decoder with state and timestamp
    decoder.handleInterrupt(digitalRead(RF_PIN), micros());
}

void setup() {
    Serial.begin(115200);
    pinMode(RF_PIN, INPUT);
    attachInterrupt(digitalPinToInterrupt(RF_PIN), handleInterrupt, CHANGE);
}

void loop() {
    // 3. Check if packet is ready
    if (decoder.getState() == HT600_STATE::DONE) {
        
        // Get Data (Mapping Z to 0)
        uint16_t data = decoder.getReceivedValue();
        // Get Z-Mask (1 where the bit is Floating/Open)
        uint16_t z_mask = decoder.getTristateValue();

        Serial.print("Data: "); Serial.println(data, BIN);
        Serial.print("Z-Mask: "); Serial.println(z_mask, BIN);

        decoder.resetAvailable();
    }
}