# 3. DHT11 Sensor Interfacing

### 3.1 Sensor Fundamentals

A sensor captures physical phenomena from the real world and converts them into electrical signals (analog or digital) for computation.

For DHT11 specifically:

- It measures temperature and humidity.
- It is commonly used in educational and basic room-monitoring applications.
- Typical module-noted range:
- Temperature: 0 to 50 deg C
- Humidity: 20% to 90% RH

### 3.2 DHT11 Variants and Wiring

DHT11 can be found as:

- bare sensor package,
- breakout module.

Breakout modules are typically easier to wire and often include support components.

### 3.3 DHT11 Protocol and Timing

DHT11 uses a single-wire, pulse-width-based digital protocol. Communication sequence:

1. MCU sends start signal (low pulse around 18-20 ms, then high).
2. DHT11 sends response pulse.
3. DHT11 transmits 40-bit serial data.
4. Bit value is represented by pulse width:
- short high pulse = 0
- long high pulse = 1

### 3.4 40-bit Data Format

| Byte | Data Field |
| :--- | :--- |
| 1 | Humidity integer part |
| 2 | Humidity decimal part |
| 3 | Temperature integer part |
| 4 | Temperature decimal part |
| 5 | Checksum |

Checksum rule:

Checksum = (Byte1 + Byte2 + Byte3 + Byte4) & 0xFF

### 3.5 Why Accurate Delay Matters

DHT11 decoding depends on timing precision. If delays are too short or too long:

- start handshake may fail,
- bit sampling can shift,
- decoded bytes can become invalid,
- checksum mismatch may occur.

### 3.6 DHT11 Assembly Example

```asm
;------------------------
; Assembly Code
;------------------------
#define __SFR_OFFSET 0x00
#include "avr/io.h"
;------------------------
.global main
;=================================================================
main:
;------------
    LDI   R17, 0xFF
    OUT   DDRC, R17     ;set port C for o/p
    OUT   DDRD, R17     ;set port D for o/p
;-----------------------------------------------------------------
agn:RCALL delay_2s      ;wait 2s for DHT11 to get ready
;-----------------------------------------------------------------
;send start signal
;------------
    SBI   DDRB, 1       ;pin PB0 as o/p
    CBI   PORTB, 1      ;first, send low pulse
    RCALL delay_20ms    ;for 20ms
    SBI   PORTB, 1      ;then send high pulse
;-----------------------------------------------------------------
;wait for response signal
;---------------
    CBI   DDRB, 1       ;pin PB0 as i/p
w1: SBIC  PINB, 1
    RJMP  w1            ;wait for DHT11 low pulse
w2: SBIS  PINB, 1
    RJMP  w2            ;wait for DHT11 high pulse
w3: SBIC  PINB, 1
    RJMP  w3            ;wait for DHT11 low pulse
;-----------------------------------------------------------------
    RCALL DHT11_reading ;read humidity (1st byte of 40-bit data)
    MOV   R19, R18
    RCALL DHT11_reading
    RCALL DHT11_reading ;read temp (3rd byte of 40-bit data)
;-----------------------------------------------------------------
    OUT   PORTD, R19    ;o/p temp byte to port C
    OUT   PORTC, R18    ;o/p humidity byte to port D
    RJMP  agn           ;go back & get another sensor reading
;=================================================================
DHT11_reading:
    LDI   R17, 8        ;set counter for receiving 8 bits
    CLR   R18           ;clear data register
    ;-------------------------------------------------------
w4: SBIS  PINB, 1
    RJMP  w4            ;detect data bit (high pulse)
    RCALL delay_timer0  ;wait 50us then check bit value
    ;-------------------------------------------------------
    SBIS  PINB, 1       ;if bit=1, skip next instruction
    RJMP  skp           ;else bit=0
    SEC                 ;C=1
    ROL   R18           ;shift in 1
    RJMP  w5
skp:LSL   R18           ;shift in 0
    ;-------------------------------------------------------
w5: SBIC  PINB, 1
    RJMP  w5            ;wait for low pulse
    ;-------------------------------------------------------
    DEC   R17
    BRNE  w4
    RET
```