DAY49 DS18B20 Single-Wire Digital Temperature Acquisition

DS18B20 Single-Wire Digital Temperature Acquisition

I. DS18B20 Core Features & Hardware Basics

1. Key Parameters (Must Remember!)

Parameter	Specifications
Measurement Range	-55℃ ~ +125℃ (Full industrial coverage)
Accuracy	±0.5℃ within -10℃~+85℃, ≤±2℃ full range
Resolution	Adjustable 9~12 bits (default 12-bit): 9-bit=0.5℃, 10-bit=0.25℃, 11-bit=0.125℃, 12-bit=0.0625℃
Operating Voltage	3V~5.5V (Compatible with 51 MCU 3.3V/5V power, no extra regulator needed)
Communication Interface	Single-wire GPIO bus (Only 1 I/O pin + GND, extremely low hardware cost)
Core Advantages	No external components, strong anti-interference, supports multi-sensor networking, retains configuration during power loss

2. Pin Definitions & Wiring

DS18B20 uses TO-92 package with 3 pins. 51 MCU wiring:

DS18B20 Pin	Function	51 MCU Connection
VDD	Power pin	Connect to 3.3V/5V (external power mode) or leave floating (parasitic power mode)
DQ	Data I/O	Connect to any GPIO (code uses P3.7) + 4.7KΩ pull-up resistor
GND	Ground	Connect to MCU GND (must share ground to prevent signal interference)

Critical Note: Single-wire bus must have 4.7KΩ pull-up resistor to ensure high level when idle for stable communication.

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II. DS18B20 Core Timing Principles (Communication Key!)

DS18B20 communication relies on strict timing protocols. All operations (reset, write, read) must follow single-wire bus timing rules - the core of successful acquisition.

1. Reset Timing (Initialization)

All communication starts with reset:

1. Host (51 MCU) pulls bus low≥480μs(reset pulse);
2. Host releases bus, switches to input mode after pulling high;
3. DS18B20 detects rising edge, delays 15~60μs, then pulls bus low60~240μs(presence pulse) to signal “ready”;
4. DS18B20 releases bus, returns to high level, enters idle state.

Code implementation:ds18b20_reset()usesDelay10us(70)for 700μs reset,Delay10us(6)to wait for presence pulse.

2. Write Timing (Host→DS18B20)

Host sends “0” or “1” via different low-level durations (LSB first, 8 bits = 1 byte):

• Write 0: Pull bus low≥60μs→ release (pull high); DS18B20 samples within 60μs, low level = “0”;
• Write 1: Pull bus low1~15μs→ release (pull high); DS18B20 samples high level = “1”;
• Minimum 1μs recovery between writes.

Code implementation:write_ds18b20()usesdat&1to determine bit, short delay for 1, long delay for 0.

3. Read Timing (DS18B20→Host)

Host triggers read by pulling bus low, then DS18B20 controls bus level:

1. Host pulls bus low≥1μs→ immediately releases (pull high);
2. Host samples bus level within 15μs (high=1, low=0);
3. Single read duration ≥60μs, minimum 1μs between reads.

Code implementation:read_ds18b20()pulls low then quickly releases, detects level viaDQ_CHECK, stores indat.

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III. Core Command Analysis (DS18B20 Operation Soul)

DS18B20 executes operations based on 8-bit host commands. Code uses 3 core commands:

Command Byte	Command Name	Function
0xCC	Skip ROM	Bypasses reading DS18B20’s 64-bit ROM code (preferred for single-sensor scenarios)
0x44	Convert T	Initiates temperature conversion (12-bit takes ~750ms), bus must stay high during conversion
0xBE	Read Scratchpad	Reads DS18B20’s 9-byte scratchpad (first 2 bytes contain temperature data)

Extension: Multi-sensor networks require0x55(Match ROM) + 64-bit ROM code to target specific sensors.

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IV. Complete 51 MCU Code Analysis

Code implements “Reset → Measure → Read → Data Processing” via P3.7 pin, ready for compilation.

1. Header & Macros (ds18b20.h)

#ifndef__DS18B20_H__#define__DS18B20_H__#include<reg51.h>// Function declarationsintds18b20_reset(void);// Reset DS18B20voidwrite_ds18b20(unsignedchardat);// Write 1 byte to DS18B20unsignedcharread_ds18b20(void);// Read 1 byte from DS18B20floatget_temp(void);// Get temperature (returns float)#endif

2. Core Function Implementation (ds18b20.c)

#include<reg51.h>#include<intrins.h>#include"ds18b20.h"#include"delay.h"// Macro definitions: P3.7 as DQ pin#defineDQ_DOWN(P3&=~(1<<7))// Pull DQ low#defineDQ_HIGH(P3|=(1<<7))// Pull DQ high#defineDQ_CHECK((P3&(1<<7))!=0)// Check DQ level/** * @brief DS18B20 reset initialization * @retval 1-Reset successful, 0-Reset failed */intds18b20_reset(void){intt=0;// 1. Send reset pulse (pull low ≥480μs)DQ_DOWN;Delay10us(70);// 70×10μs=700μs, meeting ≥480μs requirementDQ_HIGH;// Release the busDelay10us(6);// Wait 60μs to receive presence pulse// 2. Detect presence pulse (DS18B20 pulls bus low)while(DQ_CHECK&&t<30)// Timeout 300μs if no low level detected → failure{Delay10us(1);t++;}if(t>=30)return0;// Reset failed// 3. Wait for presence pulse to end (DS18B20 pulls bus high)t=0;while(!DQ_CHECK&&t<30)// Timeout 300μs if not pulled high → failure{Delay10us(1);t++;}if(t>=30)return0;// Reset failedreturn1;// Reset successful}/** * @brief Write 1 byte to DS18B20 (LSB first) * @param dat: Byte to send */voidwrite_ds18b20(unsignedchardat){inti=0;for(i=0;i<8;i++)// Loop 8 times, writing 1 bit each time{if(dat&1)// Write 1: Pull low for 1~15μs{DQ_DOWN;_nop_();// Short delay (~1μs)_nop_();DQ_HIGH;// Release the busDelay10us(5);// Wait 45μs to ensure DS18B20 sampling}else// Write 0: Pull low ≥60μs{DQ_DOWN;Delay10us(6);// 60μsDQ_HIGH;// Release the bus}dat>>=1;// Right shift 1 bit, preparing to write next bit (LSB first)}}/** * @brief Read 1 byte from DS18B20 (LSB first) * @retval Byte read */unsignedcharread_ds18b20(void){unsignedchardat=0;inti=0;for(i=0;i<8;i++)// Loop 8 times, reading 1 bit each time{DQ_DOWN;// Pull low ≥1μs to trigger read operation_nop_();_nop_();DQ_HIGH;// Release the bus, letting DS18B20 control the level_nop_();_nop_();_nop_();// Delay ~3μs, preparing to sampleif(DQ_CHECK)// Sample level: high=1, low=0{dat|=(1<<i);// Store corresponding bit (LSB first)}Delay10us(6);// Single read operation ≥60μs}returndat;}/** * @brief Get temperature value * @retval Float temperature (precision 0.0625℃) */floatget_temp(void){unsignedchartl=0;// Temperature low byte (LSB)unsignedcharth=0;// Temperature high byte (MSB, including sign bit)shortt=0;// Combined 16-bit temperature data// 1. Reset→Skip ROM→Start temperature conversionif(ds18b20_reset()==0)return-99.9;// Return error value if reset failswrite_ds18b20(0xCC);// Skip ROM (single sensor)write_ds18b20(0x44);// Start temperature conversionDelay1ms(1000);// Wait for conversion to complete (≥750ms for 12-bit resolution)// 2. Reset→Skip ROM→Read scratchpadds18b20_reset();write_ds18b20(0xCC);// Skip ROMwrite_ds18b20(0xBE);// Read scratchpad// 3. Read temperature data (first 2 bytes are temperature value, LSB first)tl=read_ds18b20();// Low byteth=read_ds18b20();// High byte// 4. Combine temperature data (16-bit signed two's complement)t=th<<8;// High byte left shift 8 bitst|=tl;// Combine low byte// 5. Temperature conversion: 12-bit resolution→1LSB=0.0625℃returnt*0.0625;}

3. Delay Function Support (delay.c/h)

DS18B20 timing requires high-precision delays (10μs, 1ms level with 11.0592MHz crystal):

// delay.h#ifndef__DELAY_H__#define__DELAY_H__voidDelay10us(unsignedintn);voidDelay1ms(unsignedintn);#endif// delay.c#include<reg51.h>voidDelay10us(unsignedintn){unsignedinti,j;for(i=n;i>0;i--)for(j=2;j>0;j--);// ≈10μs at 11.0592MHz}voidDelay1ms(unsignedintn){unsignedinti,j;for(i=n;i>0;i--)for(j=110;j>0;j--);// ≈1ms at 11.0592MHz}

4. Main Function Call Example (main.c)

#include<reg51.h>#include"ds18b20.h"#include"uart.h"// Assuming UART send functions are implementedvoidmain(void){floattemp;uart_init();// Initialize UART (for printing temperature)while(1){temp=get_temp();// Get temperatureif(temp!=-99.9)// Successful acquisition{// UART print temperature (floating-point to string function omitted here)uart_sendstr("Current temperature: ");// Example: Print integer part + decimal part}Delay1ms(2000);// Sample every 2 seconds}}

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5. Key Practical Considerations

1. Pull-up resistor is essential: A 4.7KΩ pull-up resistor is critical for stable single-wire bus communication; its absence can cause reset failures or data errors.
2. Delay precision must meet requirements: Timing parameters (e.g., 480μs reset, 60μs write 0) must strictly match; excessive delay errors will cause acquisition failures.
3. Wait for temperature conversion: After issuing the conversion command (0x44), sufficient time must be allowed (≥750ms for 12-bit resolution); otherwise, old data will be read.
4. Parasitic power mode note: If DS18B20 uses parasitic power (VDD floating), the bus must remain high during conversion, and no other operations should be performed.
5. Multi-sensor networking: Use0x55(Match ROM) command + sensor’s unique 64-bit ROM code to avoid data conflicts.

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6. Temperature Data Parsing Principle

DS18B20 stores temperature data as 16-bit signed two’s complement:

Bit 15 (MSB)	Bits 14-11	Bits 10-4	Bits 3-0 (LSB)
Sign bit (0=positive, 1=negative)	Integer part	Fractional part	Fractional part (12-bit resolution)

• Positive numbers: Directly convert using “integer part × 1 + fractional part × 0.0625”.
• Negative numbers: Convert using two’s complement rules (invert + 1) before calculation, with negative sign.
• Example: 25.5℃ → Binary00000000 00011001.1000→ Hex0x00198→ Convert to 25 + 8×0.0625=25.5℃.

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Summary

DS18B20’s core advantages are “single-wire communication + minimal hardware”. Key learning focuses ontiming protocolandcommand parsing: Reset is the communication prerequisite, write/read timing forms the data transmission foundation, and temperature conversion/scratchpad read commands are core operations. Mastering the code and principles here enables easy expansion to multi-sensor networks, temperature alarms (using TH/TL registers), UART temperature uploads, etc., making it suitable for embedded applications like environmental monitoring and equipment temperature control.