DAY49 DS18B20 Single-Wire Digital Temperature Acquisition

DS18B20 Single-Wire Digital Temperature Acquisition

I. DS18B20 Core Features & Hardware Basics

1. Key Parameters (Must Remember!)

2. Pin Definitions & Wiring

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

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

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.

III. Core Command Analysis (DS18B20 Operation Soul)

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

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

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}}

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.

6. Temperature Data Parsing Principle

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

• 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℃.

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.