Temperature and humidity data logger using PIC18F46K22 and SD card

This topic shows how to build a temperature and humidity data logger using PIC18F46K22 microcontroller, SD card and DHT22 (AM2302) digital humidity and temperature sensor.
The PIC18F46K22 MCU reads temperature (in °C) and humidity (in %rH) values from the DHT22 sensor and saves them to a text file stored in the SD card. DS3231 real time clock chip is used to get time and date information.
Time, date, temperature and humidity values are displayed on 20×4 LCD screen.
MikroC PRO for PIC compiler is used in this project.

FAT32 Library for mikroC compiler:
MikroElektronika provides a nice library for FAT32 file system (& SD card). This library can be downloaded from the following link:
FAT32 Library for mikroC compiler

The extension of the downloaded file is .mpkg, and in order to open and install the library we need a small software named: Package Manager, it’s also a free software provided by MikroElektronika, download link is below:
Package Manager download

To see how to interface PIC18F46K22 with SD card, visit the following post:
PIC18F46K22 Interface with SD card – Write & read files | mikroC Projects

Hardware Required:
This is a list of all components required to build this project.

  • PIC18F46K22 microcontroller   —->  datasheet
  • FAT32 formatted SD card (MMC, SD, SDHC, microSD ….)
  • micro SD card module
  • USB-to-serial UART converter (like: FT232RL module)
  • 20×4 LCD screen
  • DHT22 sensor (AM2302, RHT03 …)   —-> DHT22 datasheet
  • DS3231 board                            —->  DS3231 datasheet
  • 4.7k ohm resistor
  • 330 ohm resistor
  • 2 x push button
  • 10k ohm variable resistor or potentiometer
  • 3V coin cell battery
  • Breadboard
  • 5V source
  • Jumper wires

Temperature and humidity data logger using PIC18F46K22 circuit:
The following image shows project hardware circuit diagram.

As a small hint, it’s highly recommended to add a voltage level translator to MISO line in order to step up the 3.3V that comes from the SD card into 5V which goes to SDI1 pin (#23) of the PIC18F46K22 microcontroller, here we can use the integrated circuit 74HCT125.

In this project I used micro SD card module, this module is supplied from circuit 5V source, it contains the AMS1117-3V3 voltage regulator which is used to supply the SD card with 3.3V. Also this module contains an IC which is 74LVC125A and it is used as level translator (from 5V to 3.3V).

PIC18F46K22 DHT22 SD card DS3231 data logger for temperature and humidity

All the grounded terminals are connected together.

The PIC18F46K22 microcontroller does have 2 MSSP modules: MSSP1 and MSSP2 (MSSP: Master Synchronous Serial Port), each one of them may work as hardware SPI module or hardware I2C module. The SD card uses SPI protocol, it’s connected to MSSP1 module (used as SPI) hardware pins (SCK1, SDI1 and SDO1). So the micro SD card module is connected to the circuit as follows (from left to right):

The first pin of the micro SD card module (GND) is connected to circuit ground.
The second pin of the micro SD card module (VCC) is connected to circuit +5V.
The third pin of the micro SD card module (MISO) is connected to pin SDI1 (RC4) of the PIC18F46K22.
The fourth pin of the micro SD card module (MOSI) is connected to pin SDO1 (RC5) of the PIC18F46K22.
The fifth pin of the micro SD card module (SCK) is connected to pin SCK1 (RC3) of the PIC18F46K22.
The last pin of the micro SD card module (CS) is connected to pin RC2 of the PIC18F46K22.

The DS3231 RTC chip uses I2C protocol, its board data pins (SCL and SDA) are connected to MSSP2 module which is configured to operate in I2C mode. So, the SCL and SDA pins of the DS3231 board are respectively connected to SCL2 (RD0) and SDA2 (RD1) of the PIC18F46K22 microcontroller.

In the circuit there are two push buttons: B1 and B2, they are connected to pins RB0 (#33) and RB1 (#34) respectively, these buttons are used to set time and date of the real time clock.

The DHT22 sensor has 4 pins (from left to right): VCC, data, not connected pin and GND where:
VCC is connected to +5V
Data pin is connected to pin RB2 (#35) of the PIC18F46K22
GND is connected to circuit ground (0V).

The 20×4 LCD screen (4 rows and 20 columns) is used to display time, date, humidity and temperature where:
RS —> pin RD2
E   —> pin RD3
D4 —> pin RD4
D5 —> pin RD5
D6 —> pin RD6
D7 —> pin RD7
VSS, RW, D0, D1, D2, D3 and K are connected to circuit ground
VEE to the variable resistor (or potentiometer) output
VDD to +5V and A to +5V through 330 ohm resistor.

VEE pin is used to control the contrast of the LCD. A (anode) and K (cathode) are the back light LED pins.

The FT232RL board (USB to serial converter) RX pin is connected to PIC18F46K22 TX1 pin (RC6) which is USART1 module transmission pin (the PIC18F46K22 has 2 hardware USART modules). This board is used just to send some data from the microcontroller to PC.

In this project the PIC18F46K22 microcontroller runs with its internal oscillator @ 16 MHz and MCLR pin is configured as input.

Temperature data logger using PIC18F46K22 C code:
The C code below is for mikroC PRO for PIC compiler, it was tested with version 7.2.0.

To be able to compile project C code with no error, a driver (library) for the DS3231 RTC is required, download link is below. After you download the driver file which named DS3231.c, add it to your project folder.
DS3231 mikroC library

For more details about the DS1307 library, visit the following post:
DS3231 RTC Library for mikroC Compiler | mikroC Projects

The SD card chip select pin (SS) is connected to pin RC2 of the PIC18F46K22, it’s defined in the C code as:

The connection of the two push buttons are defined in the code as shown below:

The microcontroller reads temperature & humidity values from the DHT22 sensor and saves them (with time and date) to the SD card every second, for that I used the following if condition:

where the variable p_second is used to save only one value every 1 second.

Functions used in the code:
FAT32_Init(): this function initializes the FAT32 library as well as the SD card, it returns 0 if OK and non-zero if error.

FAT32_Exists(“DHT22Log.txt”): search if there is a file named “DHT22Log.txt” exists in the selected directory, return 0 if the file isn’t present, otherwise it returns a non-zero value (for example the file exists).

FAT32_Open(“DHT22Log.txt”, FILE_WRITE): this function creates a new file named “DHT22Log.txt”, returns 0 if OK and non-zero if error.

FAT32_Open(“DHT22Log.txt”, FILE_APPEND): this function opens the file named “DHT22Log.txt” and moves the cursor to the end of the file, returns 0 if OK and non-zero if error.

FAT32_Write(fileHandle, “…”, 26): writes a text (for example “…” ) of length (for example 26 characters) to the file associated with fileHandler.

FAT32_Close(fileHandle): closes the file associated with fileHandler, returns 0 if OK and non-zero if error.

char debounce (): this function is for button B1 debounce, returns 1 if button is debounced.

void wait(): this function is just a delay of 500 ms except that it can be interrupted by the two push buttons (B1 and B2). In this function Timer0 module is used to count the 500 milliseconds, it is used as 16-bit timer with prescaler = 32 (==> 1 tick every 8 microseconds ==> 62500 x 8 = 500000 us = 500 ms).

char edit(char x_pos, char y_pos, char parameter): this function is for setting the real time clock, returns the edited parameter.

void dow_print(): displays day of the week (Monday, Tuesday …) on the LCD.

void rtc_print(): displays time and date on the LCD. This function calls the previous one for displaying day of the week.

void Start_Signal(void): sends start signal to the DHT22 sensor.

char Check_Response(): checks response signal of the DHT22 sensor (after sending the start signal using the previous function), returns 1 if OK and 0 if error.

void Read_Data(unsigned short* dht_data): reads 1 byte from the DHT22 sensor which is saved to the variable dht_data.

Time, date, temperature and humidity are displayed on 20×4 LCD.
If there is a problem with the DHT22 sensor (for example bad connection) the LCD will display Sensor (on the 3rd row) Error (on the 4th row), and if there is a check sum error (for example due to wrong data reading) the LCD will display Checksum (on the 3rd row) Error (on the 4th row).
These errors will be also printed to the “DHT22Log.txt” file instead of temperature and humidity values.

Rest of code is described through comments.

Full mikroC code:

This project was tested in real hardware circuit using original Samsung microSD card with capacity of 32GB. The following image shows a part from the logger file (DHT22Log.txt) created by the hardware circuit:

DHT22 temperature and humidity data logger file mikroC

Proteus simulation:
This project could be simulated using Proteus software, the following video shows the result of the simulation.
Note that Proteus simulation circuit is not the same as real hardware circuit, project hardware circuit is shown above.

Proteus simulation file download link:
PIC18F46k22 SD card DS3231 DHT22 datalogger

SD Card image file (FAT32_MBR.ima) download link:
SD Card FAT32 image

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