USB : Real Time Sensor Data

The Universal Serial Bus (USB) is technology that allows a person to connect an electronic device to a computer. It is a fast serial bus type. The STM32 performance line embeds a USB device peripheral compatible with the USB full-speed 12 Mb/s. The USB interface implements a full-speed (12 Mbit/s) function interface. It has software-configurable endpoint setting and suspend/resume support. The USB peripheral implements an interface between a full-speed USB 2.0 bus and the APB bus. The clock source must use a HSE crystal oscillator.

Connection Diagram

Not all STM32 has this USB feature. USB 2.0 version support 3 different speed i.e. low speed 1.5Mb/s, full speed 12Mb/s & high speed 480Mb/s. Let’s look at the different class of USB-

In this article we use USB Communication Device class for display real time sensor data to PC. Here we use simple sensor AHT20. AHT20, as a new generation of temperature and humidity sensors, has established a new standard in size and intelligence. Its uses communication protocol as I²C & is very simple to operate.

The power supply range of AHT20 is 2.0-5.5V, and the recommended voltage is 3.3V. Please read the data sheet first. According to the data sheet the device has I²C address of 0x38. In order to read temperature & humidity only three step will need-

In is tutorial we use STM32F103RCT6 microcontroller. I use I2C1 for OLED display purpose & I2C2 for AHT20 temperature & humidity measurement. You can use only 1 I2C protocol. Let’s look at the code for interfacing which is very simple.

#define AHT20_ADD    0x38
#define AHT20_INIT   0xBE
#define AHT20_RESET  0xBA
#define AHT20_TRIG   0xAC
uint8_t Tx_Soft[1]={AHT20_RESET};
uint8_t Tx_INIT[3]={AHT20_INIT,0x08,0x00};
uint8_t Tx_Trig[3]={AHT20_TRIG,0x33,0x00};
uint8_t Rx_Data[6];
HAL_Delay(1000);
HAL_I2C_Master_Transmit(&hi2c2,AHT20_ADD<<1,Tx_INIT,3,200);
HAL_I2C_Master_Transmit(&hi2c2,AHT20_ADD<<1,Tx_Trig,3,200);
HAL_Delay(100);
HAL_I2C_Master_Receive(&hi2c2,(AHT20_ADD<<1|0x01),Rx_Data,6,200);

Here Rx_Data[6] represents the temperature & humidity data. Rx_Data[0] -> bit 7 represent measurement flag & bit 3 represent calibration flag.  Rx_Data[1], Rx_Data[2] & Rx_Data[3] -> upper 4 bits represent humidity. Rx_Data[4] -> lower 4 bits, Rx_Data[5] & Rx_Data[6] represents temperature. For calculation of

Temperature = (S_T /2²⁰ )*200-50 Centigrade & Humidity = (S_RH /2²⁰ )*100 % , 2²⁰ = 1048576

uint32_t Temperature,humidity;
float tem,hum;
humidity=(Rx_Data[1]<<12)+(Rx_Data[2]<<4)+(Rx_Data[3]>>4);
hum=(float)humidity/1048576*100;
Temperature=((Rx_Data[3]&0x0f)<<16)+(Rx_Data[4]<<8)+(Rx_Data[5]);
tem=((float)Temperature/1048576)*200-50;

Here I use internal RTC feature for Real time weather forecasting. Here I use OLED for display purpose, if you don’t want to display ignore OLED section. Let’s initialize the RTC & use some string function for data transmitting to PC/Laptop.

RTC_TimeTypeDef sTime;
RTC_DateTypeDef sDate;  
char display_time[16],display_date[16], display_AHT20[16];
char Time_Zone[32],Weather_Data[32], PC_Monitor[64];
char *day_name[8]={"Sun","Mon", "Tue", "Wed", "Thu", "Fri", "Sat"};
HAL_RTC_GetTime(&hrtc,&sTime,RTC_FORMAT_BCD);
HAL_RTC_GetDate(&hrtc,&sDate,RTC_FORMAT_BCD);
sprintf(display_time," %02X:%02X:%02X ",sTime.Hours,sTime.Minutes,sTime.Seconds);
sprintf(display_date," %02X/%02X/%02X %s",sDate.Date,sDate.Month,sDate.Year,day_name[sDate.WeekDay]);
sprintf(Time_Zone,"%s Time %s",display_date,display_time);
sprintf(Weather_Data,"Weather %02.2f C %02.2f %c HR",tem,hum,37);
sprintf(PC_Monitor,"%s%s\r",Time_Zone,Weather_Data);

Our final step is to transfer the string to PC/Laptop using STM32 USB Communication Device class. The code for USB Communication Device class-

#include "usbd_cdc_if.h"
CDC_Transmit_FS(PC_Monitor, strlen(PC_Monitor));

Data will send to PC without any external driver. Just Connect to PC/laptop according to the block diagram below. In PC-> Device manager see the Virtual Port number & in Realterm connect according to the port number with any baud rate. For setting all parameter please follow the youtube tutorial.