TN1228 Technical note How to interpret pressure and temperature readings in the LPS25HB pressure sensor Introduction The purpose of this technical note is to provide a review of two's complement notation for ease of design in the LPS25HB pressure sensor and to guide the user in the interpretation of pressure and temperature values in the device registers. MEMS pressure sensors measure absolute pressure (P) and 2 temperature (t) and store the values in two's complement registers which can be read via the I C or SPI host interface. The pressure sensor stores pressure as a 24-bit word. Both values are stored as two's complement integers as illustrated below. Figure 1: Pressure sensor analog-to-digital data flow Digital Domain Analog Domain P t Low Noise Analog Front End Sensors and Bias (AFE) Analog to Digital Conversion (ADC) Addr(h) November 2015 Digital Control Signal Processing (DSP) & Register Bank I2C SPI Byte Description 24-bit Pressure count Two’s complement value 28h 29h 2Ah PressureOut_XL (LSB) PressureOut_L (MIDDLE) PressureOut_H (MSB) 16-bit Temperature count Two’s complement value 2Bh 2Ch TemperatureOut_L (LSB) TemperatureOut_H (MSB) DocID028580 Rev 1 1/8 www.st.com Contents TN1228 Contents 1 Why we use two's complement ...................................................... 3 2 How to obtain pressure values in mbar ......................................... 4 3 How to obtain temperature values in °C ........................................ 5 4 5 Support material .............................................................................. 6 Revision history .............................................................................. 7 2/8 DocID028580 Rev 1 TN1228 1 Why we use two's complement Why we use two's complement Measuring analog values and storing the data in a digital domain creates an interesting dilemma. How do we represent negative analog values in a digital system? How do we handle the number zero? A computing system uses two's complement to simplify the processing logic required to handle negative numbers and subtractions. A circuit designed for addition can handle negative operands by using two’s complement. This minimizes the need for additional circuitry capable of subtraction or additional switching circuitry based on the sign. Most modern computers, MCU’s, and DSP’s use two’s complement notation. If a register is defined as two’s complement, typically the most significant bit (msb) of the most significant byte (MSB) indicates the sign as shown in the figure below. If the msb of the register is 1, the number is negative and we use two’s complement. If the bit is 0, the integer is positive and no translation is necessary. Figure 2: Sign bit indicates whether a value is positive or negative PressOut_H (0x2A) Bit 23 Bit 22 Bit 21 Bit 20 Bit 19 Bit Bit 18 17 Bit 16 PressOut_L (0x29) Bit 15 Bit 14 Bit 13 Bit 12 Bit 11 Bit Bit 10 9 PressOut_XL (0x28) Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 bit 3 bit 2 bit 1 bit 0 S I G N 0 = POS 1 = NEG pressure magnitude DocID028580 Rev 1 3/8 How to obtain pressure values in mbar 2 TN1228 How to obtain pressure values in mbar The pressure sensor stores the pressure value in raw counts in 3 registers: PressOut_H, PressOut_L, and PressOut_XL. The most significant bit of the PressOut_H register indicates the polarity. If the sign bit is zero, then the value is positive and the pressure in mbar is determined by dividing the decimal value by the scaling factor of 4096. A sign bit of 1 indicates a negative value, so we first take the two’s complement of the complete word and then divide by 4096. Keep in mind that the sign bit determines whether we should perform the two’s complement operation or not. The 3 bytes are concatenated to form a 24-bit word and the complete word is represented in two’s complement (not the single bytes). When reading the pressure value, it is important to note the byte ordering. This is especially important in auto-increment mode when the address is incremented automatically. The designer should assemble the proper 24-bit endian format (pressure count = 2Ah & 29h & 28h). The device may be configured to report a delta pressure by using the auto-zero feature. The actual pressure output value is copied in the REF_P_H, REF_P_L, and REF_P_XL registers and the PressOut() registers contain the difference between the current pressure and the reference pressure. So the PressOut() registers could contain a negative value. Negative values can also occur when a device is defective. Hence it can be used for selftest during a boot-up operation and when the auto-zero feature is not used. An example of a pressure calculation when the sign bit is 0 is shown in the following figure. Figure 3: Pressure example: sign bit is 0, this is a positive value PressOut_H (0x2A) Bit 23 Bit 22 Bit 21 Bit 20 Bit 19 0 0 1 1 1 3 Bit Bit 18 17 1 1 F PressOut_L (0x29) PressOut_XL (0x28) Bit 16 Bit 15 Bit 14 Bit 13 Bit 12 Bit 11 Bit Bit 10 9 Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 bit 3 bit 2 bit 1 bit 0 1 1 1 1 1 0 1 1 1 0 0 0 1 1 0 1 F 0 5 8 D Pressure Counts = 2Ah & 29h & 28h = 3FF58Dh = 4191629(dec) Pressure millibar = # counts Scaling factor = 4191629 counts 4096 counts/mbar = 1023.3 mbar The auto-zero feature can be enabled by setting bit 1 in CTRL_REG2 (0x21h). 4/8 DocID028580 Rev 1 TN1228 3 How to obtain temperature values in C How to obtain temperature values in °C The LPS25HB also contains a temperature sensor which is used for compensation of the pressure sensor. The temperature data is also available for outside applications. The temperature raw count is stored in registers Temp_Out_H (2Ch) and Temp_Out_L (2Bh). The temperature in °C can be determined by calculating the two’s complement, if necessary, and perform the scaling. The temperature in °C is calculated as follows: 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝑖𝑖𝑖𝑖 °𝐶𝐶 = �𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂 + #𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 #𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 � = �42.5 °𝐶𝐶 + � 480 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐/°𝐶𝐶 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 An example of a temperature calculation when the sign bit is 0 is shown below. Figure 4: Temperature example: sign bit is 0, this is a positive value Temp_Out_L (0x2B) Temp_Out_H (0x2C) Bit 15 Bit 14 Bit 13 0 1 1 Bit 12 Bit 11 Bit 10 Bit 9 Bit 8 Bit 7 Bit 6 Bit 5 0 1 0 1 1 1 1 0 B 6 Bit 4 1 Bit 3 0 D Bit 2 0 Bit 1 Bit 0 0 0 0 Temperature Counts = 2Ch & 2Bh = 6BD0h = 27600(dec) Temperature in °C = Offset + # counts scale = 42.5 + 27600 counts 480 counts / °C = 100 °C Likewise, a negative value is indicated when the MSB in the temperature word is equal to 1. To obtain the temperature, we first take the two’s complement of the complete word and then perform the scaling and offset operation. When reading the temperature values it is also important to note the byte ordering. This is especially true in auto-increment mode in which case address 2Bh is read first, followed by address 2Ch. The designer should assemble the proper 16-bit endian format (temperature count = 2Ch & 2Bh). The following registers in the device are two’s complement. All other registers in the device are either unsigned or bitwise representations (not negative). Table 1: Pressure sensor two's complement registers Function PRESS_OUT() TEMP_OUT() Byte 2Ah & 29h & 28h 2Bh & 2Ch REF_P() 0Ah & 09h & 08h RPDS() 39h & 3Ah Description 24-bit absolute pressure data or this contains the difference in pressure between Ref_P() and Press_Out() when auto-zero mode is used. 16-bit temperature data 24-bit pressure data subtracted from the sensor output measurement in auto-zero mode The pressure offset value is a 16-bit data that can be used to implement One-Point Calibration (OPC) after soldering. DocID028580 Rev 1 5/8 Support material 4 TN1228 Support material Table 2: Related design support material Document type Part number Datasheet/ data brief LPS25HB MEMS pressure sensor: 260-1260 mbar absolute digital output barometer STEVALMKI109V2 eMotion: ST MEMS adapter motherboard based on STM32. Compatible with all ST MEMS adapters based on STM32F103. STEVALMKI165V1 LPS25HB adapter board for standard DIL24 socket Evaluation board User manual Application note 6/8 Title UM0412 Getting started with DfuSe USB device firmware upgrade STMicroelectronics extension UM0979 STEVAL-MKI109V1 and STEVAL-MKI109V2 - eMotion motherboards for MEMS adapter boards UM1049 Unico graphical user interface (GUI) UM1064 Software guide for Unico lite AN4672 LPS25HB digital pressure sensor: HW guidelines for system integration AN4503 Environmental sensors: Hardware abstraction layer for Android DocID028580 Rev 1 TN1228 5 Revision history Revision history Table 3: Document revision history Date Revision 09-Nov-2015 1 Changes Initial release. DocID028580 Rev 1 7/8 TN1228 IMPORTANT NOTICE – PLEASE READ CAREFULLY STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, enhancements, modifications, and improvements to ST products and/or to this document at any time without notice. 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