Features and connections<\/h3>\n![\"DHT11](\"https:\/\/playembedded.org\/wp-content\/uploads\/2016\/04\/art_006_DHT11_mech.jpg\")
<\/a>DHT11 mechanical draft.<\/figcaption><\/figure>\nTypical connections<\/h4>\n
Typical circuit requires a pull-up resistor on data pin. In our case Data out<\/b> is connected directly to Port A, pad 8 of a ST Nucleo F4 using a simple short wire, and it works perfectly. Anyway in a definitive circuit is suggested to connect a pull-up resistor of 4,7 kΩ as shown at page 4 of datasheet.<\/p>\nPin Description<\/h4>\n\n- the VDD power supply 3.5~5.5V DC<\/li>\n
- DATA serial data, a single bus<\/li>\n
- NC, empty pin<\/li>\n
- GND ground, the negative power<\/li>\n<\/ol>\n
Data format<\/h4>\n
When the DHT11<\/b> is queried , it replies using a 40bit length data packet. Data is organized as:<\/p>\n\n- 8bit humidity integer data + 8bit humidity decimal data<\/li>\n
- 8bit temperature integer data + 8bit temperature decimal data<\/li>\n
- 8bit parity bit<\/li>\n<\/ul>\n
Humidity rate data is provided as percentage with resolution 1%. Temperature is provided as Celsius with resolution of 1°C. That means on DHT11 data packet, decimal data is always zero. This is quite different for DHT22 that provides same data packet but with higher resolution and in this case decimal data is non-zero.<\/p>\n
Data timing<\/h4>\n![\"DHT11](\"https:\/\/playembedded.org\/wp-content\/uploads\/2016\/04\/art_006_DHT11_timing.jpg\")
<\/a>A timing diagram for the DHT11 one wire protocol communication.<\/figcaption><\/figure>\nAccording to data sheet DHT11 must be queried from host(our MCU) pulling down data pin for at least 18ms, than sensor lowers data pin for 80us as the response signal, followed by an high logical state output of 80 micro-seconds as notification of starting to send the data packet: so sensor replies with data. DHT11 actually sends out square waves and bit could be recognized measuring the time during which signal stays high: if that time is 26-28us it is a 0-bit, if it is 70 us is a 1-bit.<\/p>\n![\"DHT11](\"https:\/\/playembedded.org\/wp-content\/uploads\/2016\/04\/art_006_DHT11_timing2.jpg\")
<\/a>Differences between ‘0’ and ‘1’ bit format.<\/figcaption><\/figure>\nSummering getting data from DHT11 requires:<\/p>\n
\n- Set a pin as output and pull down that output to start communication.<\/li>\n
- Set the same pin as input to capture the square wave measuring high and low logical state period.<\/li>\n
- Eventually control if communication succeeds using check sum.<\/li>\n<\/ol>\n
Using DHT11 with ChibiOS\/HAL 3.0<\/h3>\n
Rereading above, the question is: what we need for to use DHT11 with ChibiOS? Basically one-wire protocol requires a pin that could be used as output push-pull as well as capture input. HAL has a driver that can capture an input (typically a square wave) and measure time period as well as time during which signal is in logical high state or logical low state. This driver is Input Capture Unit (ICU) ad is associated to a TIM like PWM.<\/p>\n
Basically every pin could be used as digital output, but only some pin are connected to a TIM. We have to choose a pin internally connected to a timer channel. Reading Alternate Function table on data-sheet we could see that PA8 could be used as TIM1 Channel 1 thought AF 1 (If you are not familiar with PAL driver checkout the tutorial Hello ChibiOS<\/a>). Reading ST Nucleo user manual we could learn that PA8 is connected to D7 pin of Arduino Connector.
\nLet outlining our program flow:<\/p>\n\n- Set PA8 mode as output push-pull and request data to DHT11 lowering Data out<\/b> for at least 18ms.<\/li>\n
- Set PA8 as AF1 and start ICU with a clock of at least 1MHz (This way we have a resolution of 1us).<\/li>\n
- Start capturing data with ICU. Using ICU width callback acquire 41 widths.<\/li>\n
- For each acquisition we need to write some variable according to widths.<\/li>\n
- Eventually stop the ICU and restore PA8 mode to default configuration (input pull-up)<\/li>\n<\/ol>\n
Note that first acquisition that should be 80 ticks (80us since our resolution is 1us) and others should be 26-28 ticks for a 0-bit or 70 ticks for 1-bit.<\/p>\n
Creating this application from scratch<\/h3>\n
First step as always is duplicate a working project (If you don’t know how to do that checkout the tutorial Quickview of Chibistudio<\/a> paying particular attention to video. Once we have created a new project, we have to enable ICU in halconf.h<\/i>.<\/p>\n\/**\n * @brief Enables the ICU subsystem.\n *\/\n#if !defined(HAL_USE_ICU) || defined(__DOXYGEN__)\n#define HAL_USE_ICU TRUE\n#endif<\/pre>\nSince we want to use PA8 and so TIM1 CH1 we need to enable ICUD1 in mcuconf.h<\/i><\/p>\n\/*\n * ICU driver system settings.\n *\/\n#define STM32_ICU_USE_TIM1 TRUE<\/pre>\nIf we want to use chprintf() to print data on a serial we have to edit makefile adding memstreams.c and chprintf.c to CSRC and including related folder modifying INCDIR<\/p>\n
# C sources that can be compiled in ARM or THUMB mode depending on the global\n# setting.\nCSRC = $(STARTUPSRC) \\\n $(KERNSRC) \\\n $(PORTSRC) \\\n $(OSALSRC) \\\n $(HALSRC) \\\n $(PLATFORMSRC) \\\n $(BOARDSRC) \\\n $(TESTSRC) \\\n $(CHIBIOS)\/os\/hal\/lib\/streams\/memstreams.c \\\n $(CHIBIOS)\/os\/hal\/lib\/streams\/chprintf.c \\\n main.c\n...\nINCDIR = $(STARTUPINC) $(KERNINC) $(PORTINC) $(OSALINC) \\\n $(HALINC) $(PLATFORMINC) $(BOARDINC) $(TESTINC) \\\n $(CHIBIOS)\/os\/hal\/lib\/streams \\\n $(CHIBIOS)\/os\/various<\/pre>\nNow we are ready to write main.c<\/i>. First of all we have to include chprintf.h declare a base sequential stream to use with chprintf(). Furthermore we could define some constants.<\/p>\n\/*\n * Enable if your terminal supports ANSI ESCAPE CODE\n *\/\n#define ANSI_ESCAPE_CODE_ALLOWED TRUE\nstatic BaseSequentialStream * chp = (BaseSequentialStream*) &SD2;\n\/*===========================================================================*\/\n\/* DHT11 related defines *\/\n\/*===========================================================================*\/\n\/*\n * Width are in useconds\n *\/\n#define MCU_REQUEST_WIDTH 18000\n#define DHT_ERROR_WIDTH 200\n#define DHT_START_BIT_WIDTH 80\n#define DHT_LOW_BIT_WIDTH 28\n#define DHT_HIGH_BIT_WIDTH 70\n\/*===========================================================================*\/\n\/* ICU related code *\/\n\/*===========================================================================*\/\n#define ICU_TIM_FREQ 1000000<\/pre>\nNow we can write an ICU configuration according to ChibiOS documentation. Since we need to measure time during high logical states we need to set ICU input as Active High. That means ICU width will measure time between signal rising edge and signal falling edge. Measure could have an uncertainly of ± 1 tick (In this case 1us). Other configuration are width, period and overflow callbacks, channels selected and value of DIE Register of TIM. We choose only width callback and enable channel 1.<\/p>\n
static ICUConfig icucfg = {\n ICU_INPUT_ACTIVE_HIGH,\n ICU_TIM_FREQ, \/* 1MHz ICU clock frequency. *\/\n icuwidthcb,\n NULL,\n NULL,\n ICU_CHANNEL_1,\n 0\n};<\/pre>\nICU width callback must be declared before ICU configuration. According to our configuration (ICU_INPUT_ACTIVE_HIGH), width callback is called on falling edge. Here we can get a width measure from driver ICU, and make some elaboration. Lets take a look to the code.<\/p>\n
static uint8_t TEMP, HR, CHECK_SUM, tmp, bit_counter = 0;;\nstatic icucnt_t widths [40];\nstatic void icuwidthcb(ICUDriver *icup) {\n icucnt_t width = icuGetWidthX(icup);\n if(width >= DHT_START_BIT_WIDTH){\n \/* starting bit resetting the bit counter *\/\n bit_counter = 0;\n }\n else{\n \/* Recording current width. Just for fun *\/\n widths[bit_counter] = width;\n if(width > DHT_LOW_BIT_WIDTH){\n tmp |= (1 << (7 - (bit_counter % 8)));\n }\n else{\n tmp &= ~(1 << (7 - (bit_counter % 8)));\n }\n \/* When bit_counter is 7, tmp contains the bit from 0 to 7 corresponding to\n The Humidity Rate integer part (Decimal part is 0 on DHT 11) *\/\n if(bit_counter == 7)\n HR = tmp;\n \/* When bit_counter is 23, tmp contains the bit from 16 to 23 corresponding to\n The Temperature integer part (Decimal part is 0 on DHT 11) *\/\n if(bit_counter == 23)\n TEMP = tmp;\n \/* When bit_counter is 39, tmp contains the bit from 32 to 39 corresponding to\n The Check sum value *\/\n if(bit_counter == 39)\n CHECK_SUM = tmp;\n bit_counter++;\n }\n}<\/pre>\nThis code gets the latest width, checking if it is related to a start bit (80us) or to one of the forty bits sent from DHT11. If current bit is the start one the counter bit_counter<\/i> is reset. During the forty callback occurencies related to data bits, width is stored into a buffer (just for debug purposes).<\/p>\n
<\/a>Typical connections<\/h4>\n
Typical circuit requires a pull-up resistor on data pin. In our case Data out<\/b> is connected directly to Port A, pad 8 of a ST Nucleo F4 using a simple short wire, and it works perfectly. Anyway in a definitive circuit is suggested to connect a pull-up resistor of 4,7 kΩ as shown at page 4 of datasheet.<\/p>\n When the DHT11<\/b> is queried , it replies using a 40bit length data packet. Data is organized as:<\/p>\n Humidity rate data is provided as percentage with resolution 1%. Temperature is provided as Celsius with resolution of 1°C. That means on DHT11 data packet, decimal data is always zero. This is quite different for DHT22 that provides same data packet but with higher resolution and in this case decimal data is non-zero.<\/p>\n According to data sheet DHT11 must be queried from host(our MCU) pulling down data pin for at least 18ms, than sensor lowers data pin for 80us as the response signal, followed by an high logical state output of 80 micro-seconds as notification of starting to send the data packet: so sensor replies with data. DHT11 actually sends out square waves and bit could be recognized measuring the time during which signal stays high: if that time is 26-28us it is a 0-bit, if it is 70 us is a 1-bit.<\/p>\n Summering getting data from DHT11 requires:<\/p>\n Rereading above, the question is: what we need for to use DHT11 with ChibiOS? Basically one-wire protocol requires a pin that could be used as output push-pull as well as capture input. HAL has a driver that can capture an input (typically a square wave) and measure time period as well as time during which signal is in logical high state or logical low state. This driver is Input Capture Unit (ICU) ad is associated to a TIM like PWM.<\/p>\n Basically every pin could be used as digital output, but only some pin are connected to a TIM. We have to choose a pin internally connected to a timer channel. Reading Alternate Function table on data-sheet we could see that PA8 could be used as TIM1 Channel 1 thought AF 1 (If you are not familiar with PAL driver checkout the tutorial Hello ChibiOS<\/a>). Reading ST Nucleo user manual we could learn that PA8 is connected to D7 pin of Arduino Connector. Note that first acquisition that should be 80 ticks (80us since our resolution is 1us) and others should be 26-28 ticks for a 0-bit or 70 ticks for 1-bit.<\/p>\n First step as always is duplicate a working project (If you don’t know how to do that checkout the tutorial Quickview of Chibistudio<\/a> paying particular attention to video. Once we have created a new project, we have to enable ICU in halconf.h<\/i>.<\/p>\n Since we want to use PA8 and so TIM1 CH1 we need to enable ICUD1 in mcuconf.h<\/i><\/p>\n If we want to use chprintf() to print data on a serial we have to edit makefile adding memstreams.c and chprintf.c to CSRC and including related folder modifying INCDIR<\/p>\n Now we are ready to write main.c<\/i>. First of all we have to include chprintf.h declare a base sequential stream to use with chprintf(). Furthermore we could define some constants.<\/p>\n Now we can write an ICU configuration according to ChibiOS documentation. Since we need to measure time during high logical states we need to set ICU input as Active High. That means ICU width will measure time between signal rising edge and signal falling edge. Measure could have an uncertainly of ± 1 tick (In this case 1us). Other configuration are width, period and overflow callbacks, channels selected and value of DIE Register of TIM. We choose only width callback and enable channel 1.<\/p>\n ICU width callback must be declared before ICU configuration. According to our configuration (ICU_INPUT_ACTIVE_HIGH), width callback is called on falling edge. Here we can get a width measure from driver ICU, and make some elaboration. Lets take a look to the code.<\/p>\n This code gets the latest width, checking if it is related to a start bit (80us) or to one of the forty bits sent from DHT11. If current bit is the start one the counter bit_counter<\/i> is reset. During the forty callback occurencies related to data bits, width is stored into a buffer (just for debug purposes).<\/p>\nPin Description<\/h4>\n
\n
Data format<\/h4>\n
\n
Data timing<\/h4>\n
<\/a><\/a>\n
Using DHT11 with ChibiOS\/HAL 3.0<\/h3>\n
\nLet outlining our program flow:<\/p>\n\n
Creating this application from scratch<\/h3>\n
\/**\n * @brief Enables the ICU subsystem.\n *\/\n#if !defined(HAL_USE_ICU) || defined(__DOXYGEN__)\n#define HAL_USE_ICU TRUE\n#endif<\/pre>\n
\/*\n * ICU driver system settings.\n *\/\n#define STM32_ICU_USE_TIM1 TRUE<\/pre>\n
# C sources that can be compiled in ARM or THUMB mode depending on the global\n# setting.\nCSRC = $(STARTUPSRC) \\\n $(KERNSRC) \\\n $(PORTSRC) \\\n $(OSALSRC) \\\n $(HALSRC) \\\n $(PLATFORMSRC) \\\n $(BOARDSRC) \\\n $(TESTSRC) \\\n $(CHIBIOS)\/os\/hal\/lib\/streams\/memstreams.c \\\n $(CHIBIOS)\/os\/hal\/lib\/streams\/chprintf.c \\\n main.c\n...\nINCDIR = $(STARTUPINC) $(KERNINC) $(PORTINC) $(OSALINC) \\\n $(HALINC) $(PLATFORMINC) $(BOARDINC) $(TESTINC) \\\n $(CHIBIOS)\/os\/hal\/lib\/streams \\\n $(CHIBIOS)\/os\/various<\/pre>\n
\/*\n * Enable if your terminal supports ANSI ESCAPE CODE\n *\/\n#define ANSI_ESCAPE_CODE_ALLOWED TRUE\nstatic BaseSequentialStream * chp = (BaseSequentialStream*) &SD2;\n\/*===========================================================================*\/\n\/* DHT11 related defines *\/\n\/*===========================================================================*\/\n\/*\n * Width are in useconds\n *\/\n#define MCU_REQUEST_WIDTH 18000\n#define DHT_ERROR_WIDTH 200\n#define DHT_START_BIT_WIDTH 80\n#define DHT_LOW_BIT_WIDTH 28\n#define DHT_HIGH_BIT_WIDTH 70\n\/*===========================================================================*\/\n\/* ICU related code *\/\n\/*===========================================================================*\/\n#define ICU_TIM_FREQ 1000000<\/pre>\n
static ICUConfig icucfg = {\n ICU_INPUT_ACTIVE_HIGH,\n ICU_TIM_FREQ, \/* 1MHz ICU clock frequency. *\/\n icuwidthcb,\n NULL,\n NULL,\n ICU_CHANNEL_1,\n 0\n};<\/pre>\nstatic uint8_t TEMP, HR, CHECK_SUM, tmp, bit_counter = 0;;\nstatic icucnt_t widths [40];\nstatic void icuwidthcb(ICUDriver *icup) {\n icucnt_t width = icuGetWidthX(icup);\n if(width >= DHT_START_BIT_WIDTH){\n \/* starting bit resetting the bit counter *\/\n bit_counter = 0;\n }\n else{\n \/* Recording current width. Just for fun *\/\n widths[bit_counter] = width;\n if(width > DHT_LOW_BIT_WIDTH){\n tmp |= (1 << (7 - (bit_counter % 8)));\n }\n else{\n tmp &= ~(1 << (7 - (bit_counter % 8)));\n }\n \/* When bit_counter is 7, tmp contains the bit from 0 to 7 corresponding to\n The Humidity Rate integer part (Decimal part is 0 on DHT 11) *\/\n if(bit_counter == 7)\n HR = tmp;\n \/* When bit_counter is 23, tmp contains the bit from 16 to 23 corresponding to\n The Temperature integer part (Decimal part is 0 on DHT 11) *\/\n if(bit_counter == 23)\n TEMP = tmp;\n \/* When bit_counter is 39, tmp contains the bit from 32 to 39 corresponding to\n The Check sum value *\/\n if(bit_counter == 39)\n CHECK_SUM = tmp;\n bit_counter++;\n }\n}<\/pre>\n