94 lines
2.8 KiB
C
94 lines
2.8 KiB
C
/* dht11.c -- DHT11 humidity and temperature sensor reading */
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/* Copyright (c) 2020-2021 Renaud Fivet */
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#include "dht11.h" /* implements DHT11 API */
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#include "system.h" /* usleep(), gpioa_*() */
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#define DIO 13
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#define dht11_input() gpioa_input( DIO)
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#define dht11_output() gpioa_output( DIO)
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#define dht11_bread() gpioa_read( DIO)
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#define MAX_RETRIES 200 /* at 48 MHz, 160 retries for 80 us HIGH */
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#define is_not_LOW( a) a != LOW
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#define is_not_HIGH( a) a == LOW
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#define wait_level( lvl) \
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retries = MAX_RETRIES ; \
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while( is_not_##lvl( dht11_bread())) \
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if( retries-- == 0) \
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return DHT11_FAIL_TOUT
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/* 5 .. 95 %RH, -20 .. 60 C */
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unsigned char dht11_humid ; /* 5 .. 95 %RH */
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signed char dht11_tempc ; /* -20 .. 60 C */
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unsigned char dht11_tempf ; /* .0 .. .9 C */
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void dht11_init( void) {
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/* At startup A13 is ALT DIO with Pull Up enabled */
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dht11_input() ;
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}
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dht11_retv_t dht11_read( void) {
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unsigned char values[ 5] ;
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/* Host START: pulls line down for > 18ms then release line, pull-up raises to HIGH */
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dht11_output() ;
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usleep( 18000) ;
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dht11_input() ;
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/* DHT START: takes line, 80us low then 80us high */
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int retries ; /* retry counter */
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wait_level( LOW) ; /* HIGH -> LOW, starts 80us low */
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wait_level( HIGH) ; /* LOW -> HIGH, ends 80us low, starts 80us high */
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/* DHT transmits 40 bits, high bit first
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* 0 coded as 50us low then 26~28us high
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* 1 coded as 50us low then 70us high
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*/
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wait_level( LOW) ; /* HIGH -> LOW, ends 80us high, starts 50us low */
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int threshold = (MAX_RETRIES + retries) / 2 ;
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unsigned char sum = 0 ;
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unsigned char v = 0 ;
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for( int idx = 0 ; idx <= 4 ; idx += 1) {
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sum += v ;
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v = 0 ;
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for( unsigned char curbit = 128 ; curbit ; curbit >>= 1) {
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/* Measure duration of HIGH level */
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wait_level( HIGH) ; /* LOW -> HIGH, ends 50us low, starts timed high */
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wait_level( LOW) ; /* HIGH -> LOW, timed high ends, starts 50us low */
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/* Set bit based on measured HIGH duration */
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if( retries < threshold) /* 0 == 26~28us, 1 == 70us */
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v |= curbit ;
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}
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values[ idx] = v ;
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}
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/* DHT STOP: releases line after 50us, pull-up raises to HIGH */
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wait_level( HIGH) ; /* LOW -> HIGH, ends 50us low, DHT has released the line */
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if( sum != values[ 4])
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return DHT11_FAIL_CKSUM ;
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dht11_humid = values[ 0] ;
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dht11_tempc = values[ 2] ;
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dht11_tempf = values[ 3] ;
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if( dht11_tempf & 0x80) {
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dht11_tempc *= -1 ;
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dht11_tempf = 10 - ( dht11_tempf & 0x7F) ;
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if( dht11_tempf == 10) {
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dht11_tempc -= 1 ;
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dht11_tempf = 0 ;
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}
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}
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return DHT11_SUCCESS ;
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}
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/* end of file dht11.c */
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