update readme.md (#21)

* update heater code
* update readme.md

Author: RobTillaart

README.md

@@ -1,13 +1,17 @@
 
 [![Arduino CI](https://github.com/robtillaart/SHT31/workflows/Arduino%20CI/badge.svg)](https://github.com/marketplace/actions/arduino_ci)
-[![GitHub release](https://img.shields.io/github/release/RobTillaart/SHT31.svg?maxAge=3600)](https://github.com/RobTillaart/SHT31/releases)
+[![JSON check](https://github.com/RobTillaart/SHT31/actions/workflows/jsoncheck.yml/badge.svg)](https://github.com/RobTillaart/SHT31/actions/workflows/jsoncheck.yml)
+[![Arduino-lint](https://github.com/RobTillaart/SHT31/actions/workflows/arduino-lint.yml/badge.svg)](https://github.com/RobTillaart/SHT31/actions/workflows/arduino-lint.yml)
 [![License: MIT](https://img.shields.io/badge/license-MIT-green.svg)](https://github.com/RobTillaart/SHT31/blob/master/LICENSE)
+[![GitHub release](https://img.shields.io/github/release/RobTillaart/SHT31.svg?maxAge=3600)](https://github.com/RobTillaart/SHT31/releases)
 
 
 # SHT31
 
 Arduino library for the SHT31 temperature and humidity sensor
 
+Relates to the SHT85 library - https://github.com/RobTillaart/SHT85
+
 
 ## Description
 
@@ -17,10 +21,11 @@ This library should also work for SHT30 and SHT35 but these are
 not tested yet.
 
 | SENSOR | Temperature accuracy | Humidity accuracy |
-|:----:|:----:|:----:|
-| SHT30 | ~0.3 | 2 |
-| SHT31 | ~0.3 | 1.5 |
-| SHT35 | ~0.2 | 1.5 |
+|:------:|:------:|:-----:|
+| SHT30  |  ~0.3  |  2.0  |
+| SHT31  |  ~0.3  |  1.5  |
+| SHT35  |  ~0.2  |  1.5  |
+| SHT85  |  ~0.2  |  1.5  |
 
 
 An elaborated library for the SHT31 sensor can be found here
@@ -31,90 +36,109 @@ https://github.com/hawesg/SHT31D_Particle_Photon_ClosedCube
 
 #### Base interface
 
-- **SHT31()**  constructor.
-- **begin(address, dataPin, clockPin)** begin function for ESP8266 & ESP32;
+- **SHT31()** constructor.
+- **bool begin(address, dataPin, clockPin)** begin function for ESP8266 & ESP32;
 returns false if device address is incorrect or device cannot be reset.
-- **begin(address)** for single I2C bus platforms, e.g UNO.
-- **begin(address,  TwoWire \*wire)** for platforms with multiple I2C busses.
-- **read(bool fast = true)** blocks 4 (fast) or 15 (slow) milliseconds + actual read + math
-- **isConnected()** check sensor is reachable over I2C. Returns false if not connected.
-- **uint16_t readStatus()** details see datasheet and **Status fields** below
+- **bool begin(address)** for single I2C bus platforms, e.g UNO.
+- **bool begin(address,  TwoWire \*wire)** for platforms with multiple I2C busses.
+- **bool read(bool fast = true)** blocks 4 (fast) or 15 (slow) milliseconds + actual read + math.
+Does read both the temperature and humidity.
+- **bool isConnected()** check sensor is reachable over I2C. Returns false if not connected.
+- **uint16_t readStatus()** details see datasheet and **Status fields** below.
 - **uint32_t lastRead()** in milliSeconds since start of program.
-- **reset(bool hard = false)** resets the sensor, soft reset by default. Returns false if fails.
-- **getHumidity()** computes the relative humidity in % based off the latest raw reading, and returns it
-- **getTemperature()** computes the temperature in °C based off the latest raw reading, and returns it
-- **getRawHumidity()** returns the raw two-byte representation of humidity directly from the sensor
-- **getRawTemperature()** returns the raw two-byte representation of temperature directly from the sensor
+- **bool reset(bool hard = false)** resets the sensor, soft reset by default. Returns false if fails.
+- **float getHumidity()** computes the relative humidity in % based off the latest raw reading, and returns it.
+- **float getTemperature()** computes the temperature in °C based off the latest raw reading, and returns it.
+- **uint16_t getRawHumidity()** returns the raw two-byte representation of humidity directly from the sensor.
+- **uint16_t getRawTemperature()** returns the raw two-byte representation of temperature directly from the sensor.
 
 Note that the temperature and humidity values are recalculated on every call to getHumidity() and getTemperature(). If you're worried about the extra cycles, you should make sure to cache these values or only request them after you've performed a new reading.
 
 
 #### Error interface
 
-- **getError()** returns last set error flag and clear it. 
+- **int getError()** returns last set error flag and clear it. 
 Be sure to clear the error flag by calling **getError()** before calling any command as the error flag could be from a previous command.
 
-| Error | Symbolic | Description |
-|:----:|:----|:----|
-| 0x00 | SHT31_OK              | no error                    |
-| 0x81 | SHT31_ERR_WRITECMD    | I2C write failed            |
-| 0x82 | SHT31_ERR_READBYTES   | I2C read failed             |
-| 0x83 | SHT31_ERR_HEATER_OFF  | Could not switch off heater |
-| 0x84 | SHT31_ERR_NOT_CONNECT | Could not connect           |
-| 0x85 | SHT31_ERR_CRC_TEMP    | CRC error in temperature    |
-| 0x86 | SHT31_ERR_CRC_HUM     | CRC error in humidity       |
-| 0x87 | SHT31_ERR_CRC_STATUS  | CRC error in statusfield    |
+| Error | Symbolic                  | Description
+|:-----:|:--------------------------|:----------------------------|
+| 0x00  | SHT31_OK                  | no error                    |
+| 0x81  | SHT31_ERR_WRITECMD        | I2C write failed            |
+| 0x82  | SHT31_ERR_READBYTES       | I2C read failed             |
+| 0x83  | SHT31_ERR_HEATER_OFF      | Could not switch off heater |
+| 0x84  | SHT31_ERR_NOT_CONNECT     | Could not connect           |
+| 0x85  | SHT31_ERR_CRC_TEMP        | CRC error in temperature    |
+| 0x86  | SHT31_ERR_CRC_HUM         | CRC error in humidity       |
+| 0x87  | SHT31_ERR_CRC_STATUS      | CRC error in statusfield    |
+| 0x88  | SHT31_ERR_HEATER_COOLDOWN | Heater need to cool down    |
+| 0x88  | SHT31_ERR_HEATER_ON       | Could not switch on heater  |
 
 
 #### Heater interface
 
-Use the heater for max **180** seconds, and let it cool down an equal period of time.
+**WARNING:** Do not use heater for long periods. 
+
+Use the heater for max **180** seconds, and let it cool down **180** seconds = 3 minutes. 
+Version 0.3.3 and up guards the cool down time by preventing switching the heater on 
+within **180** seconds of the last switch off. Note: this guarding is not reboot persistent. 
 
-**WARNING:** Do not use heater for long periods.
+**WARNING:** The user is responsible to switch the heater off manually!
 
-**WARNING:** The user is responsible to switch the heater off manually! 
 The class does **NOT** do this automatically.
+Switch off the heater by directly calling **heatOff()** or indirectly by calling **isHeaterOn()**.
 
-- **setHeatTimeout(uint8_t seconds)** Set the time out of the heat cycle.
+- **void setHeatTimeout(uint8_t seconds)** Set the time out of the heat cycle.
 This value is truncated to max 180 seconds. 
-- **heatOn()** switches heat cycle on.  Returns false if fails.
-- **heatOff()** switches  heat cycle off. Returns false if fails.
-- **isHeaterOn()** is the sensor still in heating cycle? replaces **heatUp()**.
-Will switch heat off if max heating time has passed. 
-- **heatUp()** will be obsolete in the future.
+- **uint8_t getHeatTimeout
+- **bool heatOn()** switches heat cycle on if not already on.
+Returns false if fails, setting error to **SHT31_ERR_HEATER_COOLDOWN** 
+or to **SHT31_ERR_HEATER_ON**. 
+- **bool heatOff()** switches heat cycle off. 
+Returns false if fails, setting error to **SHT31_ERR_HEATER_OFF**.
+- **bool isHeaterOn()** is the sensor still in heating cycle? replaces **heatUp()**.
+Will switch the heater off if max heating time has passed. 
+- **bool heatUp()** will be obsolete in the future. replaced by **isHeaterOn()**
 
 
 #### Async interface
 
 See async example for usage
 
-- **requestData()** requests a new measurement. Returns false if this fails.
-- **dataReady()** checks if enough time has passed to read the data. (15 millis)
-- **readData(bool fast = true)** fast skips CRC check. Returns false if reading fails or in case of a CRC fail. 
+- **bool requestData()** requests a new measurement. Returns false if this fails.
+- **bool dataReady()** checks if enough time has passed to read the data. (15 milliseconds)
+- **bool readData(bool fast = true)** fast = true skips the CRC check. 
+Returns false if reading fails or in case of a CRC failure. 
 
 
 ## Status fields
 
-| BIT | Description | values |
-|:----:|:----|:----|
-| 15  | Alert pending status | '0': no pending alerts|
-|     |                      | '1': at least one pending alert - default |
-| 14  | Reserved | '0' |
-| 13  | Heater status  | '0’ : Heater OFF - default |
-|     |                | '1’ : Heater ON |
-| 12  | Reserved | '0' |
-| 11  | Humidity tracking alert | '0’ : no alert - default |
-|     |                         | '1’ : alert |
-| 10  | Temperature tracking alert | '0’ : no alert - default |
-|     |                            | '1’ : alert |
-| 9-5 | Reserved | '00000' |
-|  4  | System reset detected  | '0': no reset since last ‘clear status register’ command |
-|     |                        | '1': reset detected (hard or soft reset command or supply fail) - default |
-| 3-2 | Reserved | '00' |
-|  1  | Command status | '0': last cmd executed successfully |
-|     |                | '1': last cmd not processed. Invalid or failed checksum |
-| 0   | Write data checksum status | '0': checksum of last write correct |
-|     |                            | '1': checksum of last write transfer failed |
+| BIT  | Description                | value   | notes |
+|:-----|:---------------------------|:--------|:------|
+| 15   | Alert pending status       |  0      | no pending alerts
+|      |                            |  1      | at least one pending alert - default
+| 14   | Reserved                   |  0      |
+| 13   | Heater status              |  0      | Heater OFF - default
+|      |                            |  1      | Heater ON 
+| 12   | Reserved                   |  0      |
+| 11   | Humidity tracking alert    |  0      | no alert - default
+|      |                            |  1      | alert
+| 10   | Temperature tracking alert |  0      | no alert - default
+|      |                            |  1      | alert
+| 9-5  | Reserved                   |  00000  |
+|  4   | System reset detected      |  0      | no reset since last ‘clear status register’ command
+|      |                            |  1      | reset detected (hard or soft reset command or supply fail) - default
+| 3-2  | Reserved                   |  00     |
+|  1   | Command status             |  0      | last cmd executed successfully
+|      |                            |  1      | last cmd not processed. Invalid or failed checksum
+|  0   | Write data checksum status |  0      | checksum of last write correct
+|      |                            |  1      | checksum of last write transfer failed
+
+
+## Future
+
+- merge with other SHT sensors if possible
+- direct Fahrenheit formula ?
+- improve error handling / status. (all code paths)
 
 
 ## Operation

SHT31.cpp

@@ -1,7 +1,7 @@
 //
 //    FILE: SHT31.cpp
 //  AUTHOR: Rob Tillaart
-// VERSION: 0.3.2
+// VERSION: 0.3.3
 //    DATE: 2019-02-08
 // PURPOSE: Arduino library for the SHT31 temperature and humidity sensor
 //          https://www.adafruit.com/product/2857
@@ -26,6 +26,8 @@
 //  0.3.0   2021-01-04  arduino-ci 
 //  0.3.1   2021-05-27  arduino-lint fixes
 //  0.3.2   2021-08-05  expose raw data from sensor
+//  0.3.3   2021-08-24  fix #22 prevent heater to switch on too fast.
+//                      update readme
 
 
 #include "SHT31.h"
@@ -43,15 +45,18 @@
 
 #define SHT31_HEAT_ON           0x306D
 #define SHT31_HEAT_OFF          0x3066
-
+#define SHT31_HEATER_TIMEOUT    180000UL  // milliseconds
 
 SHT31::SHT31()
 {
-  _addr           = 0;
+  _address        = 0;
   _lastRead       = 0;
   rawTemperature  = 0;
   rawHumidity     = 0;
+  _heatTimeout    = 0;
   _heaterStart    = 0;
+  _heaterStop     = 0;
+  _heaterOn       = false;
   _error          = SHT31_OK;
 }
 
@@ -63,7 +68,7 @@ bool SHT31::begin(const uint8_t address, const uint8_t dataPin, const uint8_t cl
   {
     return false;
   }
-  _addr = address;
+  _address = address;
 
   _wire = &Wire;
   if ((dataPin < 255) && (clockPin < 255))
@@ -89,8 +94,8 @@ bool SHT31::begin(const uint8_t address,  TwoWire *wire)
   {
     return false;
   }
-  _addr = address;
-  _wire = wire;
+  _address = address;
+  _wire    = wire;
   _wire->begin();
   return reset();
 }
@@ -109,7 +114,7 @@ bool SHT31::read(bool fast)
 
 bool SHT31::isConnected()
 {
-  _wire->beginTransmission(_addr);
+  _wire->beginTransmission(_address);
   int rv = _wire->endTransmission();
   if (rv != 0) _error = SHT31_ERR_NOT_CONNECT;
   return (rv == 0);
@@ -184,46 +189,56 @@ bool SHT31::reset(bool hard)
 
 void SHT31::setHeatTimeout(uint8_t seconds)
 {
-  _heatTimeOut = seconds;
-  if (_heatTimeOut > 180) _heatTimeOut = 180;
+  _heatTimeout = seconds;
+  if (_heatTimeout > 180) _heatTimeout = 180;
 }
 
 
 bool SHT31::heatOn()
 {
+  if (isHeaterOn()) return true;
+  if ((_heaterStop > 0) && (millis() - _heaterStop < SHT31_HEATER_TIMEOUT))
+  {
+    _error = SHT31_ERR_HEATER_COOLDOWN;
+    return false;
+  }
   if (writeCmd(SHT31_HEAT_ON) == false)
   {
+    _error = SHT31_ERR_HEATER_ON;
     return false;
   }
   _heaterStart = millis();
+  _heaterOn    = true;
   return true;
 }
 
 
 bool SHT31::heatOff()
 {
+  // always switch off the heater - ignore _heaterOn flag.
   if (writeCmd(SHT31_HEAT_OFF) == false)
   {
     _error = SHT31_ERR_HEATER_OFF;  // can be serious!
     return false;
   }
-  _heaterStart = 0;
+  _heaterStop = millis();
+  _heaterOn   = false;
   return true;
 }
 
 
 bool SHT31::isHeaterOn()
 {
-  if (_heaterStart == 0)
+  if (_heaterOn == false)
   {
     return false;
   }
   // did not exceed time out
-  if (millis() - _heaterStart < (_heatTimeOut * 1000UL))
+  if (millis() - _heaterStart < (_heatTimeout * 1000UL))
   {
     return true;
   }
-  heatOff();     // should this be done here?
+  heatOff();
   return false;
 }
 
@@ -268,7 +283,7 @@ bool SHT31::readData(bool fast)
   }
 
   rawTemperature = (buffer[0] << 8) + buffer[1];
-  rawHumidity = (buffer[3] << 8) + buffer[4];
+  rawHumidity    = (buffer[3] << 8) + buffer[4];
 
   _lastRead = millis();
 
@@ -307,7 +322,7 @@ uint8_t SHT31::crc8(const uint8_t *data, uint8_t len)
 
 bool SHT31::writeCmd(uint16_t cmd)
 {
-  _wire->beginTransmission(_addr);
+  _wire->beginTransmission(_address);
   _wire->write(cmd >> 8 );
   _wire->write(cmd & 0xFF);
   if (_wire->endTransmission() != 0)
@@ -321,7 +336,7 @@ bool SHT31::writeCmd(uint16_t cmd)
 
 bool SHT31::readBytes(uint8_t n, uint8_t *val)
 {
-  int rv = _wire->requestFrom(_addr, (uint8_t) n);
+  int rv = _wire->requestFrom(_address, (uint8_t) n);
   if (rv == n)
   {
     for (uint8_t i = 0; i < n; i++)