Arduino: Simple LCD Thermometer with USB interface

Attention: this page is work in progress.

My motivation for "Simple LCD Thermometer with USB interface" is my server room. It is important to keep temperature and humidity in correct range. I am using automatic monitoring to be sure that everything is in "green" state. Previously I was using Conrad TFD 128 logger. It is good product but it have several design problems. To fulfill my needs I created my own device.

Technical specification

• Temperature and humidity collected in 10 second interval
• LCD interface with warnings (Temperature: hot / cold, Humidity: wet / dry, Dew point: Too high / Too low)
• LCD interface with 5 tactile keys
• History and diagnostic information on LCD
• USB interface with simple text oriented protocol
• Historical data stored in RAM memory readable via LCD and USB interface
• Historical data with 60 minutes interval (display) and 30 minutes interval (USB)
• Historical data up to 24 hours
• Minimum and Maximum since last boot-up or clear
• Temperature range: +1C - +50C, Resolution: 0.1C
• Humidity range: 1% - 99%, Resolution: 1% (display), 0.1% (USB)
• Dew point range: -65C - +50C, Resolution: 0.1C
• Internal monitoring of voltage for mCPU and sensor (Vcc between 4.8V and 5.2V reported as good)
• Range indicating "normal" condition:
  • temperature: 20 - 25C
  • humidity: 40 - 70%
  • dew point: 10 - 16C
• Device is pooling sensor for new data each 10 seconds. Displayed data may be up to 20 seconds old due to nature of selected sensor chip.

Note 1: Selected temperature range is based on expected usage pattern (indoor only). Temperature sensor can measure wider range of temperature but this may complicate code a bit (especially negative temperature).

Note 2: Selected temperature and humidity range is not proof that Arduino UNO board will survive it (e.g. high temperature and/or humidity).

Note 3: Values outside of range are internal used to indicate problem with sensor reading or reading outside of bounds.

LCD Interface

Most of the details are accessible via LCD interface. "Startup screen" is displayed during boot-up. After successful initialization device move to "Normal screen".

Startup screen

During boot-up device show source code and build date. I may change this to some kind of version numbers in future.

+----------------+
|sTemp_LCD_USB   |
|Jan 17 2016     |
+----------------+

Device show also reboot source. If there is more that one reboot source set then device cycle through all of them. It is common to see BORF and PORF after device power up.

+----------------+ +----------------+ +----------------+ +----------------+
|sTemp_LCD_USB   | |sTemp_LCD_USB   | |sTemp_LCD_USB   | |sTemp_LCD_USB   |
|(Watchdog)  WDRF| |(Brown-out) BORF| |(External) EXTRF| |(Power-on)  PORF|
+----------------+ +----------------+ +----------------+ +----------------+

Presence of boot loader (if detected) is indicated by separate screen.

+----------------+
|sTemp_LCD_USB   |
|(Loader)  LOADER|
+----------------+

Note: For details about boot loader see Note 1 at end of page.

Normal screen

Displayed during regular operation. Left part of second line show device status:

• First part:
  • Everything looks good -> ♡ altering with ♥ after each successful sensor read
  • Minor problem reported (warning) -> exclamation mark
  • Serious problem reported (error) -> exclamation mark using negative font

• Second part:
  • Up and Down arrows are on display when there is activity on serial (USB) interface.

+----------------+ +----------------+ +----------------+
|T: 21.3C H: 50% | |T: 28.8C H: 90% | |T: 10.2C H: 10% |
|♡↑↓  NORM   NORM| |♥↑↓  HOT    WET | |♡↑↓  COLD   DRY |
+----------------+ +----------------+ +----------------+

Note 1: If keyboard is not used for 10 minutes and display is not on "Normal screen" then device automatically return to it.

Note 2: All out of range text indicators (Hot, Cold, etc.) are also blinking to indicate out of normal range condition.

Dew point

Show current dew point value.

+----------------+ +----------------+ +----------------+
|Dew point:  14.1| |Dew point:  20.2| |Dew point: - 9.1|
|     Comfortable| |        Too high| |         Too low|
+----------------+ +----------------+ +----------------+

Note: minus sign to indicate negative value is on fixed position. It is not aligned with number. I will not change this in near future

History + min/max

This set of screens to show minimum, maximum and historical values. Left down corner of screen show selected mode:

Temperature and humidity:

+----------------+ +----------------+ +----------------+ +----------------+
|T: 21.3C H: 50% | |T: 21.3C H: 50% | |T: 21.3C H: 50% | |T: 21.3C H: 50% |
|-01h NORM   NORM| |-02h NORM   NORM| |-23h NORM   NORM| |-24h NORM   NORM|
+----------------+ +----------------+ +----------------+ +----------------+

+----------------+ +----------------+
|T: 20.0C H: 30% | |T: 25.0C H: 30% |
|MIN  NORM   NORM| |MAX  NORM   NORM|
+----------------+ +----------------+

Dew point:

+----------------+ +----------------+ +----------------+ +----------------+
|Dew point: 14.1C| |Dew point: 14.1C| |Dew point: 14.1C| |Dew point: 14.1C|
|-01h Comfortable| |-02h Comfortable| |-23h Comfortable| |-24h Comfortable|
+----------------+ +----------------+ +----------------+ +----------------+

+----------------+ +----------------+
|Dew point: 12.1C| |Dew point: 16.1C|
|MIN  Comfortable| |MAX  Comfortable|
+----------------+ +----------------+ 

Diagnostic screen

System support following diagnostic screens:

Diag: uptime

Time since last reboot (uptime):

+----------------+
|Diag: uptime    |
|   1234d 21h 45m|
+----------------+

Note: Values are Day, Hour, Minute

Diag: sensor

Current status and status in past:

+----------------+
|Diag: sensor    |
|OK        Mising|
+----------------+

• Text on left show current status (last reading).
• Test on right show if there was problem in past.

Possible values:

• OK -> sensor is properly wired and responding
• Missing -> no response from sensor at all
• CRC Err -> all data received but CRC don't match
• Timeout -> timeout while receiving data

Diag: history

Numbers of consecutive history banks populated with valid data:

+----------------+
|Diag: history   |
|      03        |
+----------------+

All history banks contain valid data:

+----------------+
|Diag: history   |
|      ok        |
+----------------+

Diag: reboot by

Show reboot source type.

• W-- reboot source is not indicated by hardware.
• WDR reboot source is indicated by hardware.

+----------------+ 
|Diag: reboot by |
|W-- B-- E-- P-- |
+----------------+

+----------------+
|Diag: reboot by |
|WDR BOR EXT POR<|
+----------------+

Note 1: this is example. There is no possibility that hardware indicate all reboot sources at once.

Note 2: "<" is shown when device detect bootloader.

Note 3: check Startup screen for more details.

Diag: Power

Show voltage for mCPU and sensor measured by Armel itself.

• Normal condition:

+----------------+
|Diag: Power     |
|Vcc: 4.9 V  OK  |
+----------------+

• Error condition (voltage too low):

+----------------+
|Diag: Power     |
|Vcc: 4.0 V Err  |
+----------------+

Diag: version

Build date of code. This may change to some kind of version number in future.

+----------------+
|Diag: version   |
|Jan 17 2016     |
+----------------+

Screen position

Screen can be selected by tactile keyboard and position of screens are following:

                       return back to       return back to 
                      temp/hum display     temp/hum display
                             | |                  | |
                     +----------------+   +----------------+
                     |Min/Max cleared |   |Min/Max cleared |
                     | any key to ret.|   | any key to ret.|
                     +----------------+   +----------------+
                             | |                  | |       
                     +----------------+   +----------------+
                     |T: 25.0C H: 50% | - |Dew point:  16.1|
                     |MAX  NORM   NORM| - |MAX  Comfortable|
                     +----------------+   +----------------+
                             | |                  | |       
                     +----------------+   +----------------+
                     |T: 20.0C H: 30% | - |Dew point:  12.1|
                     |MIN  NORM   NORM| - |MIN  Comfortable|
  Device boot-up     +----------------+   +----------------+
        | |                  | |                  | |       
+----------------+   +----------------+   +----------------+   +----------------+
|sTemp_LCD_USB   | - |T: 21.3C H: 50% | - |Dew point:  14.1| - |Diag: uptime    | -                        - "Normal screen"
|(Power-on)  PORF| - |♡↑↓  NORM   NORM| - |     Comfortable| - |    123d 21h 45m| - all other diag screens -
+----------------+   +----------------+   +----------------+   +----------------+
                             | |                  | |       
                     +----------------+   +----------------+
                     |T: 21.3C H: 50% | - |Dew point:  14.1|
                     |-01h NORM   NORM| - |-01h Comfortable|
                     +----------------+   +----------------+
                             | |                  | |       
                       history up to        history up to 
                          24 hours      -      24 hours
                       for temperature  -  for dew point
                        and humidity

If there is no action for 10 minutes then screen move back to "Normal screen".

Serial (USB) interface

Device use virtual serial port over USB. Protocol is text oriented and request is made by sending upper case character. All undefinied characters are silently ignored. Additional requests are ignored while device is sending response. Configuration of serial port is 9600, 8-N-1, flow control: no. Device send blank (space) character before responding to query. Maximum size of response is: 64 characters.

Format of response

Example: Temp: 10.1 C, HUM: 36.1 %, Dew: - 4.2 C, 1 *E62D

• all values are send in format: "type: value unit"
• more values in one message are separated by coma (,)
• last value in message is sequential id. Number from 0 to 3.
• CRC is separated by asterisk (*) and is calculated as CRC-16.

Message id start at 0. Increment sequentially to 3 and then wrap around to 0. This ensure that each message is unique even when temperature / humidity reading result in same number.

Commands / data query

Communication rules:

• Wait for "silence" before requesting data.
• Request data by issuing one upper case character.
• Device schedule response using internal scheduler. Response is send once all higher priority tasks are done.
• Additional response lines are can be requested by issuing same request again.
• Version command can be used as "ping" to check if device is alive (it use smallest amount of CPU cycles to provide response). It can be also used to reset internal counter on multi line responses.

Help

List of commands:

? -> Help 1/2: Ver., Reboot, Temp., Alarms, History, 0 *7F6D
? -> Help 2/2: MinMax, Diag, Lcd, 1 *236C

Version

Build date / time. I may change this to version number in future.

V -> Version: Jan 17 2016, 20:48:03, 2 *6018

Reboot

This command force device reboot by disabling interrupts and initiating endless loop. In such a condition device is automatically restarted by watchdog.

R -> (no response by device)

Note: there is no direct response to this command.

Temperature / Humidity / Dew point

Last temperature, humidity and dew point reading.

T -> Temp: 21.1 C, HUM: 47.1 %, Dew:   9.4 C, 3 *542B

Alarms

Current alarm condition.

A -> Temp: NORM, Hum: NORM, Dew: Too low    , 0 *66F0

History

Historical data (up to 24 hours in 30 minutes increments).

H -> -00.5h, Temp: 14.6 C, Hum: 40.6 %, Dew:   1.4 C, 1 *E823
H -> -01.0h, Temp: 14.1 C, Hum: 40.1 %, Dew:   0.8 C, 2 *1B46
   .
   .
H -> -23.5h, Temp: 15.6 C, Hum: 41.6 %, Dew:   2.6 C, 3 *863E
H -> -24.0h, Temp: 15.1 C, Hum: 41.1 %, Dew:   2.0 C, 0 *83E4

Minimum / Maximum + clear

Minimum and maximum values + command to clear data.

M -> Temp/MIN: 10.1 C, Hum/MIN: 36.1 %, Dew/MIN: - 4.2C, 1 *13B0                         
M -> Temp/MAX: 22.2 C, Hum/MAX: 42.1 %, Dew/MAX:   8.7C, 2 *0B01
M -> Clear MIN/MAX ?, 3 *3AB0
M -> MIN/MAX cleared, 0 *118E

Diag

Show diagnostic data in similar format as on screen.

D -> UpD: 0002, UpH: 02, UpM: 56, UpS: 38, 1 *F4B3
D -> Sensor: OK     , Missing, 3 *7810
D -> History: ok, 2 *B419
D -> Power: 4.9 V, OK , 3 *956E
D -> Status: Warning, 0 *4F74

Note: See Diagnostic screen for more details.

Internal details

I decided to use fixed connection layout to simplify code. System is internally "task" oriented.

Physical connections

Serial (USB) interface (connection)

Serial converter is connected to PD0 / PD1 on micro-controller. Internal serial logic handle serial interface.

Display

Character display (16x2) in 4bit mode is connected in following way:

PB0 -> RS
PB1 -> E

PD4 -> DB4
PD5 -> DB5
PD6 -> DB6
PD7 -> DB7

Keyboard

Keyboard is connected to analog input on pin AD0. Internally it acts as voltage divider.

Function	Resistor H	Resistor L	Voltage (expected)	Voltage (measured)	ADC readout
Select	1K	0R	0.00 V	0.01 V	000
Right	1K	330R	1.24 V	1.26 V	063
Up	1K	950R	2.44 V	2.47 V	125
Down	1K	1K95	3.31 V	3.33 V	169
Left	1K	5K25	4.20 V	4.23 V	215

Note: Apparently I have at least one LCD Shield for Arduino with different internal connection (No, it is not Chines one).

Development status

Source file	Module	Completed	WIP	Note
global.c	global variables and number to string functions	100 %	-	-
history.c	historical data management	100 %	-	-
keyboard.c	keyboard read / de-bounce	100 %	-	-
lcd.c	lcd interface	100 %	-	-
main.c	board initialization / scheduler / watchdog	100 %	-	-
menu.c	menu handler + code to prepare data for display	100 %	-	-
sensor.c	sensor read / dew point calculation	10 %	dew point sanity check	-
serial.c	serial interface including text protocol	100 %	-	-
timer.c	timer for scheduler / up-time clock	100 %	-	-