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--- en:pfw:dht22 [2025-04-25 01:29] – [All about noForth] mka
+++ en:pfw:dht22 [2025-04-26 16:16] (current) – [Numerical representation of the measured values] mka
@@ Line 37: / Line 37: @@
 The microcontroller needs to carefully time these pulses to correctly interpret the data sent by the DHT22. Libraries for popular platforms like Arduino and Raspberry Pi handle these timing-critical operations.
 [[https://wiki.forth-ev.de/lib/exe/fetch.php/pfw:am2302-en-aosong.pdf|am2302-en-aosong.pdf]] This Aosong document has the correct data, although it is a creepy English translation.
+On page 2, under 4, "The definition of single-bus interface," pin 2 of the sensor is called **SDA, Serial Data**. A table and a figure show the pin assignment of the AM2302.
 {{ :en:pfw:dht22-response.png?nolink |Screenshot of the 40-bit response on the SDA line. (PulsView Logic Analyser)}}
@@ Line 76: / Line 78: @@
 </code>
-==== Source Code ====
+==== Numerical representation of the measured values ====
+=== Humidity ===
+The first 16 bits coming from the DHT22 are the relative humidity in tenths of a percent. They can be processed directly with the 16-bit noForth and correspond to a positive integer. Rounded to whole digits for display purposes.
+Here is an excerpt from the noforth source code:
+<code>
+: .hum ( hum -- )   10 / . ." %rel" space  ;
+</code>
+=== Temperature ===
+The next 16 bits are the temperature in tenths of a degree Celsius. Temperatures from zero degrees and higher can be processed directly using the 16-bit noForth; they correspond to a positive integer.
+Temperatures below zero degrees are represented by the DHT22 as follows: The MSB of the 16-bits is set, but otherwise the temperature is specified as a positive integer. This number format //does not correspond// to the two's complement of noForth. Here, the MSB must first be evaluated to determine the sign. The sign is prepended for output. The measured value can then be further processed as a positive integer.  Whole degrees are displayed to the left of the decimal point, tenths to the right. The unit °C is specified.
+<code>
+: .tmp ( tmp -- )
+  dup hx 8000 and hx 8000 =  \ check MSB
+      if [char] -
+      else [char] +
+      then emit
+  hx 7fff and ( tmp -- +tmp )    \ reset MSB
+/mod 3 .r [char] . emit .    \ --> TTT.T
+  [char] f decemit  [char] C emit  \ print °C
+  space
+  ;
+ </code>
+  * This is already included in the following source code.\\
+  * And here you can see how this was discovered: [[en:pfw:dht22tmpformat|The temperature format of the DHT22]]
+==== Source Code - and what it was used for.====
 [[en:pfw:dht22-msp430g2553-noforth|DHT22 with Launchpad MSP430G2553 and noForth]]
+This program was used to test DHT22 modules before installation. The DHT22 modules were tested for functionality at room temperature, pre-sorted in this way, and then frozen in a bag of moist sand in the freezer and tested again. One module displayed incorrect temperature values in frost conditions, the display of negative values did not comply with the specifications.
 ==== More information on the MSP430G2553 ====
@@ Line 100: / Line 136: @@
 To work with the embedded noForth system I use //e4thcom// by Manfred Mahlow.
 https://wiki.forth-ev.de/doku.php/projects:e4thcom
+Sometimes I also use Windows and Teraterm, for example, to log longer MCU outputs to a file. I also prefer it for testing parts of the source code using copy and paste.
 === Logic Analyser ===