wokwi-ds1820-custom-chip

• DS18B20 chip data sheet
• DS18S20 chip data sheet

The DS18B20 Custom Chip simulates the Dallas Semi (Maxim Integrated) One Wire Temperature Sensor of the same names.

Currenly supported are the DS18B20 and DS18S20 chips.

The chip has the following pin groups

Name	Description
VCC	Power pin
GND	ground pin
DQ	Data Pin, used as I/O pin

Addressing

The device uses OneWire compatible addresses, configurable as described below

OneWire Comms

The device uses OneWire for comms with the MCU. This custom chip implements a OneWire Slave (only)

A good Arduino library to use on the Arduino side is the OneWire and the Dallas Temperature Libraries (TBD)

Implementation details

This chip supports all the functions of the hardware part. Commands are listed below

Reset

Write only. Resets the chip to power on.

Search

The device responds to the search command.

Read

The device replies with its serial no bytes.

Match

The device performs a match to the address sent by the master.

Skip

The device skips to wait for function command.

Alarm Search

If the last temperature conversion resulted in a temperature outside the range specified using Th and Tl, the device will respond with its address. Otherwise, only the first bit is transmitted.

Convert

The device stores the temperature that is configured through diagram.json into the scratch pad.

Write Scratchpad

The device writes the Th, Tl and Cfg bytes into the scratch pad.

Read Scratchpad

The device transmits the contents of the scratch pad (9 bytes) to the master

Copy Scratchpad

The device copies the contents of the Th, Tl (and Cfg for DS18B20) bytes from the scratchpad to its eeprom

Recall

The device copies the contents of the Th, Tl (and Cfg for DS18B20) bytes from its eeprom to the scratchpad

Read Power Supply

The device behavior depends on the value of deviceMode attribute see below When configured in Parasitic mode, the chip will pull down the wire when requested to indicate as much. When configured in VCC mode, the chip will leave the wire floating.

Simulation behaviour

Since the implementation is intended to be used together with wokwi, a simulation environment, the chip supports automatically changing the reported temperature according to a wave form at a particular frequency.

The implemented frequency range is 0.0001 - 100 Hz. There are four supported wave forms - Fixed, Sine, Square, Triangle.

By default Fixed is used and the temperature is fixed; in this case the value of the temperature attribute determines the reported temperature. The temperature can be changed during run time by clicking on the element. If the wave form is one of other available forms and the frequency is within the range, the temperature will vary between the values of the attributes minTemp and maxTemp at the frequency specified by the tempWaveFreq attribute.

Attributes

The chip defines a number of attributes that alter the behavior of the operation when used in Wokwi.

Name	Description	Default value
owDebug	controls debug output for base one wire link layer code	"0"
genDebug	controls debug output for the chip code	"0"
deviceID	Specifies the unique 48bit device serial number. This is a string and the value should be limited to precisely 12hex digits Note the device serial's CRC is calculated during init	"010203040506"
familyCode	Specifies the device family code. Supported values include 0x10, 0x22, 0x28 Note that the values have to be specified as decimal and not hex, so 0x28 -> 40, 0x10 -> 16 etc.	"0x10"
temperature	Specifies the reported temperature. Float attribute should be in the range -55 .. 125	"0"
minTemp	Specifies the minimum temperature in the range. Float attribute should be in the range -55 .. 125	"0"
maxTemp	Specifies the maximum temperature in the range. Float attribute should be in the range -55 .. 125	"0"
tempWaveForm	Specifies the temperature wave form. String attribute with the following values: fixed - fixed temperature value set to the value of the temperature attribute. sine - a variable temperature changing using a sine wave form square - a variable temperature changing using a square wave form triangle - a variable temperature changing using a triangle wave form	"fixed"
tempWaveFreq	Specifies the frequency in Hz the temperature changes in. Float attribute should be in the range 0.0001 .. 100.	"0"

Simulator examples

• DS18B20 Custom Chip

Limitations and ommissions

Convert Temperature in Parasitic mode

In this mode, the master is pulling the bus high for a period of at least 10us during which no activity takes place on the bus. The current implementation completes the conversion immediately and is going back to the init sequence. This is likely not an issue, but it deviates from the spec somewhat

Operations in powered mode

If the chip is powered (VCC is high), the current behaviour is to report, when needed (for example when converting/copying scratch or recalling it) that we've completed the operation after 1 bit is transmitted (always '1'). This is likely not representative of real devices who may take some time and perhaps will be implemented at some point later...