CGMMSTICK 1-Wire Interface Example

As an example of the operation of the 1-wire function of the CGMMSTICK1, this example interfaces to a DS1822 temperature device.

From the DS1822 datasheet:

The DS1822 digital thermometer provides 9- to 12-bit centigrade temperature measurements and has an alarm function with NV user-programmable upper and lower trigger points. The DS1822 communicates over a 1-Wire bus that by definition requires only one data line (and ground) for communication with a central microprocessor. It has an operating temperature range of –55°C to +125°C and is accurate to ±2.0°C over the range of –10°C to +85°C.

Unique 1-Wire® interface requires only one port pin for communication.

Each device has a unique 64-bit serial code stored in an on-board ROM.

The hardware couldn't be simpler.

The DS1822 pin 1 was connected to ground, pin 3 connected to 5V, and the center pin, pin 2, connected to the CGMMSTICK I/O pin 20. The center pin is the data pin of the 1-wire device. I/O pin 20 has a 5k ohm (two 10k in parallel) pullup.

From the software perspective reading from a 1-wire device isn't as simple as just sending a read command. You have to write to the device first to tell it you want to read.

Furthermore it takes a moment for the device to start and complete the temperature measurement process.

First the 'bus' should be reset. In this case the 1-wire bus is I/O Pin 20 and only contains the single DS1822 device.

' Reset the 1w bus, DS1822 device

OWRESET 20

The reset command selects the pin (bus) to reset and optionally can receive an indication that there are 1 or more devices present on the bus. This puts a reset pulse on the bus.

All devices have a serial number, so that if there are multiple devices on a single I/O Pin (bus), any individual device can be addressed for access.

In this example there is only a single device, so rather than talking to it by addressing it specifically, a command is used that skips the need for a serial number. This is called “Skip ROM” in the 1-wire terminology.

From the data sheet:

SKIP ROM [CCh]

The master can use this command to address all devices on the bus simultaneously without sending out any ROM code information. For example, the master can make all DS1822s on the bus perform simultaneous temperature conversions by issuing a Skip ROM command followed by a Convert T [44h] command.

Note that the Read Scratchpad [BEh] command can follow the Skip ROM command only if there is a single slave device on the bus. In this case time is saved by allowing the master to read from the slave without sending the device’s 64-bit ROM code. A Skip ROM command followed by a Read Scratchpad command will cause a data collision on the bus if there is more than one slave since multiple devices will attempt to transmit data simultaneously.

The DS1822 will be told to start a temperature conversion.

' Start temperature conversion

' Pin 20, send reset first, send two bytes

' &hCC - Skip ROM

' &h44 - Start conversion

OWWRITE 20, 1, 2, &hCC, &h44

The OWWRITE command selects a pin/bus to write to. A reset pulse again is sent to that bus and then two bytes are sent. &hCC is the command byte to skip the serial number, and &h44 tells the DS1822 to start the temperature conversion process.

The conversion process takes a little while to complete, so the program pauses to allow that conversion to finish.

' Wait a second for conversion process to finish

PAUSE 100

After that pause, a command is sent that tells the DS1822 that the next communication will be a read.

' Read data/temperature command

' Pin 20, send reset first, send two bytes

' &hCC - Skip ROM

' &hBE - read data command

OWWRITE 20, 1, 2, &hCC, &hBE

This again resets the bus and uses “Skip ROM” since this example uses only a single 1-wire device. &hBE is the command to read.

The read command is next to read the temperature value.

' Read data/temperature from DS1822

' Pin 20, send reset after, read two bytes

OWREAD 20, 2, 2, LowTemp, HighTemp

Note that the bus is NOT reset at the start of the read command. The DS1822 expects the read to follow the command to read without a reset between the commands.

OWREAD reads the selects bus line. The options to the command allow for a selected number of bytes to be read. In this case we need to read only the first two bytes of the multi-byte register, the registers that contain the temperature data. A bus reset happens after the OWREAD command.

Math is performed to combine the two bytes that were read from the temperature device according to the DS1822 data sheet.

' Combine and adjust according to data sheet

Cel = ((HighTemp And &b111) * 256 + LowTemp) / 16

The resulting Celsius temperature is printed, then converted to Fahrenheit and that is also printed.

' Print Celsius and convert/print Fahrenheit

Print "Celsius" Cel

Print "Fahrenheit" ( 9 / 5) * Cel + 32;

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