TC77 Temperature Sensor Interface

The TC77 device operates as a temperature sensor via an SPI interface. Its circuit connections are very simple, and the device has a temperature range of -55°C to +125°C.

The Positron8 BASIC Compiler has built in SPI commands, but the program below shows how to construct a software SPI interface using BASIC commands. It is very fast and very efficient.

The TC77's datasheet can be downloaded from here: TC77.pdf

The circuit for the TC77 is shown below:

The interfacing code is listed below:

' Interface to a TC77 temperature sensor

' Transmits the temperature (in degrees Centigrade) on a serial terminal

'

' For the Amicus18 board, which uses a PIC18F25K20 device operating at 64MHz

' Written by Les Johnson for the Positron8 BASIC Compiler

'

Include "Amicus18.inc" ' Configure the compiler to use a PIC18F25K20 at 64MHz. i.e. An Amicus18 board

Declare Float_Display_Type = Fast ' Set for faster and more accurate floating point displaying

'

' Create some variables

'

Dim TC_fTemperature As Float ' Holds the temperature

Dim TC_bBitCounter As Byte ' Used to count the bits in the bit-bashed SPI interface

Dim TC_wTemperature As SWord ' Used to hold the raw data from the device

'

' Define some pins for the SPI interface to the TC77 device

'

$define TC_SCK_Pin PORTB.0 ' Connects to the TC77's SCK pin

$define TC_CS_Pin PORTB.1 ' Connects to the TC77's CS pin

$define TC_SIO_Pin PORTB.2 ' Connects to the TC77's SIO pin

'-------------------------------------------------------------------------------

GoTo Main ' Jump to the main program loop

'-------------------------------------------------------------------------------

' Setup the TC77 device's SPI interface

' Input : None

' Output : None

' Notes : None

'

$define TC77_Init() '

High TC_CS_Pin '

Low TC_SCK_Pin '

Input TC_SIO_Pin

'----------------------------------------------------------------------

' Read the data from the TC77 device's SPI interface

' Input : None

' Output : TC_wTemperature holds the 16-bit temperature value

' Notes : None

'

Proc TC77_SPI_Read()

PinClear TC_CS_Pin ' Enable the SPI interface

PinClear TC_SCK_Pin ' Pull TC_SCK_Pin low

TC_bBitCounter = 15 ' Set bit counter for 16-bits

Repeat ' Create a loop for the bits

PinSet TC_SCK_Pin ' TC_SCK_Pin rising edge

TC_wTemperature.0 = TC_SIO_Pin ' Receive the bit from the device

PinClear TC_SCK_Pin ' Pull the TC_SCK_Pin low

TC_wTemperature = TC_wTemperature << 1 ' Rotate the bit

Dec TC_bBitCounter

Until TC_bBitCounter = 0

PinSet TC_CS_Pin ' Disable the SPI interface

EndProc

'--------------------------------------------------------------------------------

' Interface to a TC77 temperature sensor

' Input : None

' Output : Returns the temperature (in degrees Centigrade)

' Notes : The TC77 Temperature Register is a 16-bit read only register.

' The temperature data format is a 13-bit two's complement digital

' word (bits 15:3). The least significant bit (LSb) is equal to

' 0.0625 degrees C. Bit 2 is set to a logic '1' after the completion

' of the first temperature conversion following a power-up or reset

' event. Bits 1:0 are tri-stated.

'

Proc TC77_GetTemperature(), Float

TC77_SPI_Read() ' Read the data from the device into TC_wTemperature

TC_wTemperature = TC_wTemperature >> 6 ' Right adjust 13-bit 2's complement in TC_wTemperature

If TC_wTemperature.9 = 1 Then ' Is it a negative temperature?

TC_wTemperature.Byte1 = TC_wTemperature.Byte1 | $FE ' Yes. So sign extend bits 15 to 10 in TC_wTemperature

TC_wTemperature = ~TC_wTemperature ' \ 2's complement

Inc TC_wTemperature ' /

TC_wTemperature = -TC_wTemperature ' Make negative

EndIf

Result = TC_wTemperature / 2 ' Divide by 2 for the temperature

EndProc

'--------------------------------------------------------------------------------

' The main program loop starts here

' Read the temperature and transmit the value as ASCII to a serial terminal

'

Main:

TC77_Init() ' Initialise the TC77 interface

Do

TC_fTemperature = TC77_GetTemperature() ' Get the temperature

HRSOutLn Dec1 TC_fTemperature ' Transmit the temperature serially

DelayMS 500

Loop

Below is the program running in the Proteus simulator:

The above simulation files can be downloaded from here:

TC77_Proteus_Design.zip