Application notes 68HC11
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Interfacing the DS1620 to the Motorola SPI Bus
Dallas Semiconductor makes Thermal Management easy with its line of direct-to-digital temperature sensors. These sensors provide a digital reading of temperature directly, eliminating the need for A/D converters dedicated to temperature to digital conversions. Factory-calibrated to relieve the user of linearity corrections and other compensation, Dallas Semiconductor's sensors provide a range and accuracy unparalleled in the industry.
The DS1620 Digital Thermometer and Thermostat provides 9-bit temperature readings which indicate the temperature of the device. It is available with three thermostatic outputs which change based on where the temperature is in relation to user-programmed trip points. These trip points are stored in onboard EEPROM memory and are nonvolatile.
For multi-point sensing applications, there are temperature sensors, DS1820 series, which can bemultidropped through the Dallas 1-Wire interface. 
The DS1620 has a single bi-directional data pin (DQ) which can directly communicate with the SPI interface found on many Motorola microcontrollers.The method to obtain this, is to use the MOSI and MISO of the SPI system wired together and to control the direction of the MOSI from software.In addition, the DS1620 transfers the LSB first, while SPI uses the MSB as the first bit.Using the SPI protocol CPOL,CPHA = 11, there is no contention between MOSI and the DQ output.
This application note describes a straightforward way to connect several DS1620's (and other 3-wire components) to the SPI system.
* Test program for DS1620 ------ 68HC11** DQ -------- MOSI and MISO tied together* The MOSI can easily be turned off in* the DDRD register.* When a read is performed, the DQ* pin goes to a high impedance state* while the clock is high * (CPOL,CPHA=11 is used).* CLK ------- SCK* RST1 ------ PB0 DS1620 #1* RST2 ------ PB1 DS1620 #2 etc.* You can easily connect several DS1620's** The program reads a DS1620 and converts the temperature* to a readable value in reg Y*************** Equates:*-------------IOBASE equ $1000PORTB equ $1004DDRD equ $1009SPCR equ $1028SPDR equ $102Aportb equ $04 offsetsddrd equ $09spsr equ $29RST1 equ %00000001RST2 equ %00000010 etc.MOSI equ %00001000SPIF equ %10000000St_conv equ %01110111 (mirrored)Rd_temp equ %01010101 (mirrored)Wr_conf equ %00110000 (mirrored)Stack equ $00ff*********************** Initializations:*--------------------- org $f800 RESET lds #Stack jsr InitSPI jsr InitDS************************ Main loop*----------------------Loop jsr Read_DS1 jsr Mirror jsr BtoCent jsr Read_DS2 jsr Mirror jsr BtoCent bra Loop*********************************************** Subroutine Read_DS1* Reads DS1620 #1 and leaves the value* in accB*---------------------------------------------Read_DS1 pshx ldx #IOBASE bset portb,X,RST1 Select DS1620 #1 bsr Read_DS Common routine bclr portb,X,RST1 Deselect #1 pulx rts*********************************************** Subroutine Read_DS2* Reads DS1620 #2 and leaves the value* in accB*---------------------------------------------Read_DS2 pshx ldx #IOBASE bset portb,X,RST2 Select DS1620 #2 bsr Read_DS Common routine bclr portb,X,RST2 Deselect #2 pulx rts*********************************************** Subroutine Read_DS* Reads the DS1620 pointed out by* the RST signal* Leaves the value in accB, sign in accA*---------------------------------------------Read_DS bset ddrd,X,MOSI MOSI on ldab #Rd_temp Read temp command stab SPDR brclr spsr,X,SPIF,* bclr ddrd,X,MOSI MOSI off stab SPDR brclr spsr,X,SPIF,* ldab SPDR stab SPDR brclr spsr,X,SPIF,* ldaa SPDR rts*********************************************** Subroutine InitSPI*---------------------------------------------InitSPI ldaa #%110000 As default, MOSI is turned off staa DDRD ldaa #%01011100 CPOL,CPHA = 11 staa SPCR rts*********************************************** Subroutine InitDS* Initiates both DS1620's for continous* conversion**********************************************InitDS ldx #IOBASE bset portb,X,RST1+RST2 bset ddrd,X,MOSI MOSI on ldaa #Wr_conf Write config staa SPDR brclr spsr,X,SPIF,* ldaa #%01000000 CPU mode, continous conversion staa SPDR brclr spsr,X,SPIF,* bclr portb,X,RST1+RST2 bset portb,X,RST1+RST2 ldaa #St_conv Start convert staa SPDR brclr spsr,X,SPIF,* bclr ddrd,X,MOSI MOSI off bclr portb,X,RST1+RST2 rts******************************************** Subroutine BtoCent* The value in B is converted to Centigrade* in reg Y.*------------------------------------------BtoCent psha pshb pshx clra ldx #20 Divide with 20 idiv pshx pula pula Tenths --> A lslb lslb lslb Units --> B_high bitb #%00001000 Test B_low for %1000 or %0000 beq no if %1000, make it %0101 (5) subb #3no xgdy Put A:B into Y pulx pulb pula rts**************************** Subroutine Mirror* Mirrors the values in B*--------------------------Mirror psha rorb rola rorb rola rorb rola rorb rola rorb rola rorb rola rorb rola rorb rola tab pula rts org $fffe fdb RESET end
Stefan Nyman © Micronym 1999