barcode reader application in asp.net PIC18 RTOS DESIGN in Software

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PIC18 RTOS DESIGN
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Figure 13.6 The value of the bit written to the DS1820 depends on the amount of time the data line is held low.
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Figure 13.7 Data coming from the DS1820 is in a similar format to the data written to the chip.
REAL-TIME OPERATING SYSTEMS
PICmicro Initiated Reset
16 Bits of Data Sent to DS1820
DS1820 Response
1) Ch1
5 Volt
500 us
Figure 13.8 Resetting the DS1820 consists of holding down the data line for at least 500 ms.
pulling the line low for about 60 s after the line has gone high. After the reset pulse, the DS1820 should not be accessed again for 1 ms. Figure 13.8 shows the reset pulse along with the DS1820 response for a temperature read operation. Note that the DS1820 response is at a slightly higher voltage level than the PIC microcontroller-initiated reset. This is an example of bus contention, which is caused by the PIC microcontroller still driving the line high and the DS1820 trying to pull it down low. In the code that follows and that on the CD-ROM, the PIC microcontroller s I/O pin is not driving the line for the 1 ms of the reset operation to allow the DS1820 to drive the line without interference from the PIC18C452. When I rst coded this application, I created three subroutines for operating the DS1820 from the DS87C520 from this application. The rst routine was DSReset, which pulled the line down for 480 s and then waited 1 ms before returning to its caller:
DSReset: bcf movlw addlw btfss bra bsf bcf movlw addlw DS1820 125 0x0FF STATUS, Z $ - (2 * 2) DS1820 DSTRIS 0 0x0FF ; Reset the DS1820 ; Hold the DS1820 Low for 500 ; ; secs to reset
Add -1 until Reset is Equal to Zero 4 Cycles for Each Loop
Wait 1 ms before sending a command
PIC18 RTOS DESIGN
btfss bra bsf bsf return
STATUS, Z $ - (2 * 2) DSTRIS DS1820
The transmit routine, DSSend, was coded as a simple subroutine with no hooks used for the RTOS:
DSSend: movwf FSR0L, 0 clrf FSR0H, 0 movlw 8 DSSendLoop: bcf INTCON, GIE bcf DS1820 rrcf FSR0L, f, 0 btfsc STATUS, C, 0 bsf DS1820 bsf FSR0H, 3, 0 decfsz FSR0H, f, 0 bra $ - (2 * 1) bra $ + (2 * 1) bsf DS1820 bsf INTCON, GIE bsf FSR0H, 3, 0 decfsz FSR0H, f, 0 bra $ - (2 * 1) addlw 0x0FF btfss STATUS, Z, 0 bra DSSendLoop return ; Send the Byte in WREG to the DS1820
; ; ; ; ;
Make Sure Operation isn t interrupted Drop the DS1820 s Control Line Load Carry with Contents of the If 1 Sent, Restore After 4 Cycles Loop for 24 Cycles
; Put in a Full 30 Cycle Delay ; The Line is High ; Restore the Interrupts ; Loop Another 24 Cycles for Execution Delay
Subtract 1 from the Count
Finished, Return to Caller
There is one thing to note in both DSSend and DSRead (the data receive routine from the DS1820), and that is that the rst thing that I do is to mask interrupts. This was done to ensure that the TMR0 interrupt does not interrupt the timed data transfers when the DS1820 line is low. If the TMR0 interrupt occurs after the data bit is transferred, then there won t be any problems, and execution can pick up just where it left off. Reading data from the DS1820 was accomplished by pulsing the line low and then seeing how long it would stay low:
DSRead: movlw 8 DSReadLoop: bcf INTCON, GIE bcf DS1820 ; Receive the Byte from the DS1820 and put ; in WREG ; ; Make Sure Operation isn t interrupted Drop the DS1820 s Control Line
REAL-TIME OPERATING SYSTEMS
bsf nop bsf btfss bcf rrcf bsf clrf decfsz bra bsf bcf bsf addlw btfss bra movf return
DSTRIS
; ; ; ; ; ;
Turn Port into a simulated Open Drain Output What is Being Returned If Line is high, a 1 Shift in the Data bit Can Interrupt from here
STATUS, C, 0 DS1820 STATUS, C, 0 FSR0L, f, 0 INTCON, GIE FSR0H, 0 FSR0H, f, 0 $ - (2 * 1) DS1820 DSTRIS DS1820 0x0FF STATUS, Z, 0 DSReadLoop FSR0L, w, 0
Loop Around for another Bit
Return the Byte Read in Finished, Return to Caller
To read data from the DS1820, I carried out the following instruction process:
1 2 3 4 5 6 7 8
Reset the DS1820. Send 0CCh followed by 044h to begin the temperature sense and conversion. Wait 480 s for the temperature conversion to complete. Send another reset to the DS1820. Send 0CCh and 0BEh to read the temperature. Wait 100 s before reading the rst byte in the DS1820. Read the rst, or SP0, byte of the DS1820. Wait another 100 s before reading the second, or SP1, byte of the DS1820.
The DS1820 has a unique serial number burned into it. This allows multiple temperature sensors (and other devices using the Dallas Semiconductor one-wire protocol) to be placed on the same pulled-up bus. To avoid rst having to read the serial number out of the device and then referencing it each time a command is being sent to it, a 0CCh byte is sent rst. This indicates to the DS1820 that it is to send the temperature without checking for a valid serial number being sent. I found that for the DS1820 to work properly, I had to use the typical values for the data writes and not extremes in the speci cation. When I rst started working with the DS1820, I used the minimums (15 s for a 0) because I wanted to keep the time the microcontroller was running with interrupts masked to a minimum. When I did this, I found that there were problems with the DS1820 not recognizing correct data. After changing the timings to the typical values in the datasheets, the DS1820 seemed to respond properly, but instead of a single, long pulse for each bit returned, I received a valid mix of short and longer pulses.
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