Interfacing the DS1620 to the

文章摘要：Software for the InterfaceWhile the hardware for the interface is relatively straightforward, the rest of the SPI/DS1620 interface must be handled by software. The following example shows a way to do this in the case of reading the temperature from the DS1620. This code fragment assumes that the DS1

Interfacing the DS1620 to the,标签：计算机控制技术,工厂电气控制技术,http://www.88dzw.com

Software for the Interface

While the hardware for the interface is relatively straightforward, the rest of the SPI/DS1620 interface must be handled by software. The following example shows a way to do this in the case of reading the temperature from the DS1620. This code fragment assumes that the DS1620 has already been initialized, that the configuration register is set up properly, and that temperature conversions have been initiated. See the DS1620 data sheet for details on these operating modes.

Before accessing the DS1620, the DIR signal must be asserted low for a WRITE transfer to occur. RST-bar must be driven high to enable the DS1620. The SPI controller sends out the protocol (eight bits long) to the DS1620. Again, note that SPI sends information MSB first, while the DS1620 communicates LSB first. In order to accomplish this, a software "mirror" should be used to reverse the bit order. An example of such a function is given by:

unsigned char mirror(unsigned char value)
{
     unsigned char i;
     unsigned char value_mirrored = 0x00;

     for (i=0;i<=7;i++)
     {
        value_mirrored = value_mirrored | (((value>>i)&0x01)<<(7-i));
}
return (value_mirrored);
}

With the protocol sent, the DIR is changed from low to high (indicating now a READ transfer) because the DS1620 is ready to send out the 9-bit value. Note that RST-bar is still high. The SPI controller reads the first eight bits of the 9-bit value (LSB first). The software has to "mirror" the received byte. The 9th bit (followed by seven dummy bits) is pulled out by making another READ transfer and keeping DIR and RST-bar as they are. When the second byte is received, the software again mirrors it and pulls RST-bar low, terminating communication with the DS1620.

#define     RST_bit              0 /* PB0 */
#define     RST_port             PORTB
#define     DIR_bit              1 /* PB1 */
#define     DIR_port             PORTB
#define     READ_TEMP_CMD        0xAA

unsigned int read_temp(void)
{
     unsigned char temp_value_lo;
     unsigned char temp_value_hi;

     DIR_port = DIR_port & ~(1<<DIR_bit);         /* DIR = L WRITE mode */
     RST_port = RST_port | (1<<RST_bit);          /* RST = HI: DS1620 enabled */
     SPDR = mirror(READ_TEMP_CMD);                /* Send protocol to DS1620 */
     DIR_port = DIR_port | (1<<DIR_bit);          /* DIR = HI: READ mode */
     while ((SPSR & (1<<SPIF_bit)) == 0);         /* Wait for SPI flag = ready */
     temp_value_lo = mirror(SPDR);                /* Receive 8 lowest bits */
     temp_value_hi = mirror(SPDR);                /* Receive 8 highest bits */
     RST_port = RST_port & ~(1<<RST_bit);         /* RST = L Temp. reading done */
     return ((temp_value_hi<<8)+temp_value_lo);   /* Return the 9-bit value */
}

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