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#include <pic16f84a.h>
typedef unsigned char uint8_t;
__sfr __at(0x70) UART_TX;
__sfr __at(0x72) UART_STATUS;
__sfr __at(0x73) OW_DATA;
__sfr __at(0x74) OW_CMD;
__sfr __at(0x75) OW_STATUS;
#define OW_RESET 0x00
#define OW_WRITE 0x01
#define OW_READ 0x02
#define OW_BUSY 0x01
#define OW_PRESENCE 0x02
#define CMD_SKIP_ROM 0xCC
#define CMD_CONVERT_T 0x44
#define CMD_READ_SCRATCH 0xBE
/*
* Fractional nibble lookup: each step is 0.0625 deg C.
* Tables give the two decimal digits (tenths, hundredths) truncated.
* e.g. frac=1 -> 0.0625 -> prints ".06"
* frac=8 -> 0.5000 -> prints ".50"
*/
static const uint8_t frac_tens[16] = {0,0,1,1,2,3,3,4,5,5,6,6,7,8,8,9};
static const uint8_t frac_hund[16] = {0,6,2,8,5,1,7,4,0,6,2,8,5,1,7,3};
/*
* Timer0 delay. OPTION_REG 0x07: T0CS=0 (Fosc/4), PSA=0, PS=111 (1:256).
* At 50 MHz: overflow every 256*256 / (50M/4) = 5.24 ms.
* 143 overflows ≈ 750 ms.
*/
#define STATUS_RP0 0x20
#define T0IF 0x04
static void wait_overflows(uint8_t n) {
INTCON &= ~T0IF;
while (n--) {
while (!(INTCON & T0IF));
INTCON &= ~T0IF;
}
}
static void delay_750ms(void) { wait_overflows(143); }
static void uart_send(uint8_t b) {
while (UART_STATUS & 0x01);
UART_TX = b;
}
static void uart_puts(const char *s) {
while (*s) uart_send((uint8_t)*s++);
}
static void ow_wait(void) {
while (OW_STATUS & OW_BUSY);
}
static void ow_write_byte(uint8_t b) {
OW_DATA = b;
OW_CMD = OW_WRITE;
ow_wait();
}
static uint8_t ow_read_byte(void) {
OW_CMD = OW_READ;
ow_wait();
return OW_DATA;
}
static void print_uint8(uint8_t v) {
uint8_t started = 0;
if (v >= 200) { uart_send('2'); v -= 200; started = 1; }
else if (v >= 100) { uart_send('1'); v -= 100; started = 1; }
{
uint8_t t = '0';
while (v >= 10) { v -= 10; t++; }
if (started || t != '0') uart_send(t);
}
uart_send(v + '0');
}
void main(void) {
uint8_t lo, hi, sign, temp_int, frac;
uint8_t i;
/* Configure Timer0: bank 1, OPTION_REG = 0x07 (Fosc/4, prescaler 1:256) */
STATUS |= STATUS_RP0;
TMR0 = 0x07;
STATUS &= ~STATUS_RP0;
uart_puts("\x1B[2J\x1B[H");
uart_puts("DS18B20 Temperature\r\n");
uart_puts("-------------------\r\n");
while (1) {
/* Reset */
OW_CMD = OW_RESET;
ow_wait();
if (!(OW_STATUS & OW_PRESENCE)) {
uart_puts("No sensor\r\n");
delay_750ms();
continue;
}
/* Start temperature conversion */
ow_write_byte(CMD_SKIP_ROM);
ow_write_byte(CMD_CONVERT_T);
delay_750ms();
/* Reset and read scratchpad */
OW_CMD = OW_RESET;
ow_wait();
if (!(OW_STATUS & OW_PRESENCE)) {
uart_puts("Lost sensor\r\n");
continue;
}
ow_write_byte(CMD_SKIP_ROM);
ow_write_byte(CMD_READ_SCRATCH);
lo = ow_read_byte(); /* scratchpad byte 0: temp LSB */
hi = ow_read_byte(); /* scratchpad byte 1: temp MSB */
for (i = 0; i < 7; i++) ow_read_byte(); /* discard remaining bytes */
/*
* DS18B20 12-bit format:
* sign = hi bit 7 (and sign-extended through hi[7:4])
* integer part = hi[3:0]:lo[7:4]
* fractional nibble = lo[3:0] (each bit = 0.0625 deg C)
*
* Negative values are two's complement of the 16-bit word.
*/
sign = hi & 0x80;
if (sign) {
lo = ~lo;
hi = ~hi;
if (lo == 0xFF) { lo = 0; hi++; }
else { lo++; }
}
temp_int = ((hi & 0x0F) << 4) | (lo >> 4);
frac = lo & 0x0F;
uart_puts("\rTemp: ");
if (sign) uart_send('-');
print_uint8(temp_int);
uart_send('.');
uart_send('0' + frac_tens[frac]);
uart_send('0' + frac_hund[frac]);
uart_puts(" C\r\n");
}
}
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