On Thu, Mar 3, 2011 at 5:19 AM, Oli Glaser <oli.glaser@talktalk.net> wrote:=
=0A=
> On 02/03/2011 23:18, Manu Abraham wrote:=0A=
>> So, if the resistor is the STRONG_PULLUP, then why=0A=
>> mention in the software flowchart, that STRONG_PULLUP is required in=0A=
>> parasite mode ? Logically, I am more drawn to the micro pulling up the=
=0A=
>> pin using TRIS=3D0 and LAT=3D1, though I am not still very clear on that=
=0A=
>> aspect.=0A=
>=0A=
> The resistor is not the strong (enough) pull up. The datasheet=0A=
> recommends a MOSFET to Vdd but it may be fine to use a PIC pin set to=0A=
> high as the datasheet says there is no activity on the bus during the=0A=
> time the strong pull up is activated. The current needed is about 1.5 mA=
=0A=
> so it should work okay.=0A=
> The strong pull up is necessary in parasite mode as the 4k7 pull=0A=
> up/parasite power capacitor cannot supply enough current for some=0A=
> commands, namely Temperature Conversion and Write Scratchpad. The=0A=
> DS18B20 says on page 5:=0A=
>=0A=
> In parasite power mode, the 1-Wire bus and CPP can provide sufficient=0A=
> current to the DS18B20 for most=0A=
> operations as long as the specified timing and voltage requirements are=
=0A=
> met (see the DC Electrical=0A=
> Characteristics and AC Electrical Characteristics). However, when the=0A=
> DS18B20 is performing=0A=
> temperature conversions or copying data from the scratchpad memory to=0A=
> EEPROM, the operating current=0A=
> can be as high as 1.5mA. This current can cause an unacceptable voltage=
=0A=
> drop across the weak 1-Wire=0A=
> pullup resistor and is more current than can be supplied by CPP. To=0A=
> assure that the DS18B20 has sufficient=0A=
> supply current, it is necessary to provide a strong pullup on the 1-Wire=
=0A=
> bus whenever temperature=0A=
> conversions are taking place or data is being copied from the scratchpad=
=0A=
> to EEPROM. This can be=0A=
> accomplished by using a MOSFET to pull the bus directly to the rail as=0A=
> shown in Figure 4. The 1-Wire=0A=
> bus must be switched to the strong pullup within 10=ECs (max) after a=0A=
> Convert T [44h] or Copy Scratchpad=0A=
> [48h] command is issued, and the bus must be held high by the pullup for=
=0A=
> the duration of the conversion=0A=
> (tCONV) or data transfer (tWR =3D 10ms). No other activity can take place=
=0A=
> on the 1-Wire bus while the pullup=0A=
> is enabled.=0A=
=0A=
That clears up almost all the doubts I had. Much appreciated.=0A=
=0A=
I have the following pseudo code in C18, does it look sane enough with=0A=
regards to the timings ?=0A=
=0A=
#define OW_PORT			LATAbits.LATA0=0A=
#define OW_PORT_MODE		TRISAbits.TRISA0=0A=
=0A=
#define OW_STRONG_PU		(OW_PORT_MODE =3D 0, OW_PORT =3D 1)=0A=
#define OW_PORT_HIGH		OW_STRONG_PU=0A=
#define OW_PORT_LOW		(OW_PORT_MODE =3D 0, OW_PORT =3D 0)=0A=
#define OW_PORT_RPU		(OW_PORT_MODE =3D 1, OW_PORT =3D 0)=0A=
=0A=
const rom char crc_table[] =3D {=0A=
	0,94,188,226,97,63,221,131,194,156,126,32,163,253,31,65,=0A=
	157,195,33,127,252,162,64,30,95,1,227,189,62,96,130,220,=0A=
	35,125,159,193,66,28,254,160,225,191,93,3,128,222,60,98,=0A=
	190,224,2,92,223,129,99,61,124,34,192,158,29,67,161,255,=0A=
	70,24,250,164,39,121,155,197,132,218,56,102,229,187,89,7,=0A=
	219,133,103,57,186,228,6,88,25,71,165,251,120,38,196,154,=0A=
	101,59,217,135,4,90,184,230,167,249,27,69,198,152,122,36,=0A=
	248,166,68,26,153,199,37,123,58,100,134,216,91,5,231,185,=0A=
	140,210,48,110,237,179,81,15,78,16,242,172,47,113,147,205,=0A=
	17,79,173,243,112,46,204,146,211,141,111,49,178,236,14,80,=0A=
	175,241,19,77,206,144,114,44,109,51,209,143,12,82,176,238,=0A=
	50,108,142,208,83,13,239,177,240,174,76,18,145,207,45,115,=0A=
	202,148,118,40,171,245,23,73,8,86,180,234,105,55,213,139,=0A=
	87,9,235,181,54,104,138,212,149,203,41,119,244,170,72,22,=0A=
	233,183,85,11,136,214,52,106,43,117,151,201,74,20,246,168,=0A=
	116,42,200,150,21,75,169,247,182,232,10,84,215,137,107,53=0A=
};=0A=
=0A=
void ow_parasite_init(void)=0A=
{=0A=
	OW_STRONG_PU; /* initially power parasite */=0A=
	mdelay(1); /* wait a bit */=0A=
}=0A=
=0A=
int8_t ow_parasite_reset(void)=0A=
{=0A=
	int8_t detect;=0A=
=0A=
	OW_PORT_LOW;    /* DQ Low by Master */=0A=
	udelay250();    /* 480us min delay */=0A=
	udelay250();=0A=
	OW_PORT_RPU;	/* resistor pulls it up */=0A=
	udelay80();     /* wait for slave to pull it down */=0A=
	detect =3D OW_PORT; /* read Slave state */=0A=
	udelay250();	/* wait for end of Slot */=0A=
	udelay250();=0A=
	return detect;=0A=
}=0A=
=0A=
int8_t ow_parasite_get_bit(void)=0A=
{=0A=
	int8_t val;=0A=
=0A=
	OW_PORT_LOW;	/* DQ LO by Master */=0A=
	udelay1();	/* wait for 1us */=0A=
	OW_PORT_RPU;	/* release */=0A=
	udelay10();	/* master samples within 15us */=0A=
	val =3D OW_PORT;	/* read parasite state */=0A=
	udelay50();	/* wait for end of Slot */=0A=
}=0A=
=0A=
int8_t ow_parasite_set_bit(uint8_t b)=0A=
{=0A=
	OW_PORT_LOW;	/* master pulls LO */=0A=
	if (b) {=0A=
		udelay1();=0A=
		OW_PORT_HIGH;=0A=
	}=0A=
	udelay60();=0A=
	OW_PORT_RPU;=0A=
}=0A=
=0A=
uint8_t ow_parasite_read(void)=0A=
{=0A=
	uint8_t i, val;=0A=
=0A=
	for (i =3D 0; i < 8; i++) {=0A=
		if (ow_parasite_get_bit())=0A=
			val |=3D (1 << i);=0A=
		udelay50();=0A=
	}=0A=
	return val;=0A=
}=0A=
=0A=
void ow_parasite_write(uint8_t val)=0A=
{=0A=
	uint8_t i;=0A=
=0A=
	for (i =3D 0; i < 8; i++) {=0A=
		ow_parasite_set_bit((val >> i) & 0x01);	=0A=
		udelay50();=0A=
	}=0A=
}		=0A=
=0A=
void read_temperature(void)=0A=
{=0A=
	uint8_t owcrc, scratch[9];=0A=
	int16_t t;=0A=
=0A=
	ADCON1	=3D 0x0f;=0A=
=0A=
		ow_parasite_init();	/* parasite power */=0A=
		if (!ow_parasite_reset()) {	/* detect parasite */=0A=
			dev_1w =3D 1;=0A=
			ow_parasite_write(0xcc); /* skip ROM cmds */=0A=
			ow_parasite_write(0x44); /* convert temp. */=0A=
			OW_STRONG_PU;		 /* master enables strong pullup */=0A=
			mdelay(250); /* wait 750ms */=0A=
			mdelay(250);=0A=
			mdelay(250);=0A=
			OW_PORT_RPU; /* master disables strong pullup */=0A=
		}=0A=
		if (!ow_parasite_reset()) {=0A=
			ow_parasite_write(0xcc); /* skip ROM cmds */=0A=
			ow_parasite_write(0xbe); /* read scratchpad */=0A=
			for (bytes =3D 0; bytes < 8; bytes++) {=0A=
				scratch[bytes] =3D ow_parasite_read();=0A=
				crc_table[owcrc ^ scratch[bytes]];=0A=
			}=0A=
			scratch[8] =3D ow_parasite_read();=0A=
			ow_parasite_reset();=0A=
			if (scratch[8] =3D=3D owcrc) {=0A=
				t =3D scratch[1] << 8 | scratch[0];=0A=
				if (t >=3D 0)=0A=
					t =3D (t + 8) / 16;=0A=
				else=0A=
					t =3D (t - 8) / 16;=0A=
			} else {=0A=
				t =3D 0;=0A=
			}=0A=
		}=0A=
=0A=
}=0A=
=0A=
-- =0A=
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