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Subject[PATCH 6/30] W1: feature, w1_therm.c use strong pullup and documentation
Added strong pullup to thermal sensor driver and general documentation
on the sensor.

Signed-off-by: David Fries <>
Signed-off-by: Evgeniy Polyakov <>
Documentation/w1/00-INDEX | 2 +
Documentation/w1/slaves/00-INDEX | 4 +++
Documentation/w1/slaves/w1_therm | 41 ++++++++++++++++++++++++++++++++++++++
drivers/w1/slaves/w1_therm.c | 15 ++++++++++++-
4 files changed, 60 insertions(+), 2 deletions(-)

diff --git a/Documentation/w1/00-INDEX b/Documentation/w1/00-INDEX
index 5270cf4..cb49802 100644
--- a/Documentation/w1/00-INDEX
+++ b/Documentation/w1/00-INDEX
@@ -1,5 +1,7 @@
- This file
+ - Drivers that provide support for specific family codes.
- Individual chips providing 1-wire busses.
diff --git a/Documentation/w1/slaves/00-INDEX b/Documentation/w1/slaves/00-INDEX
new file mode 100644
index 0000000..f8101d6
--- /dev/null
+++ b/Documentation/w1/slaves/00-INDEX
@@ -0,0 +1,4 @@
+ - This file
+ - The Maxim/Dallas Semiconductor ds18*20 temperature sensor.
diff --git a/Documentation/w1/slaves/w1_therm b/Documentation/w1/slaves/w1_therm
new file mode 100644
index 0000000..0403aaa
--- /dev/null
+++ b/Documentation/w1/slaves/w1_therm
@@ -0,0 +1,41 @@
+Kernel driver w1_therm
+Supported chips:
+ * Maxim ds18*20 based temperature sensors.
+Author: Evgeniy Polyakov <>
+w1_therm provides basic temperature conversion for ds18*20 devices.
+supported family codes:
+W1_THERM_DS18S20 0x10
+W1_THERM_DS1822 0x22
+W1_THERM_DS18B20 0x28
+Support is provided through the sysfs w1_slave file. Each open and
+read sequence will initiate a temperature conversion then provide two
+lines of ASCII output. The first line contains the nine hex bytes
+read along with a calculated crc value and YES or NO if it matched.
+If the crc matched the returned values are retained. The second line
+displays the retained values along with a temperature in millidegrees
+Centigrade after t=.
+Parasite powered devices are limited to one slave performing a
+temperature conversion at a time. If none of the devices are parasite
+powered it would be possible to convert all the devices at the same
+time and then go back to read individual sensors. That isn't
+currently supported. The driver also doesn't support reduced
+precision (which would also reduce the conversion time).
+The module parameter strong_pullup can be set to 0 to disable the
+strong pullup or 1 to enable. If enabled the 5V strong pullup will be
+enabled when the conversion is taking place provided the master driver
+must support the strong pullup (or it falls back to a pullup
+resistor). The DS18b20 temperature sensor specification lists a
+maximum current draw of 1.5mA and that a 5k pullup resistor is not
+sufficient. The strong pullup is designed to provide the additional
+current required.
diff --git a/drivers/w1/slaves/w1_therm.c b/drivers/w1/slaves/w1_therm.c
index fb28aca..e87f464 100644
--- a/drivers/w1/slaves/w1_therm.c
+++ b/drivers/w1/slaves/w1_therm.c
@@ -37,6 +37,14 @@ MODULE_LICENSE("GPL");
MODULE_AUTHOR("Evgeniy Polyakov <>");
MODULE_DESCRIPTION("Driver for 1-wire Dallas network protocol, temperature family.");

+/* Allow the strong pullup to be disabled, but default to enabled.
+ * If it was disabled a parasite powered device might not get the require
+ * current to do a temperature conversion. If it is enabled parasite powered
+ * devices have a better chance of getting the current required.
+ */
+static int w1_strong_pullup = 1;
+module_param_named(strong_pullup, w1_strong_pullup, int, 0);
static u8 bad_roms[][9] = {
{0xaa, 0x00, 0x4b, 0x46, 0xff, 0xff, 0x0c, 0x10, 0x87},
@@ -192,9 +200,12 @@ static ssize_t w1_therm_read_bin(struct kobject *kobj,
int count = 0;
unsigned int tm = 750;

+ /* 750ms strong pullup (or delay) after the convert */
+ if (w1_strong_pullup)
+ w1_next_pullup(dev, tm);
w1_write_8(dev, W1_CONVERT_TEMP);
- msleep(tm);
+ if (!w1_strong_pullup)
+ msleep(tm);

if (!w1_reset_select_slave(sl)) {

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