path: root/Documentation
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authorKaiwan N Billimoria <>2007-07-17 04:04:05 -0700
committerLinus Torvalds <>2007-07-17 10:23:04 -0700
commit78961a5740374a8143f8fe120300f2ed160dd276 (patch)
treefccfe279a3c01f139bc845f6c1e9575e8e0efd94 /Documentation
parent4917d927809918f0070bd1077b41e3daf78643b2 (diff)
spi_lm70llp parport adapter driver
This adds a driver for the LM70-LLP parport adapter, which is an eval board for the LM70 temperature sensor. For those without that board, it may be a simpler example of a parport-to-SPI adapter then spi_butterfly. Signed-off-by: Kaiwan N Billimoria <> Doc, coding style, and interface updates; build fixes. Minor rename. Signed-off-by: David Brownell <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
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+spi_lm70llp : LM70-LLP parport-to-SPI adapter
+Supported board/chip:
+ * National Semiconductor LM70 LLP evaluation board
+ Datasheet:
+ Kaiwan N Billimoria <>
+This driver provides glue code connecting a National Semiconductor LM70 LLP
+temperature sensor evaluation board to the kernel's SPI core subsystem.
+In effect, this driver turns the parallel port interface on the eval board
+into a SPI bus with a single device, which will be driven by the generic
+LM70 driver (drivers/hwmon/lm70.c).
+The hardware interfacing on the LM70 LLP eval board is as follows:
+ Parallel LM70 LLP
+ Port Direction JP2 Header
+ ----------- --------- ----------------
+ D0 2 - -
+ D1 3 --> V+ 5
+ D2 4 --> V+ 5
+ D3 5 --> V+ 5
+ D4 6 --> V+ 5
+ D5 7 --> nCS 8
+ D6 8 --> SCLK 3
+ D7 9 --> SI/O 5
+ GND 25 - GND 7
+ Select 13 <-- SI/O 1
+ ----------- --------- ----------------
+Note that since the LM70 uses a "3-wire" variant of SPI, the SI/SO pin
+is connected to both pin D7 (as Master Out) and Select (as Master In)
+using an arrangment that lets either the parport or the LM70 pull the
+pin low. This can't be shared with true SPI devices, but other 3-wire
+devices might share the same SI/SO pin.
+The bitbanger routine in this driver (lm70_txrx) is called back from
+the bound "hwmon/lm70" protocol driver through its sysfs hook, using a
+spi_write_then_read() call. It performs Mode 0 (SPI/Microwire) bitbanging.
+The lm70 driver then inteprets the resulting digital temperature value
+and exports it through sysfs.
+A "gotcha": National Semiconductor's LM70 LLP eval board circuit schematic
+shows that the SI/O line from the LM70 chip is connected to the base of a
+transistor Q1 (and also a pullup, and a zener diode to D7); while the
+collector is tied to VCC.
+Interpreting this circuit, when the LM70 SI/O line is High (or tristate
+and not grounded by the host via D7), the transistor conducts and switches
+the collector to zero, which is reflected on pin 13 of the DB25 parport
+connector. When SI/O is Low (driven by the LM70 or the host) on the other
+hand, the transistor is cut off and the voltage tied to it's collector is
+reflected on pin 13 as a High level.
+So: the getmiso inline routine in this driver takes this fact into account,
+inverting the value read at pin 13.
+Thanks to
+o David Brownell for mentoring the SPI-side driver development.
+o Dr.Craig Hollabaugh for the (early) "manual" bitbanging driver version.
+o Nadir Billimoria for help interpreting the circuit schematic.