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Diffstat (limited to 'Documentation/spi/spi-lm70llp')
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diff --git a/Documentation/spi/spi-lm70llp b/Documentation/spi/spi-lm70llp new file mode 100644 index 000000000000..154bd02220b9 --- /dev/null +++ b/Documentation/spi/spi-lm70llp @@ -0,0 +1,69 @@ +spi_lm70llp : LM70-LLP parport-to-SPI adapter +============================================== + +Supported board/chip: + * National Semiconductor LM70 LLP evaluation board + Datasheet: http://www.national.com/pf/LM/LM70.html + +Author: + Kaiwan N Billimoria <kaiwan@designergraphix.com> + +Description +----------- +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. |
