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- 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.
- This is a SPI master controller driver. It can be used in conjunction with
- (layered under) the LM70 logical driver (a "SPI protocol driver").
- 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).
- Hardware Interfacing
- --------------------
- The schematic for this particular board (the LM70EVAL-LLP) is
- available (on page 4) here:
- http://www.national.com/appinfo/tempsensors/files/LM70LLPEVALmanual.pdf
- 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 arrangement 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.
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