linux/Documentation/spi/spi-lm70llp.rst
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   1==============================================
   2spi_lm70llp :  LM70-LLP parport-to-SPI adapter
   3==============================================
   4
   5Supported board/chip:
   6
   7  * National Semiconductor LM70 LLP evaluation board
   8
   9    Datasheet: http://www.national.com/pf/LM/LM70.html
  10
  11Author:
  12        Kaiwan N Billimoria <kaiwan@designergraphix.com>
  13
  14Description
  15-----------
  16This driver provides glue code connecting a National Semiconductor LM70 LLP
  17temperature sensor evaluation board to the kernel's SPI core subsystem.
  18
  19This is a SPI master controller driver. It can be used in conjunction with
  20(layered under) the LM70 logical driver (a "SPI protocol driver").
  21In effect, this driver turns the parallel port interface on the eval board
  22into a SPI bus with a single device, which will be driven by the generic
  23LM70 driver (drivers/hwmon/lm70.c).
  24
  25
  26Hardware Interfacing
  27--------------------
  28The schematic for this particular board (the LM70EVAL-LLP) is
  29available (on page 4) here:
  30
  31  http://www.national.com/appinfo/tempsensors/files/LM70LLPEVALmanual.pdf
  32
  33The hardware interfacing on the LM70 LLP eval board is as follows:
  34
  35   ======== == =========   ==========
  36   Parallel                 LM70 LLP
  37     Port   .  Direction   JP2 Header
  38   ======== == =========   ==========
  39      D0     2      -         -
  40      D1     3     -->      V+   5
  41      D2     4     -->      V+   5
  42      D3     5     -->      V+   5
  43      D4     6     -->      V+   5
  44      D5     7     -->      nCS  8
  45      D6     8     -->      SCLK 3
  46      D7     9     -->      SI/O 5
  47     GND    25      -       GND  7
  48    Select  13     <--      SI/O 1
  49   ======== == =========   ==========
  50
  51Note that since the LM70 uses a "3-wire" variant of SPI, the SI/SO pin
  52is connected to both pin D7 (as Master Out) and Select (as Master In)
  53using an arrangement that lets either the parport or the LM70 pull the
  54pin low.  This can't be shared with true SPI devices, but other 3-wire
  55devices might share the same SI/SO pin.
  56
  57The bitbanger routine in this driver (lm70_txrx) is called back from
  58the bound "hwmon/lm70" protocol driver through its sysfs hook, using a
  59spi_write_then_read() call.  It performs Mode 0 (SPI/Microwire) bitbanging.
  60The lm70 driver then inteprets the resulting digital temperature value
  61and exports it through sysfs.
  62
  63A "gotcha": National Semiconductor's LM70 LLP eval board circuit schematic
  64shows that the SI/O line from the LM70 chip is connected to the base of a
  65transistor Q1 (and also a pullup, and a zener diode to D7); while the
  66collector is tied to VCC.
  67
  68Interpreting this circuit, when the LM70 SI/O line is High (or tristate
  69and not grounded by the host via D7), the transistor conducts and switches
  70the collector to zero, which is reflected on pin 13 of the DB25 parport
  71connector.  When SI/O is Low (driven by the LM70 or the host) on the other
  72hand, the transistor is cut off and the voltage tied to it's collector is
  73reflected on pin 13 as a High level.
  74
  75So: the getmiso inline routine in this driver takes this fact into account,
  76inverting the value read at pin 13.
  77
  78
  79Thanks to
  80---------
  81
  82- David Brownell for mentoring the SPI-side driver development.
  83- Dr.Craig Hollabaugh for the (early) "manual" bitbanging driver version.
  84- Nadir Billimoria for help interpreting the circuit schematic.
  85