spi_lm70llp : LM70-LLP parport-to-SPI adapter

Supported board/chip:

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:

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

  • David Brownell for mentoring the SPI-side driver development.
  • Dr.Craig Hollabaugh for the (early) “manual” bitbanging driver version.
  • Nadir Billimoria for help interpreting the circuit schematic.