| Kaiwan N Billimoria | 78961a5 | 2007-07-17 04:04:05 -0700 | [diff] [blame] | 1 | spi_lm70llp : LM70-LLP parport-to-SPI adapter |
| 2 | ============================================== |
| 3 | |
| 4 | Supported board/chip: |
| 5 | * National Semiconductor LM70 LLP evaluation board |
| 6 | Datasheet: http://www.national.com/pf/LM/LM70.html |
| 7 | |
| 8 | Author: |
| 9 | Kaiwan N Billimoria <kaiwan@designergraphix.com> |
| 10 | |
| 11 | Description |
| 12 | ----------- |
| 13 | This driver provides glue code connecting a National Semiconductor LM70 LLP |
| 14 | temperature sensor evaluation board to the kernel's SPI core subsystem. |
| 15 | |
| 16 | In effect, this driver turns the parallel port interface on the eval board |
| 17 | into a SPI bus with a single device, which will be driven by the generic |
| 18 | LM70 driver (drivers/hwmon/lm70.c). |
| 19 | |
| 20 | The hardware interfacing on the LM70 LLP eval board is as follows: |
| 21 | |
| 22 | Parallel LM70 LLP |
| 23 | Port Direction JP2 Header |
| 24 | ----------- --------- ---------------- |
| 25 | D0 2 - - |
| 26 | D1 3 --> V+ 5 |
| 27 | D2 4 --> V+ 5 |
| 28 | D3 5 --> V+ 5 |
| 29 | D4 6 --> V+ 5 |
| 30 | D5 7 --> nCS 8 |
| 31 | D6 8 --> SCLK 3 |
| 32 | D7 9 --> SI/O 5 |
| 33 | GND 25 - GND 7 |
| 34 | Select 13 <-- SI/O 1 |
| 35 | ----------- --------- ---------------- |
| 36 | |
| 37 | Note that since the LM70 uses a "3-wire" variant of SPI, the SI/SO pin |
| 38 | is connected to both pin D7 (as Master Out) and Select (as Master In) |
| 39 | using an arrangment that lets either the parport or the LM70 pull the |
| 40 | pin low. This can't be shared with true SPI devices, but other 3-wire |
| 41 | devices might share the same SI/SO pin. |
| 42 | |
| 43 | The bitbanger routine in this driver (lm70_txrx) is called back from |
| 44 | the bound "hwmon/lm70" protocol driver through its sysfs hook, using a |
| 45 | spi_write_then_read() call. It performs Mode 0 (SPI/Microwire) bitbanging. |
| 46 | The lm70 driver then inteprets the resulting digital temperature value |
| 47 | and exports it through sysfs. |
| 48 | |
| 49 | A "gotcha": National Semiconductor's LM70 LLP eval board circuit schematic |
| 50 | shows that the SI/O line from the LM70 chip is connected to the base of a |
| 51 | transistor Q1 (and also a pullup, and a zener diode to D7); while the |
| 52 | collector is tied to VCC. |
| 53 | |
| 54 | Interpreting this circuit, when the LM70 SI/O line is High (or tristate |
| 55 | and not grounded by the host via D7), the transistor conducts and switches |
| 56 | the collector to zero, which is reflected on pin 13 of the DB25 parport |
| 57 | connector. When SI/O is Low (driven by the LM70 or the host) on the other |
| 58 | hand, the transistor is cut off and the voltage tied to it's collector is |
| 59 | reflected on pin 13 as a High level. |
| 60 | |
| 61 | So: the getmiso inline routine in this driver takes this fact into account, |
| 62 | inverting the value read at pin 13. |
| 63 | |
| 64 | |
| 65 | Thanks to |
| 66 | --------- |
| 67 | o David Brownell for mentoring the SPI-side driver development. |
| 68 | o Dr.Craig Hollabaugh for the (early) "manual" bitbanging driver version. |
| 69 | o Nadir Billimoria for help interpreting the circuit schematic. |