| 1 | Sensors chip driver for MAXIM MAX6650 / MAX6651 fan controllers |
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| 2 | =============================================================== |
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| 3 | |
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| 4 | Chip notes |
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| 5 | ---------- |
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| 6 | |
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| 7 | The MAX6651 is a fan controller. It has two distinct functions: |
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| 8 | |
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| 9 | 1) Speed control, with feedback, for a single fan. The code can write |
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| 10 | to a register requesting that the fan run at a certain speed, and the |
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| 11 | chip will adjust the fan voltage until it reaches that speed. This |
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| 12 | is called "closed loop" mode in the datasheet. |
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| 13 | |
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| 14 | 2) Speed measurement (tachometers) for up to four fans, including the |
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| 15 | controlled one. |
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| 16 | |
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| 17 | The MAX6650 is similar but only has one measurement tachometer. |
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| 18 | |
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| 19 | Both chips have a few general purpose i/o pins, some of which can be |
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| 20 | configured to have special functions. At the time of writing an overview |
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| 21 | of the chip, with links to the datasheet, can be seen at: |
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| 22 | |
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| 23 | http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2296/ln/en |
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| 24 | |
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| 25 | If that has moved searching for MAX6650 from Maxim's home page should |
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| 26 | work. |
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| 27 | |
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| 28 | Driver state |
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| 29 | ------------ |
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| 30 | |
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| 31 | Currently the driver provides the following entries in /proc/sys/ |
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| 32 | dev/sensors/max6650-xxx/: |
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| 33 | |
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| 34 | fan1 through fan4 (read only): the current measured fan speeds, in |
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| 35 | rpm. |
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| 36 | |
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| 37 | speed (read/write): when written, sets the chip to closed loop mode and |
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| 38 | sets the configured speed, in rpm, to the value written. When read returns |
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| 39 | the configured speed, again in rpm. |
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| 40 | |
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| 41 | xdump (read only): returns all of the chip register values, except the |
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| 42 | tachometers. Used for debug, and should probably go away or be conditionally |
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| 43 | compiled. |
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| 44 | |
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| 45 | The MAX665x uses byte wide registers with various scaling factors and |
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| 46 | some more or less complex equations (in the datasheet) relating register |
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| 47 | values to real world values. The driver takes care of all of this. |
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| 48 | |
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| 49 | Note that the measured speed ("fan1") never exactly matches the set speed |
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| 50 | ("speed") because of the coarse resolution of the underlying registers. |
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| 51 | |
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| 52 | Assumptions |
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| 53 | ----------- |
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| 54 | |
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| 55 | The driver assumes that: |
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| 56 | |
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| 57 | o The MAX665x is running from its internal 254 kHz clock, not an |
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| 58 | external clock. |
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| 59 | |
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| 60 | o The monitored fans produce two pulses per rotation (This same |
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| 61 | assumption is made by Maxim throughout the datasheet.) |
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| 62 | |
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| 63 | o The tachometer count time (register 0x16) has been configured suitably |
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| 64 | for the fans in use, perhaps by some boot code. The driver reads this |
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| 65 | register to compute rpm, but never writes to it. |
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| 66 | |
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| 67 | o The speed controller prescale (low three bits of register 0x04) has |
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| 68 | been configured suitably for the fans in use, perhaps by some boot |
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| 69 | code. The driver reads these bits to compute rpm, but never changes |
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| 70 | them. |
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| 71 | |
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| 72 | The driver currently never reads or writes any other configuration |
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| 73 | (gpio config, alarms, 5V/12V setting). |
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| 74 | |
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| 75 | Bugs |
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| 76 | ---- |
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| 77 | |
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| 78 | o The driver assumes too much: it should be possible to set more of the |
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| 79 | configuration dynamically instead of assuming that some other code has |
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| 80 | done it. |
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| 81 | |
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| 82 | o It does not differentiate between a MAX6650 and a MAX6651, as I can |
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| 83 | see no obvious way to do this. It has only been tested with a MAX6651 |
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| 84 | so if anyone gets a board with a MAX6650 to test with maybe something |
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| 85 | will be revealed, such as the three missing tachometer registers |
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| 86 | returning all ones or something. (OK, so maybe I should have called |
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| 87 | this driver max6651, and ignored the max6650 totally...) |
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| 88 | |
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| 89 | o The /proc names don't match the standards in doc/developers/proc, |
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| 90 | as this chip doesn't map well to other fan controllers, |
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| 91 | |
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| 92 | Detection notes |
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| 93 | --------------- |
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| 94 | |
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| 95 | The module will autodetect the presence of a max6650 or max6651 at any |
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| 96 | of the four possible I2C addresses for these devices. There is no ID |
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| 97 | register, but there are several registers with all zero bits at the |
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| 98 | high end. The driver checks that all of these are in fact zero before |
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| 99 | declaring the chip to be a MAX6650/1. |
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| 100 | |
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| 101 | sensors-detect |
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| 102 | -------------- |
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| 103 | |
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| 104 | Support for MAX6650/1 has been added to sensors-detect. It has been |
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| 105 | tested as correctly detecting a MAX6651 at I2C address 0x1B. Like the |
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| 106 | driver, it cannot yet distinguish a MAX6650 and MAX6651. |
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| 107 | |
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| 108 | prog/sensors |
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| 109 | ------------ |
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| 110 | |
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| 111 | Initial read-only support has been added to the "sensors" program. |
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| 112 | |
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| 113 | Testing |
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| 114 | ------- |
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| 115 | |
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| 116 | The driver and associated programs have been tested under Linux 2.4.18 |
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| 117 | running on an embedded Intel XScale processor (actually an IXP425 running |
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| 118 | in big-endian mode) with a MAX6651 chip. |
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| 119 | |
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| 120 | Author |
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| 121 | ------ |
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| 122 | |
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| 123 | John Morris (john.morris@spirentcom.com) |
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| 124 | |
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| 125 | |
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