Electronic – Read or set registers in motherboard

computersdatasheetmicrocontrollermotherboardsensor

With microcontrollers it is pretty easy to switch a LED on/off, by writing on a register.

This question is instead an attempt to write to a PCI device register in a desktop computer.

In a similar way, I would like to read/write the registers in the motherboard to control fans (on, off or even speed) or the power or hard disk LEDs, or temperature sensors.

Given – for example – an Asus motherboard and a Linux OS, is it possible? And what kind of datasheet should be readed, if it exists, to obtain the register names in the motherboard? I never found for a consumer motherboard something like the datasheet of a microcontroller.

Best Answer

The documentation no doubt exists. Not always freely available but it exists.

There is always linux and bsd source code you can read as well if they support these peripherals, there you go (yes I know that is not always an easy read).

If there is already a device driver for this peripheral then learn how to communicate with that device driver. lm-sensors as pjc50 commented is one example.

If not you can often punch through the operating system protection with mmap() and from the application talk to things directly. Depending on what you are trying to do (mother board test equipment, pcie card test equipment, etc) that may suffice. The correct way to do it is modify or write a kernel driver and talk to/through it to get at the peripheral, can be as thin or thick of a driver as you desire but at least the last mile of writes/reads come from the kernel space not application.

Note DOS is still used for motherboard and other testing being one of the best bare metal operating systems (intentional oxymoron), and if you are working closely with a motherboard company they might provide the source or snippets of code from their test programs that blink leds or read the temperature sensor, etc.

Yes motherboard documentation even if you have access is not the same as a microcontroller, the audience is different, it is often only the bios programmers (contractors) and internal test folks that need the documentation, and that is it so it could be scribbles on a whiteboard.

Being a PC what an operating system needs is defined by convention or discoverable, so you dont need documentation, you just programmatically search for things and find what is present. As needed based on what you find then you go to that specific chip vendors site to get the datasheet (which does read like a microcontrollers) for that part. some pcie bridge chip from intel, right there at intels web site. A usb bridge or master, same deal. Video card, well they might not be as open. Hard disk controller, find the datasheet. Ethernet, find the datasheet and read it. lspci is your friend. lsusb is your friend. google is your friend.

Related Solutions

Electronic – Problem in writing register maps using C structure for a TI Microcontroller

Structures are unsuitable for writing register maps, because a struct can have padding bytes added anywhere inside it, for alignment purposes. This depends on the compiler - you have to ensure that no padding is used (for example through a static assert).

Furthermore, it is easy to make mistakes when writing large structures that are supposed to reflect a register map, you have to get every single byte right or you'll break everything. This makes the struct vulnerable during maintenance.

I would strongly suggest you to write register maps through macros, such as:

#define PERIPH_A_DID0 (*(volatile uint32_t*)0x400FE000u))
#define PERIPH_A_DID1 (*(volatile uint32_t*)0x400FE004u))

This also has the advantage of being 100% portable to any C compiler.

Alternatively, you could do something more intricate such as this:

typedef volatile uint8_t* SCI_port;
#ifndef SCI0
  #define SCI0 (&SCI0BDH)
  #define SCI1 (&SCI1BDH)
#endif

#define SCIBDH(x)   (*((SCI_port)x + 0x00))   /* SCI Baud Rate Register High             */
#define SCIBDL(x)   (*((SCI_port)x + 0x01))   /* SCI Baud Rate Register Low              */
#define SCICR1(x)   (*((SCI_port)x + 0x02))   /* SCI Control Register1                   */
#define SCICR2(x)   (*((SCI_port)x + 0x03))   /* SCI Control Register 2                  */
#define SCISR1(x)   (*((SCI_port)x + 0x04))   /* SCI Status Register 1                   */
#define SCISR2(x)   (*((SCI_port)x + 0x05))   /* SCI Status Register 2                   */
#define SCIDRH(x)   (*((SCI_port)x + 0x06))   /* SCI Data Register High                  */
#define SCIDRL(x)   (*((SCI_port)x + 0x07))   /* SCI Data Register Low                   */

Then when writing your driver, you can do like this:

void sci_init (SCI_port port, ...)
{
  SCICR1(port) = THIS | THAT;
  SCICR2(port) = SOMETHING_ELSE;
  ...
}

This is very useful when you have many identical peripherals on the MCU, with the same registers, but only want one code to control them.

Piggyback off Motherboard Power Switch

For PC's made after the introduction of the ATX, the power switch is a logical input to the motherboard--the same kind of input you want to apply to the Teensy. The motherboard, in turn, outputs a separate logic signal to turn on the power supply.

In my experience, the reset switch is as good as standard, as it dates back to the original IBM PC (which didn't have the switch, but did have a connection on the motherboard for one). This is not so for the power switch. I have seen schemes where it pulls a signal up and neither side is grounded. Since you have found one side of your switch is connected to ground, that makes your connection easy. Simply attach a voltmeter to the two pins, and observe the voltage present there. Verify that this voltage drops to zero when pressing the switch. Your Teensy will see this on its input. It doesn't matter how they implemented this. If it's a floating input they will have a pull up resistor already there to make it work. Your input is just watching the resultant voltage level.

Note that the power supply is turned on when the motherboard drops the #PS_ON signal to ground. This is the infamous "green wire" on the ATX connector. If you bring this signal into a second input on your Teensy, you will now be able to tell if the button was pushed while the computer is already running (because it will show a logic 0).

Also note that certain voltages are always present even when the computer is off (or in standby). This is how the power switch manages to have power when there's no power. This is also a heads up that you may find power in other unplanned places (like some USB connections).

Best Answer

Related Solutions

Electronic – Problem in writing register maps using C structure for a TI Microcontroller

Piggyback off Motherboard Power Switch

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