Generalised Interfacing with Microprocessor System Hardware Monitors
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Proceedings of the 2007 IEEE International Conference on TuesE04 Networking, Sensing and Control, London, UK, 15-17 April 2007 Generalised Interfacing with Microprocessor System Hardware Monitors Constantine A. Murenin Department of Computer Science, East Carolina University, Greenville, NC, USA Faculty of Computing Sciences and Engineering, De Montfort University, Leicester, UK comes very handy, such that administrators can be sure that Abstract their systems are running in accordance with manufacturers’ specifications and guidelines. In this paper, we will discuss the possibilities, Vast majority of motherboards in use today already functionalities and limitations of Microprocessor System integrate chips that allow temperature, voltage and fan speed Hardware Monitors. We will provide a comparison survey monitoring, and many newer motherboards have the of their main functions, depict some weaknesses and give an circuitry for supplying fans with user-specified voltage so overview of what possibilities they provide for the end-user. that the user can control fan speed entirely from software. We will also discuss OpenBSD’s sysctl Hardware However, hardware manufacturers do not always provide Sensors framework, and describe our recent contributions to any end-user utilities to do the aforementioned monitoring the addressing scheme and underpinnings of the framework. and controlling, and when it comes to open-source software, the community is expected to come up with a software 1. Background solution on their own. This works well as long as chip manufacturers release appropriate register documentation for Power consumption of personal computers grows every their hardware. In the presence of such documentation, year along with their computational capabilities. Recently qualified software developers can write drivers for the produced microprocessors consume at times hundreds watts operating system of their choice, and these drivers are of power, and thus require massive cooling resources. Most usually shortly integrated into the official distribution of the often than not, fans are used as a low-cost solution that operating system for which they were written. provides adequate cooling in low- and medium-cost personal computers, whilst water-cooling is used in high-end systems. 2. Hardware review However, fans have several problems associated with them, such as noise when run at full speed and, once again, they Hardware monitors usually come either in a distinct chip themselves consume more electricity. package or a as a part of a Super I/O solution. Sensor Processor manufacturers have come to realise that power readings and fan-controlling capabilities may be accessed consumption of modern processors is a major drawback through different busses, mostly I2C or SMBus, ISA or LPC, from creating high-end personal systems with acceptable or PCI. power-drawn, so several power-saving technologies were I2C and SMBus are two-wire busses, which are deployed, such as deactivating some microprocessor units or essentially compatible with each other. I2C is an acronym for modules whilst the processor is in the idle loop. The Inter Integrated Circuit, a bus developed by Philips adoption of these features means that fans may be turned off Semiconductor with version 1.0 published in 1992 [1], while the overall system activity is low, and turned to full whilst SMBus is an acronym for System Management Bus, a speed when the system performs some processor-intensive bus defined by Intel in 1995. task. LPC stands for Low Pin Count Interface Specification, It is in the interest of the user to be able to control the which is a bus authored by Intel that is hardware and speed at which fans are running in order to utilise the software compatible with the existing ISA devices and power-saving features of the processor and minimise the applications. [2] overall noise of the system. Practically, this may be proven Popular hardware monitoring chips often feature both I2C extremely useful in situations where one is running an and ISA bus access, for example National Semiconductor OpenBSD-based firewall or fileserver system in the home LM78 [3] or Winbond W83627HF [4]. In such case, setting, where the overall processor activity of the server is accessing sensors over ISA bus is preferred over I2C, with going to be very minimal, whereas the noise of the system is ISA being the faster of the two. From the practical of a major concern due to the desired 24hr operation of the standpoint, however, unless special care is taken, two server within the living room area. Desktop usage of instances of the same device may be attached to the system hardware monitoring and controlling software is also of at once — one on I2C, the other one on ISA. Whilst this value, since the amount of processor idling time prevails in usually does not create any resource conflicts, most desktop applications and minimised noise is of implementations might detach the instance of the driver from particular value. In the server arena, the monitoring part the I2C bus in favour of ISA bus access to avoid any user confusion of two distinct devices being attached with Public domain. identical sensor readings. (In fact, this is what OpenBSD’s 1-4244-1076-2/07/$25.00 ©2007 IEEE 901 lm(4) driver does — when the hardware monitoring chip is monitoring chips inside their motherboard, and utilise their detected on the ISA bus, it is automatically attached on ISA voltage sensor inputs in such a way as to provide the and detached from the I2C bus.) maximum flexibility in monitoring, say, dual power-supplies It is not less common to find hardware monitors that only that provide uninterrupted power to the motherboard. have I2C interface — this is usually the case with simple However, when one thinks more about this situation, one chips that only have one or two temperature sensors. On the realises that most power supplies in the desktop market other hand, Super I/O chipsets usually offer ISA/LPC today are standard ATX power supplies, which have the very interface as they already use ISA/LPC for the purpose of same connectors from motherboard to motherboard. They connecting serial, parallel, infrared and PS/2 ports. also come in standard voltage configurations, for example, Hardware monitors that are accessible through PCI they all have a +3.3V lead, +5V regular lead, +5V lead for interface are usually enclosed in South Bridges, such as VIA ‘stand-by’ use, several +12V leads etc. [6] I.e. the hardware VT82C686A “Super South” South Bridge. At this time, only monitoring chip manufacturer should already have a pretty VIA is known to have produced this kind of “Super South” good idea of where their Super I/O chips will be used in the South Bridge with system-wide hardware monitoring vast majority of cases. capabilities. Nevertheless, because chip manufacturers still provide only recommendations of which voltages should be 2.1. Hardware documentation connected to which voltage sensor inputs, we have a problem to tackle. The problem cannot be seen until you In order to write an effective driver for a hardware realise how voltage sensors in the monitoring chip work. monitoring chip, or any chip for that matter, the Voltage sensors work as follows: each sensor has a limit manufacturer of the chip has to release appropriate of the maximum voltage that it can detect and the maximum documentation that would summarise main functions of the voltage it can withstand. Most older chips have the chip and provide some insightful information on how the maximum voltage detection level at around 4.0 volts, whilst chip can be talked-to to perform desired operations. many newer chips have it at around 2.0 volts. This maximum Unfortunately, in recent years in the computer industry it voltage detection level will only decrease over time, as the became very difficult to acquire appropriate documentation semiconductor industry manufactures integrated circuits that is required for programming the chips. According to using a smaller and smaller chip manufacturing process year Theo de Raadt, the founder and main software-architect of after year, which in turn makes chips consume less the OpenBSD project, it is the large American companies electricity, and moreover makes it impossible to transfer which usually resist from giving away their documentation high voltage over such small electrical parts inside the chip. that is required for programming the chips. [5] Datasheet Therefore, before the voltage can be connected to the unavailability is a major obstacle for open-source software, monitoring chip, it must be downscaled to no more than the because it means that driver developers have to reverse- specified maximum detection level. This is done with the engineer the devices and their interfaces in order to create help of resistors. working drivers. After the voltage is transformed to the appropriate scale Fortunately, most popular hardware monitoring chip and is connected to the sensor, the sensor usually has 8-bit, manufacturers do provide the datasheets for programming sometimes 10-bit, scale under which it can report the voltage their hardware. However, in the next subsection, we will it senses. discuss some problems that driver developers may have to In other words, the detected value from the sensor is face when interpreting provided documentation. usually stored in one byte, and assumptions are made on 2.2. Datasheet shortcomings which resistors were used in order to bring the voltage to the desired level. The resistor factors can be thought of as the In this section, we will provide a brief survey of common conversion table, and because the chip manufacturer, problems that may be encountered by driver developers in essentially, does not require the motherboard manufacturer writing drivers for most popular hardware monitoring chips. to connect specific leads from the power supply to specific Some problems mentioned in this section can equally apply sensor inputs of the monitoring chip using specific to the end user as well, for example, what expectations resistors — there is no guarantee that the value we think is should the user have for hardware monitors in general? +12V is indeed connected to the +12V line of the power supply.