System Management Bus

Input Voltage (V_IL and V_IH)

When mixing devices, the I²C specification defines the input levels to be 30% and 70% of the supply voltage V_DD,^[5]: 9 which may be 5 V, 3.3 V, or any other value. Instead of relating the bus input levels to V_DD, SMBus defines them to be fixed. SMBus 2.0 defines V_IL,max at 0.8 V and V_IH,min at 2.1 V, and supports a V_DD ranging from 3 to 5 V, while in SMBus 3.0, the levels are defined at 0.8 and 1.35 V, with a V_DD ranging from 1.8 to 5 V.^[4]

Sink Current (I_OL)

SMBus 2.0 defines a ‘High Power’ class that includes a 4 mA sink current that cannot be driven by I²C chips unless the pull-up resistor is sized to I²C-bus levels.

NXP devices have a higher power set of electrical characteristics than SMBus 1.0. The main difference is the current sink capability with V_OL = 0.4 V.

• SMBus low power = 350 μA
• SMBus high power = 4 mA
• I²C-bus = 3 mA

SMBus ‘high power’ devices and I²C-bus devices will work together if the pull-up resistor is sized for 3 mA.

Frequency (F_MAX and F_MIN)

The SMBus clock is defined from 10 to 100 kHz while I²C can be 0–100 kHz, 0–400 kHz, 0–1 MHz and 0–3.4 MHz, depending on the mode. This means that an I²C bus running at less than 10 kHz will not be SMBus compliant since the SMBus devices may time out. Many SMBus devices will however support lower frequencies.

SMBus 3.0 adds 400 kHz and 1 MHz bus speeds.

Timing

• SMBus defines a clock low time-out, TIMEOUT of 35 ms. I²C does not specify any timeout limit.
• SMBus specifies T_LOW:SEXT as the cumulative clock low extend time for a slave device. I²C does not have a similar specification.
• SMBus specifies T_LOW:MEXT as the cumulative clock low extend time for a master device. Again I²C does not have a similar specification.
• SMBus defines both rise and fall time of bus signals. I²C does not.
• The SMBus time-out specifications do not preclude I²C devices co-operating reliably on the SMBus. It is the responsibility of the designer to ensure that I²C devices are not going to violate these bus timing parameters.

ACK and NACK usage

There are the following differences in the use of the NACK bus signaling: In I²C, a slave receiver is allowed to not acknowledge the slave address, if for example it's unable to receive because it's performing some real time task. SMBus requires devices to acknowledge their own address always, as a mechanism to detect a removable device's presence on the bus (battery, docking station, etc.)

I²C specifies that a slave device, although it may acknowledge its own address, may decide, some time later in the transfer, that it cannot receive any more data bytes. I²C specifies that the device may indicate this by generating the not acknowledge on the first byte to follow.

Other than to indicate a slave's device-busy condition, SMBus also uses the NACK mechanism to indicate the reception of an invalid command or datum. Since such a condition may occur on the last byte of the transfer, it is required that SMBus devices have the ability to generate the not acknowledge after the transfer of each byte and before the completion of the transaction. This is important because SMBus does not provide any other resend signaling. This difference in the use of the NACK signaling has implications on the specific implementation of the SMBus port, especially in devices that handle critical system data such as the SMBus host and the SBS components.

SMBus protocols

Each message transaction on SMBus follows the format of one of the defined SMBus protocols. The SMBus protocols are a subset of the data transfer formats defined in the I²C specifications. I²C devices that can be accessed through one of the SMBus protocols are compatible with the SMBus specifications. I²C devices that do not adhere to these protocols cannot be accessed by standard methods as defined in the SMBus and Advanced Configuration and Power Interface (ACPI) specifications.

Address Resolution Protocol

The SMBus uses I²C hardware and I²C hardware addressing, but adds second-level software for building special systems. In particular its specifications include an Address Resolution Protocol that can make dynamic address allocations. Dynamic reconfiguration of the hardware and software allow bus devices to be ‘hot-plugged’ and used immediately, without restarting the system. The devices are recognized automatically and assigned unique addresses. This advantage results in a plug-and-play user interface. In both those protocols there is a very useful distinction made between a System Host and all the other devices in the system that can have the names and functions of masters or slaves.

In the context of motherboard PCI Express slots, the PCIe Electromechanical Specification expects ARP to be provided for the SMBus pins. However, because ARP is marked "optional" in the SMBus specification, it's commonly left unimplemented.^[7]

Time-out feature

SMBus has a time-out feature which resets devices if a communication takes too long. This explains the minimum clock frequency of 10 kHz to prevent locking up the bus. I²C can be a ‘DC’ bus, meaning that a slave device stretches the master clock when performing some routine while the master is accessing it. This will notify to the master that the slave is busy but does not want to lose the communication. The slave device will allow continuation after its task is complete. There is no limit in the I²C-bus protocol as to how long this delay can be, whereas for an SMBus system, it would be limited to 35 ms. The SMBus protocol just assumes that if something takes too long, then it means that there is a problem on the bus and that all devices must reset in order to clear this mode. Slave devices are not then allowed to hold the clock LOW too long.

Packet Error Checking

SMBus 1.1 and later define optional Packet Error Checking (PEC). In that mode, a PEC (packet error code) byte is appended at the end of each transaction. The byte is calculated as CRC-8 checksum, calculated over the entire message including the address and read/write bit. The polynomial used is x⁸+x²+x+1 (the CRC-8-ATM HEC algorithm, initialized to zero).^[8][9][10]

SMBALERT#

The SMBus has an extra optional shared interrupt signal called SMBALERT#, which can be used by slaves to tell the host to ask its slaves about events of interest. SMBus also defines a less common "Host Notify Protocol", providing similar notifications but passing more data and building on the I²C multi-master mode.