System Management Bus (SMBus) Specification, version 2.

System Management Bus (SMBus) Specification Version 2.0
2. If a pre-assigned unique ID is used, at least 16-bits must be unique. However, 32-bits is
recommended.
3. If a random number is implemented in this field, Random Number Requirements must be met.
4. Devices that support a fixed device address must still implement this field but not uniquely.
Uniqueness is important to guarantee that two like devices are identified discretely. It is the
responsibility of the device/system manufacturer to determine the possibility of like devices, and
the mechanism for providing uniqueness via the UDID and slave address fields.
5.6.1.6. Random number requirements
If a random number is utilized the following requirements must be met:
1. It must be at least 16 bits in length.
2. The device is not allowed to support a persistent slave address.
3. The device is not allowed to support fixed addresses. (If the device has a fixed address mode, the
Vendor Specific ID should be a constant, and therefore not random – this is required to guarantee that
the SMBus ARP resolution order is maintained)
4. The random number must be retained while the device has power with the exceptions described in
items 5 and 6.
5. The random number must be regenerated when the device receives the Reset Device command.
6. The random number must be regenerated when the device senses a bus collision during a read
operation directed to its Assigned Slave Address. When this happens, the device must issue a host
notify protocol if the device supports it.
5.6.2. Power-on reset
Power-on reset is described in section 3.1.4.2. In the case of ARP-capable devices, ‘operational state’
implies the ability to respond to ARP commands as required in this section.
Each slave device must fit into only one of these categories and must obey the power on reset state:
Device Type AR Flag AV Flag SMB Address UDID
PSA
CLEAR Read from Read from NVR; NO CHANGE
(Persistent Slave Address)
NVR
undefined if AV
Flag is CLEAR
Non-PSA / Non-Random Number CLEAR CLEAR undefined NO CHANGE
Non-PSA / Random Number CLEAR CLEAR undefined Generate Random
Number
Table 6: Internal state of ARP-capable devices on power-on reset
5.6.3. ARP commands
The ARP Master can issue general commands and directed commands. A general command is targeted at
all devices and is required for the address resolution process. A directed command is targeted at a single
device. All packets originated by the ARP Master use Packet Error Checking (See Section 7.5) and begin
with the basic format:
SBS Implementers Forum 38