Datasheet
2010 - 2015 Microchip Technology Inc. DS00001692C-page 33
USB251xB/xBi
5.2.4 IN-CIRCUIT EEPROM PROGRAMMING
The EEPROM can be programmed via automatic test equipment (ATE) by pulling RESET_N low (which tri-states the
hub’s EEPROM interface and allows an external source to program the EEPROM).
5.3 SMBus
The Microchip hub can be configured by an external processor via an SMBus interface (see Table 5-1 for details on
enabling the SMBus interface). The SMBus interface shares the same pins as the EEPROM interface, and therefore
the hub no longer supports the I
2
C EEPROM interface when the SMBus interface has been enabled. The hub waits
indefinitely for the SMBus code load to complete and only appears as a newly connected device on USB after the code
load is complete.
The hub’s SMBus acts as a slave-only SMBus device. The implementation only supports block write (Section 5.3.2.1)
and block read (Section 5.3.2.2) protocols, where the available registers are outlined in Section 5.1 on page 19. Refer-
ence the
System Management Bus Specification [3] for additional information.
5.3.1 SMBUS SLAVE ADDRESS
The 7-bit slave address is 0101100b. The hub will not respond to the general call address of 0000000b.
5.3.2 PROTOCOL IMPLEMENTATION
Typical block write and block read protocols are shown in figures 5-2 and 5-3. Register accesses are performed using
7-bit slave addressing, an 8-bit register address field, and an 8-bit data field. The shading shown in the figures during a
read or write indicates the hub is driving data on the SMBDATA line; otherwise, host data is on the SDA/SMBDATA line.
The SMBus slave address assigned to the hub (0101100b) allows it to be identified on the SMBus. The register address
field is the internal address of the register to be accessed. The register data field is the data that the host is attempting
to write to the register or the contents of the register that the host is attempting to read.
Note: The Hub does not have the capacity to write, or “Program,” an external EEPROM. The Hub only has the
capability to read external EEPROMs. The external EEPROM will be read (even if it is blank or non-popu-
lated), and the Hub will be “configured” with the values that are read.
Note: Data bytes are transferred MSB first.