An SGS Thompson M93C46 EEPROM, or equivalent, stores the programmable VID and PID. When the EEPROM interface is enabled (EXTMEM = 0), the EECLK and EEDATA are internally pulled down (100 μA) inside the TUSB2046x. The internal pulldowns are disabled when the EEPROM interface is disabled (EXTMEM = 1).
The EEPROM is programmed with the three 16-bit locations as shown in Table 1. Connecting terminal 6 of the EEPROM high (ORG = 1) organizes the EEPROM memory into 64×16-bit words.
|00001||VID high-byte||VID low-byte|
|00010||PID high-byte||PID low-byte|
The D and Q signals of the EEPROM must be tied together using a 1-kΩ resistor with the common I/O operations forming a single-wire bus. After system power-on reset, the TUSB2046x performs a one-time access read operation from the EEPROM if the EXTMEM terminal is pulled low and the chip select(s) of the EEPROM is connected to the system power-on reset. Initially, the EEDATA terminal is driven by the TUSB2046x to send a start bit (1) which is followed by the read instruction (10) and the starting-word address (00000). Once the read instruction is received, the instruction and address are decoded by the EEPROM, which then sends the data to the output shift register. At this point, the hub stops driving the EEDATA terminal and the EEPROM starts driving. A dummy (0) bit is then output and the first three 16-bit words in the EEPROM are output with the most significant bit (MSB) first.
The output data changes are triggered by the rising edge of the clock provided by the TUSB2046x on the EECLK terminal. The SGS-Thompson M936C46 EEPROM is recommended because it advances to the next memory location by automatically incrementing the address internally. Any EEPROM used must have the automatic internal address advance function. After reading the three words of data from the EEPROM, the TUSB2046x puts the EEPROM interface into a high-impedance condition (pulled down internally) to allow other logic to share the EEPROM. The EEPROM read operation is summarized in Figure 9. For more details on EEPROM operation, refer to SGS-Thompson Microelectronics M93C46 Serial Microwire Bus EEPROM data sheet.