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CHAPTER 4 INTERFACES
98
To access the EEPROM, the host system writes a command into the ECCR register of the
μ
PD98405.
When the
μ
PD98405 accepts the command, it executes the command via the EEPROM interface. During
the 1-word write transaction in which the command is set, the
μ
PD98405 activates TRDY_B and releases
the bus for another master regardless of whether execution of the command is complete. If the host
system starts a transaction to write another command into the ECCR register when the
μ
PD98405 has not
yet completed the processing for the command accepted by the
μ
PD98405, the
μ
PD98405 activates
STOP_B and requests a retry.
To read EEPROM data, the host system sets an address and READ command in the ECCR register.
When the
μ
PD98405 accepts the command, it starts reading the data via the EEPROM interface. While
the
μ
PD98405 is reading data, the B bit of the ERDR register is set to 1. Once the
μ
PD98405 finishes
reading the data, it sets the B bit to 0 and stores the data in the READ DATA field. After issuing the
command, the host system checks that the B bit of the ERDR register is set to 0, then obtains the data.
To write data into or erase data from the EEPROM, the host system must enable write and erase
operations using the EWEN command in advance.
When no EEPROM is connected, accessing these registers is meaningless.
(3) EEPROM format
The Subsystem vendor ID, Subsystem ID, MIN_GNT, and MAX_LAT fields of the configuration register are
stored into the EEPROM. After a hardware or software reset, the
μ
PD98405 checks whether an EEPROM
is connected. These fields are not automatically loaded if no EEPROM is connected, while they are
automatically loaded when an EEPROM is connected.
The contents of the EEPROM are shown below.
Table 4-9. EEPROM Format
Address [Hex]
Contents
00
a5a5 [Hex]
The
μ
PD98405 requires this code to check whether an EEPROM is
connected.
01
Subsystem Vendor ID
02
Subsystem ID
03
MAX LAT (8 high-order bits), MIN GNT (8 low-order bits)
04-3F
Not used. Applications can use this area in any way.