W83877TF/W83877TG
Publication Release Date: May 2006
-65- Revision 0.7
11. EXTENDED FUNCTION REGISTERS
The W83877TF/TG provides many configuration registers for setting up different types of
configurations. After power-on reset, the state of the hardware setting of each pin will be latched by
the relevant configuration register to allow the W83877TF/TG to enter the proper operating
configuration. To protect the chip from invalid reads or writes, the configuration registers cannot be
accessed by the user. There are four ways to enable the configuration.
n registers to be read or written. HEFERE (CR0C bit 5) and HEFRAS (CR16 bit 0) can be used to
select one out of these four methods of entering the Extended Function mode as follows:
HEFRAS HEFERE ADDRESS AND VALUE
0 0 write 88H to the location 250H
0 1 write 89H to the location 250H (power-on default)
1 0 write 86H to the location 3F0H twice
1 1 write 87H to the location 3F0H twice
First, a specific value must be written once (88H/89H) or twice (86H/87H) to the Extended Functions
Enable Register (I/O port address 250H or 3F0H). Second, an index value (00H-19H, 20H-29H, 2CH-
2DH, 31H-3AH, 40H-45H) must be written to the Extended Functions Index Register (I/O port address
251H or 3F0H) to identify which configuration register is to be accessed. The designer can then
access the desired configuration register through the Extended Functions Data Register (I/O port
address 252H or 3F1H).
After programming of the configuration register is finished, an additional value should be written to
EFERs to exit the Extended Function mode to prevent unintentional access to those configuration
registers. In the case of EFER at 250H, this additional value can be any value other than 88H if
HEFERE = 0 and 89H if HEFERE = 1. While EFER is at 3F0H, this additional value must be AAH. The
designer can also set bit 6 of CR9 (LOCKREG) to high to protect the configuration registers against
accidental accesses.
The configuration registers can be reset to their default or hardware settings only by a cold reset (pin
MR = 1). A warm reset will not affect the configuration registers.
11.1 Extended Functions Enable Registers (EFERs)
After a power-on reset, the W83877TF/TG enters the default operating mode. Before the
W83877TF/TG enters the extended function mode, a specific value must be programmed into the
Extended Function Enable Register (EFER) so that the extended function register can be accessed.
The Extended Function Enable Registers are write-only registers. On a PC/AT system, their port
addresses are 250H or 3F0H (as described in the above section).
