Board Initialization
42 – Configuration
VL-486-4 Reference Manual
82C721 I
NITIALIZATION
The Internal Configuration Registers (ICR) of the 82C721 are accessed using I/O ports 03F0h
and 03F1h. The initialization data must be written to the 82C721 immediately after initializing
the 82C836. A three step configuration sequence is used to prevent accidental changes by an
errant program. Any deviation from the sequence described below will return the 82C721 to its
initial idle state.
Configuration steps:
1. Enter Configuration Mode
Output two consecutive 55h to port 03F0h.
2. Initialize the Configuration Registers
a. Output the index number to port 03F0h
b. Output the initialization data to port 03F1h (see table).
3. Exit Configuration Mode
Output AAh to port 03F0h.
Table 20: Chips & Technologies 82C721 Initialization Data
Index Number
Initialization Data
Description
00h
9Bh
Configuration Register 0
01h
15h
Configuration Register 1
02h
DCh
Configuration Register 2
03h
00h
Configuration Register 3
486SXLC I
NITIALIZATION
Several registers in the 486SXLC microprocessor must be initialized for proper operation of the
VL-486-1 and VL-486-2. In DOS-based systems, the BIOS automatically initializes the CPU;
however, in non-DOS-based systems you must program the initialization sequence in ROM.
This manual does not document the details of the data written to the 486SXLC registers. Refer
to the Texas Instruments 486SXLC Data Book listed in “Other References” on page v.
To initialize the 486SXLC:
1. Output the index number to port 0022h.
2. Output the initialization data to port 0023h (see table).
3. Repeat steps 1 and 2 for all the registers.
