82375EB Datasheet, PDF (114/131 Pages) Intel Corporation – PCI-EISA BRIDGE (PCEB)

82375EB SB
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Figure 32 Copy witn Re-Drive 32-bit EISA Master Accessing a 16-bit EISA or
ISA Slave-One Word Write BE 3 0 ^e0011
9 0 BIOS TIMER
The PCEB provides a system BIOS Timer that decrements at each edge of its 1 03 1 04 MHz clock (derived
from the 8 25 8 33 MHz BCLK) Since the state of the counter is undefined at power-up the BIOS Timer
Register must be programmed before it can be used The timer can be enabled disabled by writing to the
BIOS Timer Address Register
The BIOS Timer Register can be accessed as a single 16-bit quantity or as 32-bit quantity For 32-bit access-
es the upper 16 bits are don’t care (reserved) The BIOS Timer I O address location is software programma-
ble The address is determined by the value programmed into the BTMR Register and can be located on
Dword boundaries anywhere in the 64 KByte PCI I O space
The BIOS Timer clock has a frequency of 1 03 or 1 04 MHz depending on the value of BCLK (derived either
from 25 MHz or 33 MHz PCICLK) This allows time intervals to be counted from 0 to approximately 65 ms The
accuracy of the counter is g 1 ms
9 1 BIOS Timer Operations
A write operation (either 16-bit or 32-bit) to the BIOS Timer Register initiates the counting sequence After
initialization the BIOS timer starts decrementing until it reaches zero When the value in the timer reaches
zero the timer stops decrementing and register value remains at zero until the timer is re-initialized
After the timer is initialized the current value can be read at any time The timer can be re-programmed (new
initial value written to the BIOS Timer Register) before the register value reaches zero All write and read
operations to the BIOS Timer Register should include all 16 counter bits Separate accesses to the individual
bytes of the counter must be avoided since this can cause unexpected results (incorrect count intervals)
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