82374EB datasheet(74/208 Pages) INTEL

74 / 208 page

82374EB82374SB

3226 DCMDMA CLEAR MASK REGISTER

00EhChannels 0-3

0DChChannels 4-7

Default Value

xxh

Attribute

Write Only

Size

Software Command This command clears the mask bits of all four channels enabling them to accept DMA

requests IO port 0Eh is used for Channels 0-3 and IO port 0DCh is used for Channels 4-7

Bit

Description

Clear Mask

No specific pattern Command enabled with a write to the IO port address

33 Timer Unit Registers

The ESC contains five counters that are equivalent to those found in the 82C54 Programmable Interval Timer

The Timer registers control these counters and can be accessed from the EISA Bus via IO space This

section describes the countertimer registers on the ESC The countertimer operations are further described

in Section 80 Interval Timer

331 TCWTIMER CONTROL WORD REGISTER

043hTimer 1

04BhTimer 2

Default Value

xxh

Attribute

Write Only

Size

8 Bits

The Timer Control Word specifies the counter selection the operating mode the counter byte programming

order and size of the COUNT value and whether it counts down in a 16-bit or binary-coded decimal (BCD)

format After writing the control word a new count may be written at any time The new value will take effect

according to the programmed mode

There are six programmable counting modes Typically the ESC Timer Unit Counters 0 and 2 are programmed

for Mode 3 the Square Wave Mode while Counter 1 is programmed in Mode 2 the Rate Generator Mode

Two special commands are selected through the Control Word Register The Counter Latch Command is

selected when bits 54 are both 0 The Read-Back Command is selected when bits 76 are both 1 When

either of these two commands are selected with the Control Word Register the meaning of the other bits in

the register changes Both of these special commands and the respective changes they make to the bit

definitions in this register are covered in detail under separate register descriptions later in this section

Bits 4 and 5 are also used to select the count register programming mode The programming process is

simple

1 Write a control word

2 Write an initial count for each counter

3 Load the LSB MSB or LSB then MSB

The readwrite selection chosen with the control word dictates the programming sequence that must follow

when initializing the specified counter

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