OXCB950 datasheet(59/68 Pages) ETC

59 / 68 page

Data Sheet Revision 1.0 PRELIMINARY

Page 59

OXCB950

OXFORD SEMICONDUCTOR LTD.

Now that the target has signalled that the clock is to be

maintained, the target's drive on the CCLKRUN# line must

be maintained for 2 clocks during which the host will begin

to drive the line also. Since the CCLKRUN# is also the data

to the 2-bit shift register then, when the CCLKRUN# is

driven by the control circuitry, it will take 2 clock cycles for

the new clock run status to propagate through the logic to

turn off the transistor. During this period, the host will have

detected the assertion of the CCLKRUN# by the target and

will also begin to assert the CLKRUN# line, so that when

the clock control's transistor is turned off, the CCLKRUN#

will be held in the asserted state by the host thereby

keeping the transistor off. The transistor will remain off until

the host next attempts to negate the CCLKRUN# line to

indicate a clock stop and is prevented from doing so by the

clock control logic.

For the condition when the device driver needs to place the

oxcb950 into a low powerstate, and the clock control logic

is not required, then the device driver will assign the MIO0

pin to a logic 0 that disables the clock control circuitry and

forces the clockrun transistor to the off state. For this

condition, when the host negates the CCLKRUN# line it

does not see the CCLKRUN# being asserted by the target

(as the circuitry has been disabled) and the clock is

stopped by the host after the relevant number of clocks

from the negation.

Since the UART functionality within the oxcb950 device is

not dependant upon the CCLK line for interrupt generation,

then any activity within the UART that results in an interrupt

will be conveyed to the cardbus interface asynchronously.

This will result in the CCLKRUN# being asserted by the

host (and thus enabling the clock CCLK) and control will be

handed to the device driver that will re-enable the clock

control circuitry to prevent the clock from stopping until all

tasks have been completed.

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