Confidential Draft
3/7/00
CS4281 Programming Manual
182
DS308PRM1
control for the
VOLUP/VOLDN
pins. See the Gener-
al Purpose Input/output Pins section and Figure 51
for a conceptual view of how the signal interfaces
with the extended GPIO logic.
A shadow register on the CS4281 snoops writes to
the AC Link registers and stores writes to the Mas-
ter Volume register on the Codec. Hardware vol-
ume changes then read the shadow register,
increments/decrements one step and, write the new
value to both the shadow register and the Codec
(through the AC Link), thereby keeping the hard-
ware volume relative to any software updates.
Host writes to the master volume control registers
take precedence over volume control button opera-
tion. The Host write will have the same effect as if
the user released the button just before the host
write, and then pressed the button again just after
the write. Host software that does read-modify-
writes of the volume registers will maintain any
changes from the last hardware volume update. In
addition, an interrupt can be generated on a hard-
ware volume change thereby keeping hardware
volume changes synchronized with the host soft-
ware sliders/values.
If the AC Link is down (no
ABITCLK
) the hardware
volume values are frozen.
VOLUP
/
VOLDN
button
changes are ignored and registers are NOT updat-
ed. Hardware volume registers are powered from
the core (unlike the AC ‘97 codec volume regis-
ters). If the core power is removed, the hardware
volume is lost and host software must write the
master volume to re-initialize the hardware volume
registers when the power and link come back up.
Host software can avoid this loss of synchroniza-
tion by either doing a read-modify-write on the vol-
ume (which will take into account any hardware
volume changes) or enable the hardware volume
interrupt, through the GPIOR logic, which will let
the software know when the hardware volume
changes the master volume registers.
The volume control pins have an internal 20 k
Ω
pullup to the positive supply. It is expected that the
pins would use SPST momentary switches between
the pin and ground to activate the function. No ex-
ternal debounce capacitors are required, although
EMI filter capacitors may be needed.
20.2
Clock Control
The CS4281 clocking model includes a delay
locked loop for high frequency core clock genera-
tion, and power control via selective clock gating.
D
ra
ft
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