UM10208_2
© NXP B.V. 2007. All rights reserved.
User manual
Rev. 02 — 1 June 2007
278 of 362
NXP Semiconductors
UM10208
Chapter 22: LPC2800 DDAC
19
SMUTE
After this bit is switched from 0 to 1, the gain of the interpolator is
gradually decreased (according to a raised cosine function) during
128 fs periods. When the output is fully muted, the voltage on the
output pins is (VREFN + VREFP)/2, and the MUTE bit in the
DDACSTAT register is set to 1. When this bit is switched from 1 to 0
(and after the output voltage has been ramped up to (VREFN +
VREFP)/2 after a Reset or when the PD bit is cleared), the gain of the
interpolator is gradually increased to the values indicated by the
RGAIN and LGAIN fields, during 128 fs periods.
0
21:20 MODE2FS
00 in this field selects 1 fs mode. Use this value if the input data rate is
between 8 kHz and 96 kHz and sharp filter roll-off is desired. In this
mode, all stages of the interpolation filter are used and digital
de-emphasis can be selected.
01 in this field selects 2 fs mode. Use this value if the input data rate is
96 kHz or above, and/or a slow roll-off is desired. In this mode, the
first stage of the interpolation filter is bypassed and digital
de-emphasis cannot be done.
0
23:22 ROLLOFF
The field controls sharp vs. slow rolloff. See
for allowed
combinations of values in this field and the MODE field. Do not
program combinations other than those shown.
0
24
PSLOW
This field controls how long the Dual DAC takes to power up and
down. 0 selects 512 fs periods; 1 selects 1024 fs periods.
0
25
DDAC_PD
A 1 in this bit powers down the interpolator. Setting this bit (as
described in
Section 22–6.3 “Power-down procedure” on page 282
automatically invokes the same soft-muting operation described
above for the SMUTE bit. Thereafter, the analog outputs are gradually
reduced to VREFN, in either 512 or 1024 fs periods, depending on the
PSLOW bit.
When this bit is switched from 1 to 0 (as described in
“Power-up procedure” on page 282
) the analog outputs are gradually
increased from VREFN to (VREFN+VREFP)/2, in 512 or 1024 fs
periods depending on PSLOW. If bit SMUTE is 0, this voltage ramp
sequence is followed by a soft-unmute sequence as described above
for the SMUTE bit.
0
26
DDAC_INV A 1 in this bit inverts the signal polarity of both the left and right
channels.
28:27 SILDET_T
If the ENSILDET bit is 1, this field controls how many consecutive
all-zero input values each channel’s silence-detection circuit will
require, before it sets the LSILENT or RSILENT bit in the DDACSTAT
register.
00 3200
01 4800
10 9600
11 19200
29
ENSILDET A 1 in this bit enables the silence-detection circuit.
31:30 -
Reserved. Always write 0s to these bits. The value read from reserved
bits is not defined.
Table 315. Dual DAC Control Register (DDACCTRL - 0x8020 0398)
Bit(s) Name
Description
Reset
Value