AD1846
D MA Inter r upt
T he completion of autocalibration can be determined by polling
the Autocalibrate-In-Progress (ACI) bit in the T est and Initial-
ization Register, which will be set during autocalibration. T rans-
fers enabled during autocalibration do not begin until the
completion of autocalibration.
Writing to the internal 16-bit Base Count Register sets up the
count value for the number of samples to he transferred. Note
that the number of bytes transferred for a given count will be a
function of the selected global data format. T he internal Cur-
rent Count Register is updated with the current contents of the
Upper and Lower Base Count Registers when a write occurs to
the Upper Base Count Register.
T he following summarizes the procedure for autocalibration:
• Mute left and right AUX1 and AUX2 inputs, and digital mix.
(It is unnecessary to mute the DAC outputs, as this will hap-
pen automatically.)
T he Current Count Register cannot be read by the host. Read-
ing the Base Count Registers will only read back the initializa-
tion values written to them.
• Set the Mode Change Enable (MCE) bit.
• Set the Autocalibration (ACAL) bit.
T he Current Count Register is decremented every sample pe-
riod when either the PEN or CEN bit is enabled and also either
the T ransfer Request Disable (T RD) bit or the Interrupt Status
(INT ) bit is zero. An interrupt event is generated after the Cur-
rent Count Register is zero and an additional playback sample is
transferred. T he INT bit in the Status Register always reflects
the current internal interrupt state defined above. T he external
INT pin will only go active HI if the Interrupt Enable (IEN) bit
in the Interface Configuration Register is set. If the IEN bit is
zero, the external INT pin will always stay LO, even though the
Status Register’s INT bit may be set.
• Clear the Mode Change Enable (MCE) bit.
• T he Autocalibrate-In-Progress (ACI) bit will transition from
LO to HI within five sample periods. It will remain HI for
approximately 384 sample periods. Poll the ACI bit until it
transitions from HI to LO.
• Set to desired gain/attenuation values, and unmute DAC
outputs (if muted), AUX inputs, and digital mix.
During the autocalibration sequence, data output from the
ADCs is meaningless. Inputs to the DACs are ignored. Even if
the user specified the muting of all analog outputs, near the end
of the autocalibration sequence, analog outputs very close to
VREF will be produced at the line output.
P O WER UP AND RESET
T he PWRDWN pin should be held in its active LO state when
power is first applied to the AD1846. Analog Devices recom-
mends waiting one full second after deasserting PWRDWN be-
fore commencing audio activity with the AD1846. T his will
allow the analog outputs to fully settle to the VREF voltage level
prior to system autocalibration. At any point when powered, the
AD1846 can be put into a state for minimum power consump-
tion by asserting PWRDWN LO. All analog and digital sections
are shut down. T he AD1846’s parallel interface does not func-
tion; all bidirectional signal lines are in high impedance state.
CH ANGING SAMP LE RATES
T o change the selection of the current sample rate requires a
Mode Change Enable sequence since the bits which control that
selection are in the Clock and Data Format Register. T he fact
that the clocks change requires a special sequence which is sum-
marized as follows:
• If autocalibration will take place at the end of this sequence,
then mute AUX1 and AUX2 inputs and the digital mix.
Deasserting PWRDWN by bringing it HI begins the AD1846’s
initialization. While initializing, the AD1846 ignores all writes
and all reads will yield “1000 0000 (80h).” At the conclusion of
reset initialization, all registers will be set to their default values
as listed in “Control Registers” above. T he conclusion of the
initialization period can be detected by polling the index register
for some value other than “1000 0000 (80h).”
• Set the Mode Change Enable (MCE) bit.
• In a single write cycle, change the Clock Frequency Divide
Select (CFS2:0) and/or the Clock Source Select (CSS).
• T he AD1846 now needs to resynchronize its internal states to
the new clock. Writes to the AD1846 will be ignored. Reads
will produce “1000 0000 (80h)” until the resynchronization is
complete. Poll the Index Register until something other than
this value is returned.
It is imperative to autocalibrate on power up for proper opera-
tion. See next section.
• Clear the Mode Change Enable (MCE) bit.
AUTO CALIBRATIO N
• If ACAL is set, follow the procedure described in
“Autocalibration” above.
T he AD1846 can calibrate its ADCs and DACs to minimize dc
offsets. Autocalibration occurs whenever the AD1846 returns
from the Mode Change Enable state and the ACAL bit in the
Interface Configuration register has been set. If the ACAL bit is
not set, the RAM normally containing ADC and DAC offset
compensations will he saved, retaining the offsets of the most re-
cent autocalibration. T herefore, it is imperative to autocalibrate
on power up for proper operation.
• Poll the ACI bit until it transitions LO (approximately 128
sample cycles).
• Set to desired gain/attenuation values, and unmute DAC out-
puts (if muted).
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