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參數(shù)資料
| 型號： | 92F8945 |
| 英文描述： | IC-SM D/A CONVERTER |
| 中文描述： | 集成電路釤D / A轉(zhuǎn)換 |
| 文件頁數(shù)： | 9/12頁 |
| 文件大?。?/td> | 409K |
| 代理商： | 92F8945 |
AD669
REV. A
–9–
Unipolar coding is straight binary, where all zeros (0000H) on
the data inputs yields a zero analog output and all ones
(FFFFH) yields an analog output 1 LSB below full scale.
Bipolar coding is offset binary, where an input code of 0000H
yields a minus full-scale output, an input of FFFFH yields an
output 1 LSB below positive full scale, and zero occurs for an
input code with only the MSB on (8000H).
The AD669 can be used with twos complement input coding if
an inverter is used on the MSB (DB15).
DIGITAL INPUT CONSIDERATIONS
The threshold of the digital input circuitry is set at 1.4 volts.
The input lines can thus interface with any type of 5 volt logic.
The AD669 data and control inputs will float to indeterminate
logic states if left open. It is important that
CS
and
L1
be con-
nected to DGND and Chat LDAC be tied to V
LL
if these pins
are not used.
Fanout for the AD669 is 40 when used with a standard low
power Schottky gate output device.
16-BIT MICROPROCESSOR INTERFACE
The 16-bit parallel registers of the AD669 allow direct interfac-
ing to 16-bit general purpose and DSP microprocessor buses.
The following examples illustrate typical AD669 interface
configurations.
AD669 TO ADSP-2101 INTERFACE
The flexible interface of the AD669 minimizes the required
“glue” logic when it is connected in configurations such as the
one shown in Figure 8. The AD669 is mapped into the ADSP-
2101’s memory space and requires two wait states using a 12.5
MHz processor clock.
In this configuration, the ADSP-2101 is set up to use the inter-
nal timer to interrupt the processor at the desired sample rate.
The
WR
pin and data lines D8–D23 from the ADSP-2101 are
tied directly to the
L1
and DB0 through DB15 pins of the
AD669, respectively. The decoded signal
CS1
is connected to
both
CS
and LDAC. When a timer interrupt is detected, the
ADSP-2101 automatically vectors to the appropriate service
routine with minimal overhead. The interrupt routine then in-
structs the processor to execute a data memory write to the ad-
dress of the AD669.
The
WR
pin and
CS1
both go low causing the first 16-bit latch
inside the AD669 to be transparent. The data present in the first
rank is then latched by the rising edge of
WR
. The rising edge
of
CS1
will cause the second rank 16-bit latch to become
transparent updating the output of the DAC. The length of
WR
is extended by two wait states to comply with the timing
requirements of t
LOW
shown in Figure 1b. It is important to
latch the data with the rising edge of
WR
rather than the decoded
CS1.
This is necessary to comply with the t
DH
specification of
the AD669.
A0
D8
ADSP-2101
DGND
+5V
DECODER
ADDRESS BUS
LDAC
AD669
DGND
DB0
DATA BUS
DMS
WR
CS1
CS
L1
A13
DB15
D23
V
LL
V
LL
V
OUT
a. ADSP-2101 to AD669 Interface
A13
A12
A11
DMS
CS1
b. Typical Address Decoder
Figure 8. ADSP-2101 to AD669 Interface
Figure 8b shows the circuitry a typical decoder might include.
In this case, a data memory write to any address in the range
3000H to 3400H will result in the AD669 being updated. These
decoders will vary greatly depending on the number of devices
memory-mapped by the processor.
AD669 TO DSP56001 INTERFACE
Figure 9 shows the interface between the AD669 and the
DSP56001. Like the ADSP-2101, the AD669 is mapped into
the DSP56001’s memory space. This application was tested
with a processor clock of 20.48 MHz (t
CYC
= 97.66 ns) although
faster rates are possible.
An external clock connected to the
IRQA
pin of the DSP56001
interrupts the processor at the desired sample rate. If ac perfor-
mance is important, this clock should be synchronous with the
DSP56001 processor clock. Asynchronous clocks will cause jit-
ter on the latch signal due to the uncertainty associated with the
acknowledgment of the interrupt. A synchronous clock is easily
generated by dividing down the clock from the DSP crystal. If
ac performance is not important, it is not necessary for
IRQA
to
be synchronous.
After the interrupt is acknowledged, the interrupt routine ini-
tiates a memory write cycle. All of the AD669 control inputs are
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