AD7869
–11–
MICROPROCESSOR INTERFACING
Microprocessor interfacing to the AD7869 is via a serial bus that
uses standard protocol compatible with DSP machines. The
communication interface consists of separate transmit (DAC)
and receive (ADC) sections whose operations can be either syn-
chronous or asynchronous with respect to each other. Each sec-
tion has a clock signal, a data signal and a frame or strobe pulse.
Synchronous operation means that data is transmitted from the
ADC and to the DAC at the same time. In this mode, only one
interface clock is needed, and this has to be the ADC clock out;
RCLK must be connected to TCLK. For asynchronous opera-
tion, DAC and ADC data transfers are independent of each
other; the ADC provides the receive clock (RCLK) while the
transmit clock (TCLK) may be provided by the processor or the
ADC or some other external clock source.
Another option to be considered with serial interfacing is the use
of a gated clock. A gated clock means that the device sending
the data switches on the clock when data is ready to be transmit-
ted and three states the clock output when transmission is com-
plete. Only 16 clock pulses are transmitted with the first data bit
being latched into the receiving device on the first falling clock
edge. Ideally, there is no need for frame pulses, however the
AD7869 DAC frame input (
TFS) has to be driven high between
data transmissions. The easiest method is to use RFS to drive
TFS and use only synchronous interfacing. This avoids the use
of interconnects between the processor and AD7869 frame sig-
nals. Not all processors have a gated clock facility; Figure 16
shows an example with the DSP56000.
Table I below shows the number of interconnect lines between
the processor and the AD7869 for the different interfacing
options.
The AD7869 has the ability to use different clocks for transmit-
ting and receiving data. This option, however, exists only on
some processors and normally just one clock (ADC clock) is
used for all communication with the AD7869. For simplicity, all
the interface examples in this data sheet use synchronous inter-
facing and use the ADC clock (RCLK) as an input for the DAC
clock (TCLK). For a better understanding of each of these in-
terfaces, consult the relevant processor data sheet.
AD7869–DSP56000 Interface
Figure 16 shows a typical interface between the AD7869 and
DSP56000. The interface arrangement is synchronous with a
gated clock requiring only three lines of interconnect. The
DSP56000 internal serial control registers have to be configured
for a 16-bit data word with valid data on the first falling clock
edge. Conversion starts and DAC updating are controlled by an
external timer. Data transfers, which occur during ADC conver-
sions, are between the processor receive and transmit shift regis-
ters and the AD7869’s ADC and DAC. At the end of each
16-bit transfer, the DSP56000 receives an internal interrupt in-
dicating the transmit register is empty, and the receive register is
full.
DSP56000
STD
TFS
*ADDITIONAL PINS OMITTED FOR CLARITY
DT
SCK
SRD
RCLK
DR
CONVST
RFS
TIMER
AD7869*
4.7kΩ 2kΩ 4.7kΩ
LDAC
CONTROL
TCLK
5V+
SC0
Figure 16. AD7869–DSP56000 Interface
AD7869–ADSP-2101/2102 Interface
An interface that is suitable for the ADSP-2101 or the ADSP-
2102 is shown in Figure 17. The interface is configured for syn-
chronous, continuous clock operation. The
LDAC is tied low so
the DAC gets updated on the sixteenth falling clock after
TFS
goes low. Alternatively,
LDAC may be driven from a timer as
shown in Figure 16. As with the previous interface, the proces-
sor receives an interrupt after reading or writing to the AD7869
and updates its own internal registers in preparation for the next
data transfer.
ADSP-2101/2
TFS
DT
TCLK
DT
LDAC
TFS
*ADDITIONAL PINS OMITTED FOR CLARITY
RFS
SCLK
DR
RCLK
DR
CONVST
RFS
CONTROL
TIMER
AD7869*
4.7kΩ 2kΩ 4.7kΩ
5V
–
5V
+
Figure 17. AD7869–ADSP-2101/ADSP-2102 Interface
Table I. Interconnect Lines for Different Interfacing Options
Number of
Configuration Interconnects Signals
Synchronous 4 RCLK, DR, DT and
RFS
(TCLK = RCLK, TFS = RFS)
Asynchronous* 5 or 6 RCLK, DR, RFS, DT, TFS
(TCLK = RCLK or
µP serial CLK)
Synchronous 3 RCLK, DR and DT
Gated Clock (TCLK = RCLK, TFS = RFS)
*5 LINES OF INTERCONNECT WHEN TCLK = RCLK
6 LINES OF INTERCONNECT WHEN TCLK = µP SERIAL CLK
