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AD7715
–25–
REV. C
AD7715 to 8XC51 Interface
An interface circuit between the AD7715 and the 8XC51
microcontroller is shown in Figure 10. The diagram shows the
minimum number of interface connections with
CS
on the
AD7715 hardwired low. In the case of the 8XC51 interface, the
minimum number of interconnects is just two. In this scheme,
the
DRDY
bit of the Communications Register is monitored to
determine when the Data Register is updated. The alternative
scheme, which increases the number of interface lines to three,
is to monitor the
DRDY
output line from the AD7715. The
monitoring of the
DRDY
line can be done in two ways. First,
DRDY
can be connected to one of the 8XC51’s port bits (such
as P1.0) which is configured as an input. This port bit is then
polled to determine the status of
DRDY
. The second scheme is
to use an interrupt driven system in which case, the
DRDY
output is connected to the
INT1
input of the 8XC51. For inter-
faces that require control of the
CS
input on the AD7715, one
of the port bits of the 8XC51 (such as P1.1), which is config-
ured as an output, can be used to drive the
CS
input.
The 8XC51 is configured in its Mode 0 serial interface mode.
Its serial interface contains a single data line. As a result, the
DATA OUT and DATA IN pins of the AD7715 should be
connected together with a 10 k
pull-up resistor. The serial
clock on the 8XC51 idles high between data transfers. The
8XC51 outputs the LSB first in a write operation while the
AD7715 rearranged before being written to the output serial
register. Similarly, the AD7715 outputs the MSB first during a
read operation while the 8XC51 expects the LSB first. There-
fore, the data which is read into the serial buffer needs to be
rearranged before the correct data word from the AD7715 is
available in the accumulator.
AD7715
DATA OUT
CS
8XC51
RESET
DATA IN
SCLK
P3.0
P3.1
DV
DD
DV
DD
10k
V
Figure 10. AD7715 to 8XC51 Interface
AD7715 to ADSP-2103/ADSP-2105 Interface
Figure 11 shows an interface between the AD7715 and the
ADSP-2103/ADSP-2105 DSP processor. In the interface
shown, the
DRDY
bit of the Communications Register is again
monitored to determine when the Data Register is updated. The
alternative scheme is to use an interrupt driven system, in which
case the
DRDY
output is connected to the
IRQ2
input of the
ADSP-2103/ADSP-2105. The serial interface of the ADSP-
2103/ADSP-2105 is set up for alternate framing mode. The
RFS
and
TFS
pins of the ADSP-2103/ADSP-2105 are config-
ured as active low outputs, and the ADSP-2103/ADSP-2105
serial clock line, SCLK, is also configured as an output. The
CS
for the AD7715 is active when either the
RFS
or
TFS
outputs
from the ADSP-2103/ADSP-2105 are active. The serial clock
rate on the ADSP-2103/ADSP-2105 should be limited to
3MHz to ensure correct operation with the AD7715.
AD7715
DATA OUT
CS
ADSP-2103/2105
RESET
DATA IN
SCLK
RFS
TFS
DR
DT
DV
DD
SCLK
Figure 11. AD7715 to ADSP-2103/ADSP-2105 Interface
CODE FOR SETTING UP THE AD7715
Table XVI gives a set of read and write routines in C code for
interfacing the 68HC11 microcontroller to the AD7715. The
sample program sets up the various registers on the AD7715
and reads 1000 samples from the part into the 68HC11. The
setup conditions on the part are exactly the same as those out-
lined for the flowchart of Figure 8. In the example code given
here, the
DRDY
output is polled to determine if a new valid
word is available in the data register.
The sequence of the events in this program are as follows:
1. Write to the Communications Register, setting the gain to 1
with standby inactive.
2. Write to the Setup Register, setting bipolar mode, buffer off,
no filter synchronization, confirming a clock frequency of
2.4576MHz, setting the output rate for 60Hz and initiating
a self-calibration.
3. Poll the
DRDY
Output.
4. Read the data from the Data Register.
5. Loop around doing Steps 3 and 4 until the specified number
of samples have been taken.