
REV. 0
AD7814
–6–
Read Operation
Figure 2 shows the timing diagram for a serial read from the
AD7814. The
CS
line enables the SCLK input. Ten bits of data
are transferred during a read operation. Read operations occur
during streams of 16 clock pulses. The serial data is accessed in
a number of bytes if ten bits of data are being read. At the end
of the read operation, the DOUT line remains in the state of the
last bit of data clocked out of the AD7814 until
CS
returns
high, at which time the DOUT line goes into three-state.
Write Operation
Figure 2 also shows the timing diagram for a serial write to the
AD7814. The write operation takes place at the same time as
the read operation. Data is clocked into the Control Register on
the part on the rising edge of SCLK. Only the third bit in the
data stream provides a user-controlled function. This third bit is
the power-down bit which puts the AD7814 into shutdown
mode. The first two bits of the data stream are don’t cares while
all other bits in the data stream other, than the power-down bit,
should be 0 to ensure correct operation of the AD7814. Data is
loaded to the Control Register on the fifteenth falling SCLK
edge and the data takes effect at this time i.e., if the part is pro-
grammed to go into shutdown, it does so at this point. If the
CS
is brought high before this fifteenth SCLK edge, the Control
Register will not be loaded and the power-down status of the
part will not change.
MICROPROCESSOR INTERFACING
The AD7814’s serial interface allows for easy interface to most
microcomputers and microprocessors. Figures 3 through 6 show
some typical interface circuits.
The serial interface on the AD7814 consists of four wires:
CS
,
DIN, DOUT and SCLK. All interface circuits shown utilize all
four interface lines. However, depending on the application it is
possible to operate the interface with either two or three wires.
If the SCLK line that comes to the AD7814 is not shared with
any other devices and the issue with spurious clocks outlined
previously is not a concern, it is possible to leave the
CS
line of
the AD7814 tied permanently low. If the application does not
require the power-down facility offered by the AD7814, the
DIN line can be tied permanently low. Thus, the interface can
be operated from just two wires, SCLK and DOUT.
CS
SCLK
DOUT
DIN
DON'T
CARE
DON'T
CARE
POWER-
DOWN
LEADING
ZERO
DB9
DB8
DB0
1
2
3
4
11
15
16
t
1
t
2
t
3
t
4
t
5
t
6
t
7
t
8
Figure 2. Serial Interface Timing Diagram
The serial data transfer to and from the AD7814 requires a 16-
bit read operation. Many 8-bit microcontrollers have 8-bit serial
ports and this 16-bit data transfer is handled as two 8-bit trans-
fers. Other microcontrollers and DSP processors transfer 16 bits
of data in a serial data operation.
AD7814 to MC68HC11 Interface
Figure 4 shows an interface between the AD7814 and the
MC68HC11 microcontroller. The MC68HC11 is configured in
the master mode with its CPOL bit set to a logic one and its
CPHA bit set to a logic one. When the MC68HC11 is config-
ured like this, its SCLK line idles high between data transfers.
Data is transferred to and from the AD7814 in two 8-bit serial
data operations. The diagram shows the full (four-wire) inter-
face. PC1 of the MC68HC11 is configured as an output and
used to drive the
CS
input.
AD7814*
SCLK
DOUT
DIN
CS
MC68HC11*
SCLK
MISO
MOSI
PC1
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 3. AD7814 to MC68HC11 Interface
AD7814 to 8051 Interface
An interface circuit between the AD7814 and the micro-
controller is shown in Figure 4. The 8xC51 is configured in its
Mode 0 serial interface mode. The serial clock line of the 8xC51
(on P3.1) idles high between data transfers. Data is transferred
to and from the AD7814 in two 8-bit serial data operations. The
AD7814 outputs the MSB of its data stream as the first valid
bit while the 8xC51 expects the LSB first. Thus, the data read
into the serial buffer needs to be rearranged before the correct
data word from the AD7814 is available in the accumulator.