REV. E
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
and a leading zero 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 10 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, when set to a 1, puts the AD7814
into shutdown mode. The first 2 bits of the data stream are
dont 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 fif-
teenth falling SCLK edge and the data takes effect at this time
(i.e., if the part is programmed 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 AD7814s 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, it is possible to operate the inter-
face with three wires. 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 three wires: SCLK, CS, and DOUT.
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
transfers. Other microcontrollers and DSP processors transfer
16 bits of data in a serial data operation.
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
AD7814 to MC68HC11 Interface
Figure 3 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 (4-wire) interface.
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 8051 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.
In the example shown, the AD7814 is connected to the serial
port of the 8051. Because the serial interface of the 8xC51 con-
tains only one data line, the DIN line of the AD7814 is tied low
in the interface example given in Figure 4.
For applications that require the use of the power-down feature
of the AD7814, the serial interface should be implemented
using data port lines on the 8051. This allows a full-duplex
serial interface to be implemented. The method involves bit-
banging a port line to generate a serial clock while using two
other port lines to shift data in and out with the fourth port line
connecting to CS. Port lines 1.0 through 1.3 (with P1.1 config-
ured as an input) can be used to connect to SCLK, DOUT,
DIN, and CS, respectively, to implement this scheme.
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