AD8802 Datasheet, PDF (9/16 Pages) Analog Devices – 12 Channel, 8-Bit TrimDACs with Power Shutdown

AD8802/AD8804
+5V
RS
RF
100kΩ
100kΩ
VDD
VREFH
A
AD8802/
AD8804
B
GND
VREFL
AD8804
ONLY
100kΩ
+5V
OP191
–5V
–5V TO +4.98V
+12V
OP193
0V TO +10V
100kΩ
Figure 23. Increasing Output Voltage Swing
DAC B of Figure 24 is in a noninverting gain of two configura-
tions, which increases the available output swing to +10 V. The
feedback resistors can be adjusted to provide any scaling of the
output voltage, within the limits of the external op amp power
supplies.
Microcomputer Interfaces
The AD8802/AD8804 serial data input provides an easy inter-
face to a variety of single-chip microcomputers (µCs). Many µCs
have a built-in serial data capability that can be used for com-
municating with the DAC. In cases where no serial port is pro-
vided, or it is being used for some other purpose (such as an
RS-232 communications interface), the AD8802/AD8804 can
easily be addressed in software.
Twelve data bits are required to load a value into the AD8802/
AD8804 (4 bits for the DAC address and 8 bits for the DAC
value). If more than 12 bits are transmitted before the Chip Se-
lect input goes high, the extra (i.e., the most-significant) bits are
ignored. This feature is valuable because most µCs only transmit
data in 8-bit increments. Thus, the µC will send 16 bits to the
DAC instead of 12 bits. The AD8802/AD8804 will only re-
spond to the last 12 bits clocked into the SDI port, however, so
the serial data interface is not affected.
An 8051 µC Interface
A typical interface between the AD8802/AD8804 and an 8051
µC is shown in Figure 24. This interface uses the 8051’s internal
serial port. The serial port is programmed for Mode 0 opera-
tion, which functions as a simple 8-bit shift register. The 8051’s
Port 3.0 pin functions as the serial data output, while Port 3.1
serves as the serial clock.
When data is written to the Serial Buffer Register (SBUF, at
Special Function Register location 99_H), the data is automati-
cally converted to serial format and clocked out via Port 3.0 and
Port 3.1. After 8 bits have been transmitted, the Transmit Inter-
rupt flag (SCON.1) is set and the next 8 bits can be transmitted.
The AD8802 and AD8804 require the Chip Select to go low at
the beginning of the serial data transfer. In addition, the SCLK
input must be high when the Chip Select input goes high at the
end of the transfer. The 8051’s serial clock meets this require-
ment, since Port 3.1 both begins and ends the serial data in the
high state.
+5V
0.1µF 10µF
SBUF
SERIAL DATA
SHIFT REGISTER
SHIFT CLOCK
8051 µC
RxD P3.0
TxD P3.1
P1.3
P1.2
P1.1
PORT 1
1.3 1.2 1.1
VDD VREFH
AD8802
SDI
O1
SCLK
RESET
SHDN
O12
CS
GND
Figure 24. Interfacing the 8051 µC to an AD8802/AD8804,
Using the Serial Port
Software for the 8051 Interface
A software for the AD8802/AD8804 to 8051 interface is
shown in Listing 1. The routine transters the 8-bit data stored at
data memory location DAC_VALUE to the AD8802/AD8804
DAC addressed by the contents of location DAC_ADDR.
The subroutine begins by setting appropriate bits in the Serial
Control register to configure the serial port for Mode 0 opera-
tion. Next the DAC’s Chip Select input is set low to enable the
AD8802/AD8804. The DAC address is obtained from memory
location DAC_ADDR, adjusted to compensate for the 8051’s
serial data format, and moved to the serial buffer register. At
this point, serial data transmission begins automatically. When
all 8 bits have been sent, the Transmit Interrupt bit is set, and
the subroutine then proceeds to send the DAC value stored at
location DAC_VALUE. Finally the Chip Select input is re-
turned high, causing the appropriate AD8802/AD8804 output
voltage to change, and the subroutine ends.
The 8051 sends data out of its shift register LSB first, while the
AD8802/AD8804 require data MSB first. The subroutine there-
fore includes a BYTESWAP subroutine to reformat the data.
This routine transfers the MSB-first byte at location SHIFT1 to
an LSB-first byte at location SHIFT2. The routine rotates the
MSB of the first byte into the carry with a Rotate Left Carry in-
struction, then rotates the carry into the MSB of the second byte
with a Rotate Right Carry instruction. After 8 loops, SHIFT2
contains the data in the proper format.
The BYTESWAP routine in Listing 1 is convenient because the
DAC data can be calculated in normal LSB form. For example,
producing a ramp voltage on a DAC is simply a matter of re-
peatedly incrementing the DAC_VALUE location and calling
the LD_8802 subroutine.
If the µC’s hardware serial port is being used for other purposes,
the AD8802/AD8804 DAC can be loaded by using the parallel
port. A typical parallel interface is shown in Figure 25. The se-
rial data is transmitted to the DAC via the 8051’s Port 1.6 out-
put, while Port 1.6 acts as the serial clock.
Software for the interface of Figure 25 is contained in Listing 2. The
subroutine will send the value stored at location DAC_VALUE to
the AD8802/AD8804 DAC addressed by location DAC_ADDR.
The program begins by setting the AD8802/AD8804’s Serial
Clock and Chip Select inputs high, then setting Chip Select low
REV. 0
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