AD7701
ASYNCH RO NO US CO MMUNICATIO NS (AC) MO D E
T he AC mode (MODE pin tied to –5 V) offers a UART -
compatible interface which allows the AD7701 to transmit data
asynchronously from remote locations. An external SCLK sets
the baud rate and data is transmitted in two bytes in UART -
compatible format. Using the AC mode, the AD7701 can be
interfaced direct to microprocessors with UART interfaces, such
as the 8051 and T MS70X2.
D IGITAL NO ISE AND O UTP UT LO AD ING
As mentioned earlier, the AD7701 divides its internal timing
into two distinct phases, analog sampling and settling and digital
computation. In the SSC mode, data is transmitted only during
the digital computation periods, to minimize the effects of
digital noise on analog performance. In the SEC and AC modes
data transmission is externally controlled, so this automatic
safeguard does not exist.
Data transmission is initiated by CS going low. If CS is low on a
falling edge of SCLK, the AD7701 begins transmitting an 8-bit
data byte (DB8–DB15) with one start bit and two stop bits, as
in Figure 21. T he SDAT A output will then go three-state. T he
second byte is transmitted by bringing CS low again and DB0 to
DB7 are transmitted in the same format as the first byte.
Whatever mode of operation is used, resistive and capacitive
loads on digital outputs should be minimized in order to reduce
crosstalk between analog and digital portions of the circuit. For
this reason connection to low-power CMOS logic such as one of
the 4000 series or 74C families is recommended.
It is especially important to minimize the load on SDAT A in the
AC mode, as transmission in this mode is inherently asynchro-
nous. In the SEC mode the AD7701 should be synchronized to
the digital system clock via CLKIN.
UART baud rates are typically low compared to the AD7701’s
4 kHz output update rate. If CS is low and data is still being
transmitted when a new data word becomes available, the new
data will be ignored. However, if CS has been taken high
between bytes, when a new data word becomes available, the
AD7701 could update the output register before the second byte
is transmitted. In this case, the UART would receive the first
byte of the new word instead of the second byte of the old word.
When using the AC mode, care must obviously be taken to
ensure that this does not occur.
SCLK (I)
DRDY (O)
CS (I)
HI-Z
START
BIT
STOP
STOP
START
BIT
STOP STOP
BIT BIT
DB14 DB15
DB6
DB8
DB9
SDATA (O)
DB0
DB1
DB7
BIT
BIT
Figure 21. Tim ing Diagram for Asynchronous Com m unications Mode
REV. D
–15–
ADI [ ADI ]
