REV. E–16–
AD7701
Asynchronous Communications (AC) Mode
The AC mode (MODE pin tied to –5 V) offers a UART com-
patible interface that 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 directly to microprocessors with UART interfaces,
such as the 8051 and TMS70X2.
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 to DB15) with one start bit and two stop bits,
as in Figure 21. The SDATA output will then go three-state.
The second byte is transmitted by bringing CS low again and
DB0 to DB7 are transmitted in the same format as the first byte.
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 trans-
mitted. 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.
DIGITAL NOISE AND OUTPUT LOADING
As mentioned earlier, the AD7701 divides its internal timing
into two distinct phases, analog sampling and settling and digi-
tal 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 auto-
matic safeguard does not exist.
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 SDATA 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.
SCLK (I)
SDATA (O)
DB9
START
BIT
DB8
DB14 DB15
STOP
BIT
DB0 DB1
DB6
DB7
HI-Z
STOP
BIT
START
BIT
STOP
BIT
STOP
BIT
DRDY (O)
CS (I)
Figure 21. Timing Diagram for Asynchronous Communications Mode
