AD7840
Table III. Parallel Mode Truth Table
Serial Data Format
CS
H
X
L
H
H
X
WR
X
H
L
H
X
H
LDAC
H
H
H
L
L
L
L
L
Function
}
Both Latches Latched
}
}
f
L
f
g
L
g
Input Latch Transparent
Input Latch Latched
DAC Latch Transparent
Analog Output Updated
Input Latch Transparent
DAC Latch Data Transfer Inhibited
Input Latch Is Latched
DAC Latch Data Transfer Occurs
The serial data format is selected for the AD7840 by connecting
the
CS/SERIAL
line to –5 V. In this case, the
WR/SYNC,
D13/SDATA, D12/SCLK, D11/FORMAT and D10/JUSTIFY
pins all assume their serial functions. The unused parallel inputs
should not be left unconnected to avoid noise pickup. Serial
data is loaded to the input latch under control of SCLK,
SYNC
and SDATA. The AD7840 expects a 16-bit stream of serial data
on its SDATA input. Serial data must be valid on the falling
edge of SCLK. The
SYNC
input provides the frame synchroni-
zation signal which tells the AD7840 that valid serial data will
be available for the next 16 falling edges of SCLK. Figure 8
shows the timing diagram for serial data format.
X = Don’t Care
Figure 6. Parallel Mode Timing Diagram
Figure 8. Serial Mode Timing Diagram
Figure 7. AD7840 Simplified Parallel Input Control Logic
Although 16 bits of data are clocked into the AD7840, only 14
bits go into the input latch. Therefore, two bits in the stream are
don’t cares since their value does not affect the input latch data.
The order and position in which the AD7840 accepts the 14 bits
of input data depends upon the FORMAT and JUSTIFY in-
puts. There are four different input data modes which can be
chosen (see Table I in the Pin Function Description section).
The first mode (M1) assumes that the first two bits of the input
data stream are don’t cares, the third bit is the LSB and the last
(or 16th bit) is the MSB. This mode is chosen by tying both the
FORMAT and JUSTIFY pins to a logic 0. The second mode
(M2; FORMAT = 0, JUSTIFY = 1) assumes that the first bit in
the data stream is the LSB, the fourteenth bit is the MSB and
the last two bits are don’t cares. The third mode (M3;
FORMAT= 1, JUSTIFY 0) assumes that the first two bits in
the stream are again don’t cares, the third bit is now the MSB
and the sixteenth bit is the LSB. The final mode (M4; FOR-
MAT = 1, JUSTIFY= 1) assumes that the first bit is the MSB,
the fourteenth bit is the LSB and the last two bits of the stream
are don’t cares.
REV. B
–7–
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