Datasheet

AD7938-6 Data Sheet
Rev. C | Page 24 of 32
Reading Data from the AD7938-6
With the W/
B
pin tied logic high, the AD7938-6 interface operates
in word mode. In this case, a single read operation from the
device accesses the conversion data-word on Pin DB0 to Pin
DB11. The DB8/HBEN pin assumes its DB8 function. With the
W/
B
pin tied to logic low, the AD7938-6 interface operates in
byte mode. In this case, the DB8/HBEN pin assumes its HBEN
function. Conversion data from the AD7938-6 must be accessed
in two read operations with eight bits of data provided on DB0
to DB7 for each of the read operations. The HBEN pin determines
whether the read operation accesses the high byte or the low
byte of the12-bit word. For a low byte read, DB0 to DB7 provide
the eight LSBs of the 12-bit word. For a high byte read, DB0 to
DB3 provide the four MSBs of the 12-bit word, DB4 to DB6
provide the Channel ID and DB7 is always a 0. shows
the read cycle timing diagram for a 12-bit transfer. When operating
in word mode, the HBEN input does not exist, and only the first
read operation is required to access data from the device. When
operating in byte mode, the two read cycles shown in
are required to access the full data-word from the device.
Figure 35
Figure 36
The
CS
and
RD
signals are gated internally and the level is
triggered active low. In either word mode or byte mode,
CS
and
RD
can be tied together as the timing specifications for t
10
and
t
11
are 0 ns minimum. This means the bus would be constantly
driven by the AD7938-6.
The data is placed onto the data bus a time, t
13
, after both
CS
and
RD
go low. The
RD
rising edge can be used to latch data
out of the device. After a time, t
14
, the data lines become
three-stated.
Alternatively,
CS
and
RD
can be tied permanently low and the
conversion data is valid and placed onto the data bus a time, t
9
,
before the falling edge of BUSY.
Note that if
RD
is pulsed during the conversion time this causes
a degradation in linearity performance of approximately 0.25 LSB.
Reading during conversion by way of tying
CS
and
RD
low does
not cause any degradation.
t
15
t
11
t
10
t
13
t
16
t
15
t
16
t
14
t
12
t
17
LOW BYTE HIGH BYTE
DB0 TO DB7
HBEN/DB8
RD
CS
04751-005
Figure 36. AD7938-6 Parallel Interface—Read Cycle Timing for Byte Mode Operation (W/
B
= 0)