Conversion Length
4 through 7 are forced to a zero and the four
LSB’s are enabled. The two byte format is “left
justified data” as shown above and can be con-
sidered to have a decimal point or binary to the
left of byte 1.
A conversion start transition latches the state of
A0 asshowninFigure8andTable1. Thelatched
state determines if the conversion stops with 8–
bits (A0 high) or continues for 12–bits (A0 low).
If all 12–bits are read following an 8–bit conver-
sion, the three LSB’s will be a logic “0” and DB3
will be a logic “1”. A0 is latched because it is also
involved in enabling the output buffers as ex-
plained elsewhere. No other control inputs are
latched.
A0 may be toggled without damage to the con-
verter at any time. Break–before–make action is
guaranteed between the two data bytes. This
assures that the outputs which are strapped to-
gether in Figure 11 will never be enabled at the
same time.
Stand–Alone Operation
Thesimplestinterfaceisacontrollineconnected
to R/C. The other controls must be tied to known
states as follows: CE and 12/8 are wired high, A0
and CS are wired low. The output data arrives in
words of 12–bits each. The limits on R/C duty
cycle are shown in Figures 9 and 10. The duty
cycle may be within and including the extremes
shown in the specifications. In general, data may
bereadwhenR/CishighunlessSTSisalsohigh,
indicating a conversion is in progress.
In Figure 11, it can be seen that a read operation
usually begins after the conversion is complete
and STS is low. If earlier access is needed, the
read can begin no later than the addition of times
tDD and tHS before STS goes low.
Reading Output Data
The output data buffers remain in a high imped-
ance state until the following four conditions are
met: R/C is high, STS is low, CE is high and CS
is low. The data lines become active in response
to these four conditions, and output data accord-
ing to the conditions of the control lines 12/8 and
A . The timing diagram for this process is shown
in0 Figure 11. When 12/8 is high, all 12 data
outputs become active simultaneously and the
A input is ignored. The 12/8 input is usually tied
hi0gh or low; it is TTL/CMOS compatible. When
12/8islow, theoutputisseparatedintotwo8–bit
bytes as shown below:
ADDRESS BUS
A
0
28
STS
27
26
25
24
23
22
21
20
19
2
4
12/8
DB11 (MSB)
A
0
DATA
BUS
BYTE 1
xxxx xxxx
BYTE2
xxxx 0000
SPx74A
MSB
LSB
18
17
16
15
Thisconfigurationmakesiteasytoconnecttoan
8–bit address bus as shown in Figure 7. The A
control can be connected to the least significan0t
bitofthedatabusinordertostoretheoutputdata
into two consecutive memory locations. When
A0 is pulled low, the 8 MSB’s are enabled only.
When A0 is high, the 8 MSB’s are disabled, bits
DB0 (LSB)
DIG
COM
Figure 7. Interfacing SPx74A to 8–Bit Interface Bus
12
SPECTRUM [ SPECTRUM MICROWAVE, INC. ]
