CE
CS
R/C
12/8
AO
OPERATION
None
tHRL
0
X
X
1
0
0
X
X
0
0
0
0
X
X
X
X
X
X
X
X
1
X
X
0
1
0
1
0
1
X
0
1
R/C
None
Initiate 12-bit conversion
Initiate 8-bit conversion
Initiate 12-bit conversion
Initiate 8-bit conversion
Initiate 12-bit conversion
Initiate 8-bit conversion
Enable 12-bit output
Enable 8 MSBs only
Enable 4 LSBs plus
4 trailing zeros
tDS
1
1
1
1
1
1
1
Status
0
0
0
0
0
tC
tHS
tHDR
High-Z State
1
1
1
DB11-DB0
Data Valid
Data Valid
0
0
FIGURE 4. R/C Pulse Low—Outputs Enabled After Con-
version.
TABLE III. Control Input Truth Table.
STAND-ALONE OPERATION
For stand-alone operation, control of the converter is accom-
plished by a single control line connected to R/C. In this
mode CS and AO are connected to digital common and CE
and 12/8 are connected to VLOGIC (+5V). The output data are
presented as 12-bit words. The stand-alone mode is used in
systems containing dedicated input ports which do not
require full bus interface capability.
R/C
tHRH
tDS
Status
tC
High-Z State
tHDR
tDDR
High-Z
DB11–
DB0
Data Valid
Conversion is initiated by a high-to-low transition of R/C.
The three-state data output buffers are enabled when R/C is
high and STATUS is low. Thus, there are two possible
modes of operation; conversion can be initiated with either
positive or negative pulses. In either case the R/C pulse must
remain low for a minimum of 50ns.
FIGURE 5. R/C Pulse High—Outputs Enabled Only While
R/C Is High.
an 8-bit conversion, the 3LSBs (DB0-DB2) will be low
(logic 0) and DB3 will be high (logic 1). AO is latched
because it is also involved in enabling the output buffers. No
other control inputs are latched.
Figure 4 illustrates timing when conversion is initiated by an
R/C pulse which goes low and returns to the high state
during the conversion. In this case, the three-state outputs
go to the high-impedance state in response to the falling
edge of R/C and are enabled for external access of the data
after completion of the conversion. Figure 5 illustrates the
timing when conversion is initiated by a positive R/C pulse.
In this mode the output data from the previous conversion is
enabled during the positive portion of R/C. A new conver-
sion is started on the falling edge of R/C, and the three-state
outputs return to the high-impedance state until the next
occurrence of a high R/C pulse. Table IV lists timing
specifications for stand-alone operation.
CONVERSION START
The converter is commanded to initiate a conversion by a
transition occurring on any of three logic inputs (CE, CS,
and R/C) as shown in Table III. Conversion is initiated by
the last of the three to reach the required state and thus all
three may be dynamically controlled. If necessary, all three
may change states simultaneously, and the nominal delay
time is the same regardless of which input actually starts
conversion. If it is desired that a particular input establish the
actual start of conversion, the other two should be stable a
minimum of 50ns prior to the transition of that input. Timing
relationships for start of conversion timing are illustrated in
Figure 6. The specifications for timing are contained in
Table V.
SYMBOL
PARAMETER
MIN
TYP MAX UNITS
tHRL
tDS
tHDR
tHS
tHRH
tDDR
Low R/C Pulse Width
STS Delay from R/C
Data Valid After R/C Low
STS Delay After Data Valid
High R/C Pulse Width
Data Access Time
50
ns
200
1000
150
ns
ns
ns
ns
ns
25
300
150
400
The STATUS output indicates the current state of the con-
verter by being in a high state only during conversion.
During this time the three state output buffers remain in a
high-impedance state, and therefore data cannot be read
during conversion. During this period additional transitions
of the three digital inputs which control conversion will be
ignored, so that conversion cannot be prematurely termi-
nated or restarted. However, if AO changes state after the
beginning of conversion, any additional start conversion
transition will latch the new state of AO, possibly resulting
in an incorrect conversion length (8 bits vs 12 bits) for that
conversion.
TABLE IV. Stand-Alone Mode Timing.
FULLY CONTROLLED OPERATION
Conversion Length
Conversion length (8-bit or 12-bit) is determined by the state
of the AO input, which is latched upon receipt of a conver-
sion start transition (described below). If AO is latched high,
the conversion continues for 8 bits. The full 12-bit conver-
sion will occur if AO is low. If all 12 bits are read following
®
8
ADC574A
BB [ BURR-BROWN CORPORATION ]
