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.
CONTROLLING THE ADC774
This is an abridged data sheet. For Discussion of Specifica-
tions, Installation, Calibration refer to ADC574A data sheet
or order PDS-835.
The Burr-Brown ADC774 can be easily interfaced to most
microprocessor systems and other digital systems. The
microprocessor may take full control of each conversion, or
the converter may operate in a stand-alone mode, controlled
only by the R/C input. Full control consists of selecting an
8- or 12-bit conversion cycle, initiating the conversion, and
reading the output data when ready—choosing either 12 bits
all at once, or 8 bits followed by 4 bits in a left-justified
format. The five control inputs (12/8, CS, AO, R/C, and CE)
are all TTL-/CMOS-compatible. The functions of the con-
trol inputs are described in Table I. The control function
truth table is listed in Table II.
tHRL
R/C
tDS
STS
tC
tHS
tHDR
High-Z State
DB11–DB0
Data Valid
Data Valid
FIGURE 1. R/C Pulse Low—Outputs Enabled After Con-
version.
Read footnote 5 to the Electrical Specifications table if
using ADC774 to replace the HI-774.
STAND-ALONE OPERATION
R/C
For stand-alone operation, control of the converter is ac-
complished 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.
tHRH
tDS
STS
tC
High-Z State
tHDR
tDDR
High-Z
DB11–
DB0
Data Valid
FIGURE 2. R/C Pulse High—Outputs Enabled Only While
R/C Is High.
PIN
DESIGNATION
DEFINITION
FUNCTION
CE (Pin 6)
Chip Enable
(active high)
Must be high (“1”) to either initiate a conversion or read output data. 0-1 edge may be used to initiate a
conversion.
CS (Pin 3)
R/C (Pin 5)
Chip Select
(active low)
Must be low (“0”) to either initiate a conversion or read output data. 1-0 edge may be used to initiate
a conversion.
Read/Convert
(“1” = read)
Must be low (“0”) to initiate either 8- or 12-bit conversions. 1-0 edge may be used to initiate a
conversion. Must be high (“1”) to read output data. 0-1 edge may be used to initiate a read operation.
(“0” = convert)
A
O (Pin 4)
Byte Address
Short Cycle
In the start-convert mode, AO selects 8-bit (AO = “1”) or 12-bit (AO = “0”) conversion mode. When reading
output data in two 8-bit bytes, AO = “0” accesses 8 MSBs (high byte) and AO = “1” accesses 4 LSBs and
trailing “0s” (low byte).
12/8 (Pin 2)
Data Mode Select
(“1” = 12 bits)
(“0” = 8 bits)
When reading output data, 12/8 = “1” enables all 12 output bits simultaneously. 12/8 = “0” will enable the
MSBs or LSBs as determined by the AO line.
TABLE I. ADC774 Control Line Functions.
CE
CS
R/C
12/8
AO
OPERATION
SYMBOL PARAMETER
MIN
TYP
MAX
UNITS
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
None
None
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
ns
ns
ns
ns
ns
200
375
150
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
25
150
1
1
1
1
1
1
1
150
0
0
0
0
0
TABLE III. Stand-Alone Mode Timing.
1
1
1
0
0
TABLE II. Control Input Truth Table.
®
ADC774
5
BB [ BURR-BROWN CORPORATION ]
