AD674B/AD774B
STANDALONE MODE
GENERAL A/D CONVERTER INTERFACE
“Standalone” mode is useful in systems with dedicated input
ports available and thus not requiring full bus interface capabil-
ity. Standalone mode applications are generally able to issue
conversion start commands more precisely than full-control
mode, resulting in improved accuracy.
CONSIDERATIONS
A typical A/D converter interface routine involves several opera-
tions. First, a write to the ADC address initiates a conversion.
The processor must then wait for the conversion cycle to com-
plete, since most integrated circuit ADCs take longer than one
instruction cycle to complete a conversion. Valid data can, of
course, only be read after the conversion is complete. The
AD674B and AD774B provide an output signal (STS) which
indicates when a conversion is in progress. This signal can be
polled by the processor by reading it through an external three-
state buffer (or other input port). The STS signal can also
generate an interrupt upon completion of conversion if the sys-
tem timing requirements are critical and the processor has other
tasks to perform during the ADC conversion cycle. Another
possible time-out method is to assume that the ADC will take its
maximum conversion time to convert, and insert a sufficient
number of “no-op” instructions to ensure that this amount of
processor time is consumed.
CE and 12/8 are wired HIGH, CS and A0 are wired LOW, and
conversion is controlled by R/C. The three-state buffers are
enabled when R/C is HIGH and a conversion starts when R/C
goes LOW. This gives rise to two possible control signals—a
high pulse or a low pulse. Operation with a low pulse is shown
in Figure 4a. In this case, the outputs are forced into the high
impedance state in response to the falling edge of R/C and
return to valid logic levels after the conversion cycle is completed.
The STS line goes HIGH 200 ns after R/C goes LOW and
returns low 600 ns after data is valid.
If conversion is initiated by a high pulse as shown in Figure 4b,
the data lines are enabled during the time when R/C is HIGH.
The falling edge of R/C starts the next conversion, and the data
lines return to three-state (and remain three-state) until the next
high pulse of R/C.
Once conversion is complete, the data can be read. For convert-
ers with more data bits than are available on the bus, a choice of
data formats is required, and multiple read operations are
needed. The AD674B and AD774B include internal logic to
permit direct interface to 8-bit and 16-bit data buses, selected
by the 12/8 input. In 16-bit bus applications (12/8 high) the
data lines (DB11 through DB0) may be connected to either the
12 most significant or 12 least significant bits of the data bus.
The remaining 4 bits should be masked in software. The inter-
face to an 8-bit data bus (12/8 low) is done in a left-justified for-
mat. The even address (A0 low) contains the 8 MSBs (DB11
through DB4). The odd address (A0 high) contains the 4 LSBs
(DB3 through DB0) in the upper half of the byte, followed by
four trailing zeroes, thus eliminating bit masking instructions.
CONVERSION TIMING
Once a conversion is started, the STS line goes HIGH. Convert
start commands will be ignored until the conversion cycle is
complete. The output data buffers can be enabled up to 1.2 µs
prior to STS going LOW. The STS line will return LOW at the
end of the conversion cycle.
The register control inputs, A0 and 12/8, control conversion
length and data format. If a conversion is started with A0 LOW,
a full 12-bit conversion cycle is initiated. If A0 is HIGH during a
convert start, a shorter 8-bit conversion cycle results.
During data read operations, A0 determines whether the three-
state buffers containing the 8 MSBs of the conversion result
(A0 = 0) or the 4 LSBs (A0 = 1) are enabled. The 12/8 pin
determines whether the output data is to be organized as two
8-bit words (12/8 tied LOW) or a single 12-bit word (12/8 tied
HIGH). In the 8-bit mode, the byte addressed when A0 is high
contains the 4 LSBs from the conversion followed by four trail-
ing zeroes. This organization allows the data lines to be over-
lapped for direct interface to 8-bit buses without the need for
external three-state buffers.
It is not possible to rearrange the output data lines for right-jus-
tified 8-bit bus interface.
D7
D0
XXX0 DB11
(EVEN ADDR) (MSB)
DB10 DB9 DB8 DB7 DB6 DB5 DB4
DB0
(LSB)
XXX1
(ODD ADDR)
DB3 DB2 DB1
0
0
0
0
Figure 10. Data Format for 8-Bit Bus
–10–
REV. C
ROCHESTER [ Rochester Electronics ]
