ADC912AFS View Datasheet(PDF) - Analog Devices
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ADC912AFS Datasheet PDF : 16 Pages
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ADC912A
MICROPROCESSOR INTERFACING
The ADC912A has self-contained logic for both 8-bit and 16-bit
data bus interfacing. The output data can be formatted into
either a 12-bit parallel word for a 16-bit data bus or an 8-bit
data word pair for an 8-bit data bus. Data is always right justi-
fied, i.e., LSB is the most right-hand bit in a 16-bit word. For a
two-byte read, only data outputs D7 . . . D0/8 are used. Byte
selection is governed by the HBEN input which controls an
internal digital multiplexer. This multiplexes the 12 bits of
conversion data onto the lower D7 . . . D0/8 outputs (4 MSBs or
8 LSBs) where it can be read in two read cycles. The 4 MSBs
always appear on D11 . . . D8 whenever the three-state output
drivers are turned on. See Figure 20.
Two A/D conversion modes of operation are available for both
data bus sizes: the ROM mode and the Slow-Memory mode.
ADC912A
HBEN
CS
RD
"1"
CONVERSION START
D
Q
(POSITIVE EDGE
TRIGGER)
CLR
BUSY
ACTIVE HIGH
(HBEN = "0")
ENABLE THREE-STATE
OUTPUTS
PINS: D11 ... D0/8
DATA BITS: DB11 ... DB0
ACTIVE HIGH
(HBEN = "1")
ENABLE THREE-STATE
OUTPUTS
PINS: D11 ... D8
DATA BITS: DB11 ... DB8
PINS: D7 ... D4
DATA BITS: LOGIC LOW
PINS: D3/11 ... D0/8
DATA BITS: DB11 ... DB8
Figure 20. Internal Logic for Control Inputs CS, RD, and
HBEN
In the ROM mode each READ instruction obtains new, valid
data, assuming the minimum timing requirements are satisfied.
However, since the data output from a current READ instruc-
tion was generated from a conversion initiated by a previous
READ operation, the current data may be out-of-date. To be
sure of obtaining up-to-date data, READ instructions may be
coded in pairs (with some NOPs between them); use only the
data from the second READ in each pair. The first READ starts
the conversion, the second READ gets the results.
The Slow-Memory mode is the simplest. It is the method of
choice where compact coding is essential, or where software
bugs are a hazard. In this mode, a single READ instruction will
initiate a data conversion, interrupt the microprocessor until
completion (WAIT states are introduced), then read the results.
If the system throughput tolerates WAIT states, and the hardware
is correct, then the Slow-Memory mode is virtually immune to
subsequent software modifications. Placing the microprocessor
in the WAIT state has an additional advantage of quieting the
digital system to reduce noise pickup in the analog conversion
circuitry. The 12-bit parallel Slow-Memory mode provides the
fastest analog sampling rate combined with digital data transfer
rate for sampled-data systems.
PARALLEL READ, SLOW-MEMORY MODE
(HBEN = LOW)
Figure 5 shows the timing diagram and data bus status for Par-
allel Read, Slow-Memory Mode. CS and RD going low triggers
a conversion and the ADC912A acknowledges by taking BUSY
low. Data from the previous conversion appears on the three-
state data outputs. BUSY returns high at the end of conversion,
when the output latches have been updated, and the conversion
result is placed on data outputs D11 . . . D0/8.
TWO-BYTE READ, SLOW-MEMORY MODE
For a two-byte read only the eight data outputs D7 . . . D0/8
are used. Conversion start procedure and data output status for
the first read operation is identical to Parallel Read, Slow-Memory
Mode. See Figure 6, Timing Diagram and Data Bus Status. At
the end of conversion, the low data byte (DB7 . . . DB0) is read
from the A/D converter. A second READ operation with HBEN
high places the high byte on data outputs D3/11 . . . D0/8 and
disables conversion start. Note the 4 MSBs also appear on data
outputs D11 . . . D8 during these two READ operations.
PARALLEL READ, ROM MODE (HBEN = LOW)
A conversion is started with a READ operation. The 12 bits of
data from the previous conversion are available on data outputs
D11 . . . D0/8 (see Figure 7). This data may be disregarded if
not required. A second READ operation reads the new data
(DB11 . . . DB0) and starts another conversion. A delay at least
as long as the ADC912A conversion time must be allowed be-
tween READ operations. If a READ takes place prior to the end
of 13 CLKS of the ADC conversion, the remaining bits not yet
tested will be invalid.
TWO-BYTE READ, ROM MODE
For a two-byte read only the data outputs D7 . . . D0/8 are used.
Conversion is started in the same way with a READ operation
and the data output status is the same as the Parallel Read,
ROM Mode. See Figure 8, Two-Byte Read Timing Diagram,
ROM Mode. Two more READ operations are required to obtain
the new conversion result. A delay equal to the ADC912A con-
version time must be allowed between conversion start and
places the high byte (4 MSBs) on data outputs D3/11 . . . D0/8. A
third READ operation accesses the low data byte (DB7 . . . DB0)
and starts another conversion. The 4 MSBs also appear on data
outputs D11 . . . D8 during all three read operations above.
REV. B
–11–
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