AD7824/AD7828
MODE 0
MODE 1
Figure 14 shows the timing diagram for Mode 0 operation. This
mode can only be used for microprocessors that have a WAIT
state facility, whereby a READ instruction cycle can be extended
to accommodate slow memory devices. A READ operation brings
CS
and
RD
low, which starts a conversion. The analog multiplexer
address inputs must remain valid while
CS
and
RD
are low. The
data bus (DB7–DB0) remains in the three-state condition until
conversion is complete. There are two converter status outputs on
the AD7824/AD7828, interrupt (INT) and ready (RDY), which
can be used to drive the microprocessor READY/WAIT input.
The RDY is an open-drain output (no internal pull-up device) that
goes low on the falling edge of
CS
and goes high impedance at the
end of conversion when the 8-bit conversion result appears on the
data outputs. If the RDY status is not required, the external
pull-up resistor can be omitted and the RDY output tied to GND.
The
INT
goes low when conversion is complete and returns high
on the rising edge of
CS
or
RD.
Mode 1 operation is designed for applications where the micropro-
cessor is not forced into a WAIT state. A READ operation takes
CS
and
RD
low, which triggers a conversion (see Figure 15). The
multiplexer address inputs are latched on the rising edge of
RD.
Data from the previous conversion is read from the three-state
data outputs (DB7–DB0). This data may be disregarded if not
required. Note that the RDY output (open drain output) does
not provide any status information in this mode and must be
connected to GND. At the end of conversion,
INT
goes low. A
second READ operation is required to access the new conversion
result. This READ operation latches a new address into the multi-
plexer inputs and starts another conversion.
INT
returns high at the
end of the second READ operation, when
CS
or
RD
returns high.
A delay of 2.5
µs
must be allowed between READ operations.
CS
t
CSS
RD
t
CSH
t
CSS
t
AS
ANALOG
CHANNEL
ADDRESS
ADDRESS
VALID
t
P
t
AS
ADDRESS
VALID
t
AH
RDY
t
RDY
INT
t
INTH
t
CRD
t
ACC2
DATA
VALID
t
DH
DATA
HIGH IMPEDANCE
Figure 14. Mode 0 Timing Diagram
CS
t
CSS
RD
t
RD
t
CSH
t
CSS
t
RD
t
CSH
t
P
t
AS
ANALOG
CHANNEL
ADDRESS
ADDRESS
VALID
t
AS
ADDRESS
VALID
t
AH
t
CRD
t
INTH
INT
t
AH
t
INTH
t
ACC1
DATA
OLD
VALID
t
DH
t
ACC1
NEW
VALID
t
DH
Figure 15. Mode 1 Timing Diagram
REV. F
–9–
AD [ ANALOG DEVICES ]
