AD664
Output Loads
+5V
Readback timing is tested with the output loads shown in Figure
22.
AD664
#1
AD664
#N
10kΩ
RST
RST
100nF
Figure 24. Power-On Reset
It is obvious from inspection that the scheme shown in Figure
24 is only appropriate for systems in which the RST is otherwise
not used. Should the user wish to use the RST pin, an addi-
tional logic gate may be included to combine the power-on reset
with the reset signal.
INTERFACING THE AD664 TO MICROPROCESSORS
The AD664 is easy to interface with a wide variety of popular
microprocessors. Common architectures include processors with
dedicated 8-bit data and address buses, an 8-bit bus over which
data and address are multiplexed, an 8-bit data and 16-bit
address partially muxed, and separate 16-bit data and address
buses.
Figure 22. Output Loads
Asynchronous Reset Operation
AD664 addressing can be accomplished through either
memory-mapped or I/O techniques. In memory-mapped
schemes, the AD664 appears to the host microprocessor as
RAM memory. Standard memory addressing techniques are
used to select the AD664. In the I/O schemes, the AD664 is
treated as an external I/O device by the host. Dedicated I/O pins
are used to address the AD664.
The asynchronous reset signal shown in Figure 23 may be
asserted at any time. A minimum pulse width (tRW) of 90 ns is
required. The reset feature is designed to return all DAC out-
puts to 0 volts regardless of the mode or range selected. In the
44-pin versions, the modes are reset to unipolar 10 V span (gain
of 1), and the input codes are rewritten to be “0s.” Previous
DAC code and mode information is erased.
MC6801 Interface
In Figures 25a–25d, we illustrate a few of the various methods
that can be used to connect an AD664 to the popular MC6801
microprocessor. In each of these cases, the MC6801 is intended
to be configured in its expanded, nonmultiplexed mode of
operation. In this mode, the MC6801 can address 256 bytes of
external memory over 8-bit data (Port 3) and 8-bit address
(Port 4) buses. Eight general-purpose I/O lines (Port 1) are also
available. On-board RAM and ROM provide program and data
storage space.
Figure 23a. Asynchronous Reset Operation
In Figure 25a, the three least significant address bits (P40, P41
and P42) are employed to select the appropriate on-chip
addresses for the various input registers of the AD664. Three
I/O lines (P17, P16 and P15) are used to select various operat-
ing features of the the AD664. IOS and E(nable) are combined
to produce an appropriate CS signal. This addressing scheme
leaves the five most significant address bits and five I/O lines
free for other tasks in the system.
Figure 23b. Asynchronous Reset Operation Timing
In the 28-pin versions of the AD664, the mode remains
unchanged, the appropriate input code is rewritten to reset the
output voltage to 0 volts. As in the 44-pin versions, the previous
input data is erased.
At power-up, an AD664 may be activated in either the read or
write modes. While at the device level this will not produce any
problems, at the system level it may. Analog Devices recom-
mends the addition of a simple power-on reset scheme to any
system where the possibility of an unknown start-up state could
be a problem. The simplest version of this scheme is illustrated
in Figure 24.
Figure 25b shows another way to interface an AD664 to the
MC6801. Here we’ve used the six least significant address lines
to select AD664 features and registers. This is a purely memory-
mapped scheme while the one illustrated in Figure 25a uses
some memory-mapping as well as some dedicated I/O pins. In
Figure 25b, two address lines and all eight I/O lines remain free
for other system tasks.
D
REV.
–12–
ADI [ ADI ]
