ADC0803, ADC0804
This converter has been designed to directly interface with
Interfacing 6800 Microprocessor Derivatives (6502,
etc.)
8080/85 or Z-80 Microprocessors. The three-state output
capability of the A/D eliminates the need for a peripheral
interface device, although address decoding is still required
to generate the appropriate CS for the converter. The A/D
can be mapped into memory space (using standard
The control bus for the 6800 microprocessor derivatives does
not use the RD and WR strobe signals. Instead it employs a
single R/W line and additional timing, if needed, can be derived
from the φ2 clock. All I/O devices are memory-mapped in the
6800 system, and a special signal, VMA, indicates that the
current address is valid. Figure 23 shows an interface
memory-address decoding for CS and the MEMR and
MEMW strobes) or it can be controlled as an I/O device by
using the I/OR and I/OW strobes and decoding the address
bits A0 → A7 (or address bits A8 → A15, since they will
contain the same 8-bit address information) to obtain the CS
input. Using the I/O space provides 256 additional
addresses and may allow a simpler 8-bit address decoder,
but the data can only be input to the accumulator. To make
use of the additional memory reference instructions, the A/D
should be mapped into memory space. See AN020 for more
discussion of memory-mapped vs I/O-mapped interfaces. An
example of an A/D in I/O space is shown in Figure 21.
schematic where the A/D is memory-mapped in the 6800
1
system. For simplicity, the CS decoding is shown using /
2
DM8092. Note that in many 6800 systems, an already decoded
4/5 line is brought out to the common bus at pin 21. This can be
tied directly to the CS pin of the A/D, provided that no other
devices are addressed at HEX ADDR: 4XXX or 5XXX.
In Figure 24 the ADC080X series is interfaced to the MC6800
microprocessor through (the arbitrarily chosen) Port B of the
MC6820 or MC6821 Peripheral Interface Adapter (PlA). Here
the CS pin of the A/D is grounded since the PlA is already
memory-mapped in the MC6800 system and no CS decoding
is necessary. Also notice that the A/D output data lines are
connected to the microprocessor bus under program control
through the PlA and therefore the A/D RD pin can be grounded.
The standard control-bus signals of the 8080 (CS, RD and
WR) can be directly wired to the digital control inputs of the
A/D, since the bus timing requirements, to allow both starting
the converter, and outputting the data onto the data bus, are
met. A bus driver should be used for larger microprocessor
systems where the data bus leaves the PC board and/or
must drive capacitive loads larger than 100pF.
Application Notes
NOTE #
DESCRIPTION
It is useful to note that in systems where the A/D converter is
1 of 8 or fewer I/O-mapped devices, no address-decoding
circuitry is necessary. Each of the 8 address bits (A0 to A7)
can be directly used as CS inputs, one for each I/O device.
AN016
“Selecting A/D Converters”
AN018
“Do’s and Don’ts of Applying A/D Converters”
AN020
“A Cookbook Approach to High Speed Data Acquisition
and Microprocessor Interfacing”
Interfacing the Z-80 and 8085
The Z-80 and 8085 control buses are slightly different from
that of the 8080. General RD and WR strobes are provided
and separate memory request, MREQ, and I/O request,
IORQ, signals have to be combined with the generalized
strobes to provide the appropriate signals. An advantage of
operating the A/D in I/O space with the Z-80 is that the CPU
will automatically insert one wait state (the RD and WR
strobes are extended one clock period) to allow more time
for the I/O devices to respond. Logic to map the A/D in I/O
space is shown in Figure 22. By using MREQ in place of
IORQ, a memory-mapped configuration results.
AN030
“The ICL7104 - A Binary Output A/D Converter for
Microprocessors”
Additional I/O advantages exist as software DMA routines are
available and use can be made of the output data transfer
which exists on the upper 8 address lines (A8 to A15) during
I/O input instructions. For example, MUX channel selection for
the A/D can be accomplished with this operating mode.
The 8085 also provides a generalized RD and WR strobe, with
an IO/M line to distinguish I/O and memory requests. The circuit
of Figure 22 can again be used, with IO/M in place of IORQ for
a memory-mapped interface, and an extra inverter (or the logic
equivalent) to provide IO/M for an I/O-mapped connection.
13
INTERSIL [ Intersil ]
