AD7821
MICROPROCESSOR INTERFACING
AD7821 – TMS32010 INTERFACE
The AD7821 is designed for easy interfacing to microprocessors
as a memory mapped peripheral or an I/O device. This reduces
to a minimum the amount of external logic required for
interfacing.
A typical interface to the TMS32010 is shown in Figure 16. The
AD7821 is mapped at a port address and the interface is designed
for the maximum TMS32010 clock frequency of 20 MHz. In this
case, the AD7821 is configured in the WR-RD interface mode.
This means that a write instruction starts a conversion and a read
instruction reads the result when the conversion is completed. A
precise timer or clock source is used to start a conversion in
applications requiring equidistant sampling intervals. The
scheme used, whereby the AD7821 generates an interrupt to
the TMS32010, is limited in that it does not allow the AD7821
to be sampled at its maximum rate. This is because the time
between samples has to be long enough to allow the TMS32010
to service its interrupt and read data from the AD7821.
Constant interruption of the TMS32010 by the AD7821, every time
the ADC completes a conversion, is not a very efficient use of
the processor time. To overcome these problems, some buffer
memory or FIFO could be placed between the AD7821 and the
TMS32010. The INT line of the AD7821 could be used to
trigger a pulse which drives its CS and RD lines and places the
AD7821 data into a FIFO or buffer memory. The microproces-
sor can then read a batch of data from the FIFO or buffer memory
at some convenient time. Reading data from the AD7821, after an
INT has been received, consists of a <IN A, PA> instruction
(PA is the decoded ADC address).
AD7821 – 68008 INTERFACE
Figure 14 shows an AD7821 interface to the 68008 micropro-
cessor. The ADC is configured for the RD interface mode. This
means that one read instruction starts a conversion and reads
the result when the conversion is completed. The read cycle is
stretched out over the entire conversion period by taking the
INT line back to the DTACK input of the 68008. Starting a
conversion and reading the relevant data consists of a <MOVE B
Dn, addr> instruction, where addr is the decoded ADC address and
Dn is the data register into which the result is placed.
Figure 14. AD7821 to 68008 Interface
AD7821 – 8088 INTERFACE
A typical interface to the 8088 is shown in Figure 15. The AD7821
is configured for the RD interface mode. One read instruction
starts a conversion and reads the result. The read cycle is stretched
out over the entire conversion period by taking the RDY line back
to the READY input of the 8088. Starting a conversion and
reading the result consists of a <MOV AX, (addr)> instruction,
where addr is the decoded ADC address and AX is the 8088 data
register into which the conversion result is placed.
Figure 16. AD7821 to TMS32010 Interface
AD7821 – 8051 INTERFACE
Figure 17 shows the AD7821 interface to the 8051 microcom-
puter. The AD7821 is configured in the WR-RD interface mode
and is connected to the 8051 ports. The processor starts conver-
sion and then polls INT, until it goes low, before reading the
conversion result. Data is read from the AD7821 by using the
<MOV A, 90H> instruction (90H is the address for Port 1).
Figure 15. AD7821 to 8088 Interface
Figure 17. AD7821 to 8051 Interface
REV. B
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ADI [ ADI ]
