EE-158_15 Datasheet, PDF (1/16 Pages) Analog Devices – Technical Notes on using Analog Devices DSP components and development tools

a Engineer To Engineer Note EE-158
Technical Notes on using Analog Devices’ DSP components and development tools
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ADSP-2181 EZ-Kit Lite IDMA to
PC Printer Port Interface
Contributed by HS, last change 11-Apr-2002
Introduction:
The ADSP-218x family can be used either in
host or full memory interface configuration, ADSP-
2181 and ADSP-2183 offer both interfaces at the
same time. This host port interface allows through
the IDMA port easy read / write access to the internal
memory of the DSP. With little additional software on
the DSP even DSP system registers can be
accessed or external overlays be booted.
select. As the PC printer port has only eight data
lines and four output strobes, a little glue logic (exnor
of /WR and /RD) is required to realize the interface
for /CS. The i8255 has a nasty trait of pulling all
signals low when the direction of any the data ports
PA, PB or PC is changed. To avoid troubles at the
IDMA side, all four strobe lines (IAL, /IS, /IRD and
/IWR) have therefore an additional inverter placed.
After these inverters, PA of the i8255 is used to drive
the IDMA strobe lines, while PB and PC are used as
16bit IAD data port. A complete schematic of a 5V
PC to DSP interface is presented in this document.
To make the interface compatible with other 3.3V
ADSP-218x components, level transceivers like
74LVT245 have to be put in between PA, PB and PC
and switched accordingly.
Application Idea:
The IDMA interface is an easy to handle
asynchronous 16bit wide interface and requires only
four strobe lines to operate and synchronize it to the
DSP clock. Due to IDMA long read and write
capabilities, almost every host can access this
interface.
Often DSP software is developed on a PC
platform using VisualDSP. Code verification is
usually done with the simulator. Even todayís PCs do
not match the performance of the DSP, so
regression testing by driving test vector to an easy to
use hardware platform is desired. With a little
external circuitry, which can be easily fitted to an
ADDS-2181-EZ-Lite, application code can be booted
into to DSP and test vectors driven from a bi-
directional PC parallel port (Printer Port).
Software Design:
To ease the application and to provide a generic
example of routines, a driver for PC systems
allowing direct I/O access to PC system resources is
provided.
To access the IDMA port, first the i8255 must be
set up correctly from the reset state. Afterwards, a
write to the PA port allows to drive the required
strobe signals. Further writes to the PB and PC port
will latch the data to be transmitted to the ADSP-
218x. A write sequence will look like:
1 PA: set IAL
2 PB, PC: drive address
3 PA: clear IAL
4 PA: set write strobes /IS, /IWR
Hardware Design:
5 PB, PC; drive data
6 PA: set write strobe IS; IWR
The IDMA interface of the ADSP-218x family
requires just few data and strobe lines to operate the
interface. To realize the interface, the four strobe
lines and 16 data lines must be connected to the PC
printer port by a port expander. An i8255 offers three
freely programmable data ports, which can be
switched between input and output. A fourth internal
register controls the setting of the device. To access
the port expander, the 8 data lines of the PC printer
port are connected to the I/O data port. The 8255
has two additional address line to select the internal
ports and requires besides read or write a chip
Table 1: typical write sequence
To drive multiple data words into the ADSP-218x
through IDMA interface, it is just necessary to repeat
steps 4 to 6.
A read sequence looks like the write sequence,
with a little difference: before data is read in step 5,
the PB and PC port direction must be switched
between step 3 and 4 to an input and must be
changed again past step 6.
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