CY545 Stepper System Controller
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CHAPTER 12 - EXTERNAL MEMORY SUPPORT
Memory Read/Write
The CY545 may be operated with up to 64K bytes of external memory, which hold various
sequences of commands to run. A number of CY545 commands are used to support the
external memory, allowing you to set the memory address pointer, enter commands into
memory, and execute commands from memory.
Several types of external memory may be used, including RAM, EEPROM, and EPROM. Note
that EPROM memory must be programmed separately from its use in a CY545 application, so
the CY545 Enter command and the ability to define the contents of memory are given up when
an EPROM is used. The other two forms of memory allow the CY545 to write to memory as well
as read it.
When RAM is used, the contents are lost when power is removed, unless some battery back-up
power is provided. Normally, the contents of RAM must be redefined whenever the CY545
application is started.
With EEPROM, the memory can still be written by the CY545, especially if a 5-volt-only version
is used, but the memory contents are not lost when power is removed.
An applications schematic, provided with this manual, illustrates the use of an EEPROM as the
external data memory for the CY545. The device shown is a Xicor X28256 EEPROM, which has
32K bytes of memory. Smaller devices may be used if less memory is required, and a larger
device, with a full 64K bytes of memory could be supported.
The CY545 provides all address and strobe timing signals to the memory. Signals involved are
the data bus, XMEM_SEL, ALE, WR, and RD. Special use is made of the data bus, which
provides all 16 bits of address, plus the actual data transfers. Two external latches are required
to hold the address. The example uses 74LS373s for this purpose.
When the CY545 reads or writes the external memory, it starts by placing the most significant
address byte on the data bus. It then selects the memory by driving XMEM_SEL low. This both
chip-enables the memory and latches the upper address byte into the 74LS373 which holds
address lines A8 to A15. The CY545 then performs a read or write operation.
© 2002 Cybernetic Micro Systems
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Chapter 12 - External Memory Support
ETC [ ETC ]
