EPROM types
The Southern Cross II can use three different EPROM types, 8K (2764), 16K (27128) and 32K (27256). The
EPROM type is selected by setting jumper JP2 to the correct position as marked on the circuit board.
Note that in order to save BASIC programs in EPROM, a 27256 EPROM must be used.
Southern Cross II Memory Map
This manual does not aim to provide a detailed explanation of the architecture of the 8051 family of
microcontrollers. However a short description is necessary so that the memory decoding scheme can be
explained and understood.
The 8051 family of microcontrollers implement the Harvard memory architecture. This means that these
devices have separate address spaces for Program and Data memory, each up to 64K in size. Internal
circuitry in the microcontroller accesses the correct memory based on the instruction being executed.
As well as having separate Program and Data spaces, these microcontrollers also have internal and external
Program and Data memory. The 8051/52 series of microcontrollers can have up to 8K of internal program
memory. Thus any program address in the range 0000h to 1FFFh is directed to internal Program memory.
Any other program address is directed externally. It is possible to direct all program access externally by
disabling internal program memory. This is done by connecting the /EA pin to ground. The 8031/32 series
have no internal program memory so all program access must be directed externally. The microcontrollers
running BASIC are 8052 series chips with the BASIC interpreter stored in internal program memory.
Program memory can only be read, not written to, The /PSEN signal is used as a read strobe for any external
program memory. Data memory occupies a separate address space from Program memory. The /RD and /WR
signals are used to access external Data memory as needed. Only Data memory can be written to and
programs can only run from Program memory!
It is possible to combine all or part of the Program and Data memory into one address space. This is done by
logically ORing the /PSEN and /RD signals with the decoded memory range required and using the resultant
output as a read strobe. Combining Program and Data memory is necessary on the Southern Cross II so that
users can download and run programs.
The Southern Cross II provides 16K of RAM memory that is accessible as both Program and Data memory.
This is the area of memory where user programs are downloaded into and executed from. The other 16K of
RAM is accessible as Data memory only.
The EPROM can be an 8K, 16K or 32K type. The EPROM address space is split, with 16K located at 0000h
and the other 16K located at 8000h. The 16K block at 8000h is used to store BASIC programs when the
Southern Cross II is used as a BASIC computer! Refer to the "BASIC-52 Programming" reference manuals
for more information.
All external I/O devices such as the SPI and PPI chips are located in external data memory. Remember that
only data memory can be written to!
Figure 1 gives an overall picture of the Southern Cross II memory map.
Memory and I/O decoding is provided via two programmable logic devices (GALs). Using GALs for address
mapping adds great flexibility to the circuit design as well as significantly reducing chip count. Other memory
and I/O address mapping schemes are possible by re-programming the GALs.
Contact DIY Electronics for further information.
