38
38
INTERNAL RAM
The integration of the 1K x 8 RAM in the
ARCNET core represents significant real estate
savings.
The PC board is now free of the
cumbersome external RAM, external latch, and
multiplexed
address/data
bus
and
control
functions which were necessary to interface to
the RAM.
The integration of RAM represents significant
cost savings because it isolates the system
designer from the changing costs of external
RAM and it minimizes reliability problems,
assembly time and costs, and layout complexity.
Sequential Access Memory
The internal RAM is accessed via a pointer-
based scheme.
Rather than interfering with
system memory, the internal RAM is indirectly
accessed through the Address High and Low
Pointer Registers. The data is channeled to and
from the microcontroller via the 8-bit Data
Register. For example: a packet in the internal
RAM buffer is read by the microcontroller by
writing the corresponding address into the
Address Pointer High and Low Registers (offsets
02H and 03H).
Note that the High Register
should be written first, followed by the Low
Register, because writing to the Low Register
loads the address.
At this point the device
accesses
that
location
and
places
the
corresponding data into the Data Register. The
microcontroller then reads the Data Register
(offset 04H) to obtain the data at the specified
location. If the Auto Increment bit is set to logic
"1", the device will automatically increment the
address and place the next byte of data into
the Data
Register, again to be
read by the
microcontroller. This process is continued until
the entire packet is read out of RAM. Refer to
Figure 10 for an illustration of the Sequential
Access operation.
When switching between reads and writes, the
pointer must first be written with the starting
address. At least one cycle time should separate
the pointer being loaded and the first read (see
timing parameters).
Access Speed
The ARCNET core is able to accommodate very
fast access cycles to its registers and buffers.
Arbitration to the buffer does not slow down the
cycle because the pointer based access method
allows data to be prefetched from memory and
stored in a temporary
register. Likewise, data
to be written is stored in the temporary register
and then written to memory.
A Slow Arbitration Bit is provided in the Setup
Register to slow down the arbitration clock for
buffer accesses at 5 Mbps. The SLOWARB bit
must be set to a "1" for 5 Mbps operation.
SOFTWARE INTERFACE
The 80C32 core interfaces to the ARCNET core
via software by accessing the various registers.
These actions are described in the Internal
Registers
section.
The
software
flow
for
accessing the data buffer is based on the
Sequential Access scheme. The basic sequence
is as follows:
• Disable Interrupts
• Write to Pointer Register High (specifying
Auto-Increment mode.)
• Write to Pointer Register Low (this loads the
address.)
• Enable Interrupts
• Read or write the Data Register (repeat as
many times as necessary to empty or fill the
buffer.)
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