isl3874 Intersil Corporation, isl3874 Datasheet - Page 13

• Manufacturer Part Number: isl3874
• Description: Wireless Lan Integrated Medium Access Controller With Baseband Processor With Mini-pci
• Manufacturer: Intersil Corporation

External Memory Interface
The ISL3874 provides separate external chip selects for
code space and data storage space. Code space is
accessible as data space through an overlay mechanism,
except for an internal ROM. Refer to Figures 6 , 7 and 8 for
ISL3874 memory configuration detail examples.
The maximum possible memory space size is 4Mbytes.
Most of the data store space is reserved for storage of
received and transmitted data, with some areas reserved for
use by firmware. However, a portion of the data store may
be allocated as code store. This permits higher speed
instruction execution, by using fast RAMs, than is possible
from Flash memories. The maximum size of this overlay is
the full code space address range, 128kbytes, and is
allocated in independent sections of 16KBytes each, on
16kbyte boundaries, ranging from the highest address of the
actual physical memory space and extending down.
Mapping code execution to RAM requires the RAM to have
code written into it. Typically, this is done by placing code in a
non-volatile memory such as a Flash in the code space. At
initialization, the code in the non-volatile memory transfers itself
to RAM, maps the appropriate blocks of the code space to the
RAM, and then branches to begin execution from RAM. This
allows low cost, slow Flash devices to hold an entire code
image, which can be executed much faster from RAM. If code
is not placed in an external non-volatile memory as described
here, it must be transferred to the RAM via the Host Interface.
Slow memories are not dynamically sensed. Following reset,
the instruction clock operates with a slower cycle while the
Flash is copied to RAM. Once code has been copied from
Flash to RAM, execution transfers to RAM and the clock is
raised to the normal operating frequency.
As mentioned above, it is feasible to operate without a code
image in a non-volatile memory. In such a system, the
firmware must be downloaded to RAM through the host
interface before operation can commence.
ISL3874
MISO (PJ2)
SD (PJ1)
SCLK (PJ0)
CS# (TCLKIN)
LARGE SERIAL EEPROM
PULLUP
13
SI
SCK
CS
AT45DB011
FIGURE 8. SERIAL EEPROM MEMORY INTERFACE
RESET#
WP#
SO
PULLUP
ISL3874
The external SRAM memory must be organized in a 16-bit
width to provide adequate performance to implement the
802.11 protocol at 11Mb/s rates. Systems designed for lower
performance applications may be able to use 8-bit wide
memory.
The minimum external memory is 128kbytes of SRAM,
organized 8 or 16 bits wide. Typical applications, including
802.11 station designs, use 256kbytes organized 128K x 16.
An access point application could make use of the full address
space of the device with 4Mbytes organized a 2M x 16.
The ISL3874 supports 8 or 16-bit code space, and 8 or 16-bit
data space. Code space is typically populated with the least
expensive Flash memory available, usually an 8-bit device.
Data space is usually populated with high-speed RAMs
configured as a 16-bit space. This mixing of 8/16 bit spaces is
fully supported, and may be done in any combination desired
for code and data space.
The ISL3874 supports direct control of single chip 16-bit
wide SRAMs with high/low byte enables, as well as direct
control of a 16-bit space constructed from 8-bit wide SRAMs.
The type of memory configuration is specified via the
appropriate MD pin, sensed when the ISL3874 is reset.
ISL3874 pin MA0/MWEH functions as Address 0 for 8-bit
access, (such as Flash) as MWEH (High Byte Write Enable)
when two x8 memories are configured as a single x16
space, and as the upper Byte Enable when a single x16
memory is used. No external logic is required to generate
the required signals for both types of memory configurations,
even when both exist together; all that is required is for the
ISL3874 code to configure the ISL3874 memory controller to
generate the proper signals for the particular address space
being accessed.
For 8-bit spaces, the ISL3874 dynamically configures pin
MA0/MWEH cycle-by-cycle as the address LSB.
ISL3874
CS# (TCLKIN)
SCLK (PJ0)
SMALL SERIAL DEVICES
PULLUP
(NOTE: MUST OPERATE
SDA
SCL
AT 400kHz AT 3.3V
24C08
WP
AO
A1
A2
DC
)