FUNCTIONAL DESCRIPTION
DMA block. The DMA port of the FIFO stores
DESCRIPTION OF BLOCKS
Clock Generator Block
32 bits exploiting the 32-bit data path into
memory. The Control Path consists of a set of
registers interfaced to the CPU via the BIU.
The LAN91C100's clock generator uses a 25
MHz crystal connected to pins XTAL1 XTAL2
and generates two free running clocks:
DMA Block
1) 50 MHz free running clock - Supplied to the
DMA and the ARBITER blocks.
This block accesses packet memory on the
CSMA/CD's behalf, fetching transmit data and
storing received data.
It interfaces the
2) 25 MHz free running clock - Used to run the
EPH during reset or when no TX25 is
present.
CSMA/CD Transmit and Receive FIFOs on one
side, and the Arbiter block on the other. The
data path is 32 bits wide.
Other clocks:
The DMA machine is able to support full duplex
operation. Independent receive and transmit
counters are used. Transmit and receive cycles
are alternated when simultaneous receive and
transmit accesses are needed.
3) TXCLK and RXCLK are 10 MHz clock
inputs. These clocks are generated by the
external ENDEC in 10 Mbps mode and are
only used by the CSMA/CD block.
Arbiter Block
4) TX25 is an input clock. It will be the nibble
rate of the particular PHY connected to the
MII (2.5 MHz for a 10 Mbps PHY, and 25
MHz for a 100 Mbps PHY).
The Arbiter block sequences accesses to packet
RAM requested by the BIU and by the DMA
blocks. BIU requests represent pipelined CPU
accesses to the Data Register, while DMA
requests represent CSMA/CD data movement.
The external memory devices used are 25ns
5) RX25 - This is the MII nibble rate receive
clock used for sampling received data
nibbles and running the receive state
machine (2.5 MHz for a 10 Mbps PHY, and
25 MHz for a 100 Mbps PHY).
32kx8 SRAM.
The cycle time for CPU
consecutive accesses to the Data Path is
80ns/doubleword. This time includes arbitration
and CSMA memory cycles.
6) LCLK - Bus clock - Used by the BIU for
synchronous
frequency is 50 MHz for VL BUS mode, and
8.5 MHz for EISA slave DMA.
accesses.
Maximum
The Arbiter is also responsible for controlling the
nRWE0-nRWE3 lines as a function of the bytes
being written. Read accesses are always 32
bits wide, and the Arbiter steers the appropriate
byte(s) to the appropriate lanes as a function of
the address.
CSMA/CD Block
This is a 16-bit oriented block, with fully-
independent Transmit and Receive logic. The
data path in and out of the block consists of two
6-bit wide uni-directional FIFOs interfacing the
The CPU Data Path consists of two uni-
directional FIFOs mapped at the Data Register
location. These FIFOs can be accessed in any
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