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LAN91C110

FEAST Fast Ethernet Controller for PCMCIA and Generic 16-Bit Applications

SMSC -> Microchip 

FEAST Fast Ethernet Controller for PCMCIA and Generic 16-Bit Applications

Datasheet

Chapter 4 Functional Description

4.1 Description of Blocks

4.1.1

Clock Generator Block

1. The XTAL1 and XTAL2 pins are to be connected to a 25 MHz 50 PPM crystal.

2. 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).

3. 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).

4.2

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 16-bit wide uni-directional FIFOs interfacing the DMA block. The DMA port of the

FIFO stores 32 bits to exploit the 32 bit data path into memory, but the FIFOs themselves are 16 bit wide. The

Control Path consists of a set of registers interfaced to the CPU via the BIU.

4.2.1

DMA Block

This block accesses packet memory on the CSMA/CD’s behalf, fetching transmit data and storing received

data. It interfaces the CSMA/CD Transmit and Receive FIFOs on one side, and the Arbiter block on the other.

To increase the bandwidth into memory, a 50 MHz clock is used by the DMA block, and the data path is 32

bits wide.

For example, during active reception at 100 Mbps, the CSMA/CD block will write a word into the Receive FIFO

every 160ns. The DMA will read the FIFO and accumulate two words on the output port to request a memory

cycle from the Arbiter every 320ns.

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.

4.2.2

Arbiter Block

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 used is a 25ns SRAM.

The Arbiter is also responsible for controlling the nRWE0-nRWE3 lines as a function of the bytes being

written. Read accesses are always 32 bit wide, and the Arbiter steers the appropriate byte(s) to the

appropriate lanes as a function of the address.

SMSC LAN91C110 Rev. B

Page 11

DATASHEET

Revision 1.0 (11-04-08)

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