LAN91C100FD-ST SMSC, LAN91C100FD-ST Datasheet - Page 49

• Manufacturer Part Number: LAN91C100FD-ST
• Description: Ethernet ICs Non-PCI 10/100 Ethernet MAC
• Manufacturer: SMSC
• Type: Single Chip MAC and PHY Controllerr

Specifications of LAN91C100FD-ST

• Ethernet Connection Type: 10 Base-T, 100 Base-TX
• Minimum Operating Temperature: 0 C
• Mounting Style: SMD/SMT
• Product: Ethernet Controllers
• Number Of Transceivers: 1
• Standard Supported: 802.3
• Data Rate: 10 MB, 100 MB
• Supply Voltage (max): 7 V
• Supply Voltage (min): 0.3 V
• Supply Current (max): 40 mA
• Maximum Operating Temperature: + 70 C
• Package / Case: TQFP-208
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

5.6
SMSC LAN91C100FD Rev. D
FEAST Fast Ethernet Controller with Full Duplex Capability
Memory is always requested by the side that needs to write into it, that is: the CPU for transmit or the MAC
for receive. The CPU can control the number of bytes it requests for transmit but it cannot determine the
number of bytes the receive process is going to demand. Furthermore, the receive process requests will
be dependent on network traffic, in particular on the arrival of broadcast and multicast packets that might
not be for the node, and that are not subject to upper layer software flow control.
In order to prevent unwanted traffic from using too much memory, the CPU can program a "memory
reserved for transmit" parameter. If the free memory falls below the "memory reserved for transmit" value,
MMU requests from the MAC block will fail and the packets will overrun and be ignored. Whenever
enough memory is released, packets can be received again. If the reserved value is too large, the node
might lose data which is an abnormal condition. If the value is kept at zero, memory allocation is handled
on first-come first-served basis for the entire memory capacity.
Note that with the memory management built into the LAN91C100FD, the CPU can dynamically program
this parameter. For instance, when the driver does not need to enqueue transmissions, it can allow more
memory to be allocated for receive (by reducing the value of the reserved memory). Whenever the driver
needs to burst transmissions it can reduce the receive memory allocation. The driver program the
parameter as a function of the following variables:
1.
2.
The reserved memory value can be changed on the fly. If the MEMORY RESERVED FOR TX value is
increased above the FREE MEMORY, receive packets in progress are still received, but no new packets
are accepted until the FREE MEMORY increases above the MEMORY RESERVED value.
Interrupt Generation
The interrupt strategy for the transmit and receive processes is such that it does not represent the
bottleneck in the transmit and receive queue management between the software driver and the controller.
For that purpose there is no register reading necessary before the next element in the queue (namely
transmit or receive packet) can be handled by the controller. The transmit and receive results are placed
in memory.
The receive interrupt will be generated when the receive queue (FIFO of packets) is not empty and receive
interrupts are enabled. This allows the interrupt service routine to process many receive packets without
exiting, or one at a time if the ISR just returns after processing and removing one.
There are two types of transmit interrupt strategies:
1.
2.
The strategy is determined by how the transmit interrupt bits and the AUTO RELEASE bit are used.
TX INT bit - Set whenever the TX completion FIFO is not empty.
TX EMPTY INT bit - Set whenever the TX FIFO is empty.
AUTO RELEASE - When set, successful transmit packets are not written into completion FIFO, and their
memory is released automatically.
1.
Free memory (read only register)
Memory size (read only register)
One interrupt per packet.
One interrupt per sequence of packets.
One interrupt per packet: enable TX INT, set AUTO RELEASE=0. The software driver can find the
completion result in memory and process the interrupt one packet at a time. Depending on the
completion code the driver will take different actions. Note that the transmit process is working in
parallel and other transmissions might be taking place. The LAN91C100FD is virtually queuing the
packet numbers and their status words.
DATASHEET
Page 49
Revision 1.0 (09-22-08)