LAN91C95 datasheet(70/144 Pages) SMSC

70 / 144 page

I/O SPACE - BANK2

OFFSET

NAME

TYPE

SYMBOL

MMU COMMAND REGISTER

WRITE ONLY

BUSY Bit Readable

MMUCR

This register is used by the CPU to control the memory allocation, de-allocation, TX FIFO and RX FIFO

control. The three command bits determine the command issued as described below:

HIGH

BYTE

LOW

BYTE

COMMAND

N0/BUSY

COMMAND SET:

xyz

000

NOOP - NO OPERATION

001

ALLOCATE MEMORY FOR TX - N2,N1,N0 defines the amount of memory requested as (value

+ 1) * 256 bytes. Namely N2,N1,N0 = 1 will request 2 * 256 = 512 bytes. Valid range for

N2,N1,N0 is 0 through 5. A shift-based divide by 256 of the packet length yields the appropriate

value to be used as N2,N1,N0. Immediately generates a completion code at the ALLOCATION

RESULT REGISTER. Can optionally generate an interrupt on successful completion. The

allocation time can take worst case (N2, N1, N0 + 2) * 200ns.

010

RESET MMU TO INITIAL STATE - Frees all memory allocations, clears relevant interrupts,

resets packet FIFO pointers.

011

REMOVE FRAME FROM TOP OF RX FIFO - To be issued after CPU has completed

processing of present receive frame. This command removes the receive packet number from

the RX FIFO and brings the next receive frame (if any) to the RX area (output of RX FIFO).

100

REMOVE AND RELEASE TOP OF RX FIFO - Like 3) but also releases all memory used by

the packet presently at the RX FIFO output.

101

RELEASE SPECIFIC PACKET - Frees all pages allocated to the packet specified in the

PACKET NUMBER REGISTER. Should not be used for frames pending transmission.

Typically used to remove transmitted frames, after reading their completion status. Can be

used following 3) to release receive packet memory in a more flexible way than 4).

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