Non-PCI Single-Chip Full Duplex Ethernet Controller
I/O SPACE - BANK2
OFFSET
0
NAME
MMU COMMAND REGISTER
TYPE
SYMBOL
MMUCR
WRITE ONLY
BUSY BIT
READABLE
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:
N0/
BUSY
COMMAND
x
0
N2
N1
w
y
z
0
COMMAND SET:
wxyz
0000
0010
0) - NOOP - NO OPERATION -
2) - 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 and 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)* 200µs.
0100
0110
4) - RESET MMU TO INITIAL STATE - Frees all memory allocations, clears relevant
interrupts, resets packet FIFO pointers.
6) - 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).
0111
7) - REMOVE FRAME FROM TOP OF TX FIFO- To be issued ONLY after the Host disabled
the transmitter and has completed processing of the present transmit frame.
Note:
Determining Transmit completion is done by polling the TEMPTY bit in the Transmit FIFO Port Register.
This command removes the Transmit packet number from the TX FIFO and brings the next Transmit
frame (if any) to the TX area (output of TX FIFO).
1000
8) - REMOVE AND RELEASE TOP OF RX FIFO - Like 6) but also releases all memory used
by the packet presently at the RX FIFO output.
1010
A) - 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 6 (to release receive packet memory in a more flexible way than 8).
1100
C) - ENQUEUE PACKET NUMBER INTO TX FIFO - This is the normal method of
transmitting a packet just loaded into RAM. The packet number to be enqueued is taken
from the PACKET NUMBER REGISTER.
SMSC DS – LAN91C96I
Page 47
Rev. 11/18/2004
DATASHEET
SMSC [ SMSC CORPORATION ]
