
W89C840AF
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Publication Release Date:October 2000
Revision 1.01
moved back into the host memory by the receive DMA state machine. In loopback mode, the transmission
function will work normally as it is in normal mode, but, only the data is processed by the MAC. The COL,
CRS will be ignored in loopback mode.
To program the W89C840AF into loopback mode, the Loopback Mode of C18/CNCR should be set to
01H.
Full duplex and half duplex function
The transmit DMA and the receive DMA are independently operating no matter what the
W89C840AF is set in full duplex mode or in half duplex mode. However, in MAC side, the operations are
different and depends on the full duplex or half duplex mode is selected.
In the half duplex mode, the MAC should perform the transmission or reception operation at the
different time frame. Simultaneous transmission and reception operation are not allowed. Once the half
duplex mode is selected, the MII input signals, COL and CRS , will be valid inputs for the W89C840AF.
During the time the MAC is transmitting a packet, an active signal COL on MII bus will be judged to be a
transmit collision event, occurred on the media. However, in the time duration from 10 bits time to 16 bits
time after the packet is transmitted, the active COL signal is recognized as a SQE test signal but not a
collision event. The active signal CRS will be recognized as a loopback carrier sense signal when the MAC is
transmitting a packet. The carrier sense lost status is relied on the CRS. Normally, there should not be any
carrier sense lost during transmitting if the media and devices are functional.
In the full duplex mode, the MAC can perform the transmission and receive operation at the same
time. In this case, the COL and CRS signal on MII will be ignored. There are no collision event, SQE lost and
carrier sense lost defined in the full duplex mode. Any activity on COL and CRS will not affect the operation
inside the MAC.
To select the full duplex mode or the half duplex mode, you have to set the full duplex control bit of
C18/CNCR.
Network media speed selection function
The W89C840AF is specifically designed for both 100BASE-T and 10BASE-T Ethernet. The
transmission rate is based on the clock speed of the MII input signals TXCLK and RXCLK. The MAC will
transmit the data based on the clock rate of TXCLK, and receive the MII input data based on the clock rate of
RXCLK.
In 100BASE mode, the TXCLK and RXCLK will be at 25MHz, however, in 10BASE mode, the
TXCLK and RXCLK will be at 2.5 MHz. The clock rate of the RXCLK and the TXCLK is dependent and
determined by the external physical layer device(PHY) which supports 10/100M bit rate.