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LAN9117 ๋ฐ์ดํฐ์ํธ(HTML) 81 Page - Microchip Technology |
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LAN9117 ๋ฐ์ดํฐ์ํธ(HTML) 81 Page - Microchip Technology |
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๏ฃ 2005-2016 Microchip Technology Inc. DS00002267A-page 81 LAN9117 14 Reserved 13 Hash/Perfect Filtering Mode (HPFILT). When reset (0), the LAN9117 will implement a perfect address filter on incoming frames according the address specified in the MAC address register. When set (1), the address check Function does imperfect address filtering of multicast incoming frames according to the hash table specified in the multicast hash table register. If the Hash Only Filtering mode (HO) bit is set (1), then the physical (IA) are imperfect filtered too. If the Hash Only Filtering mode (HO) bit is reset (0), then the IA addresses are perfect address filtered according to the MAC Address register 12 Late Collision Control (LCOLL). When set, enables retransmission of the collided frame even after the collision period (late collision). When reset, the MAC disables frame transmission on a late collision. In any case, the Late Collision status is appropriately updated in the Transmit Packet status. 11 Disable Broadcast Frames (BCAST). When set, disables the reception of broadcast frames. When reset, forwards all broadcast frames to the application. Note: When wake-up frame detection is enabled via the WUEN bit of the WUCSRโWake-up Control and Status Register, a broadcast wake-up frame will wake-up the device despite the state of this bit. 10 Disable Retry (DISRTY). When set, the MAC attempts only one transmission. When a collision is seen on the bus, the MAC ignores the current frame and goes to the next frame and a retry error is reported in the Transmit status. When reset, the MAC attempts 16 transmissions before signaling a retry error. 9 Reserved 8 Automatic Pad Stripping (PADSTR). When set, the MAC strips the pad field on all incoming frames, if the length field is less than 46 bytes. The FCS field is also stripped, since it is computed at the transmitting station based on the data and pad field characters, and is invalid for a received frame that has had the pad characters stripped. Receive frames with a 46-byte or greater length field are passed to the Application unmodified (FCS is not stripped). When reset, the MAC passes all incoming frames to the host unmodified. 7-6 BackOff Limit (BOLMT). The BOLMT bits allow the user to set its back-off limit in a relaxed or aggressive mode. According to IEEE 802.3, the MAC has to wait for a random number [r] of slot- times** after it detects a collision, where: (eq.1)0 < r < 2K The exponent K is dependent on how many times the current frame to be transmitted has been retried, as follows: (eq.2)K = min (n, 10) where n is the current number of retries. If a frame has been retried three times, then K = 3 and r= 8 slot-times maximum. If it has been retried 12 times, then K = 10, and r = 1024 slot-times maximum. An LFSR (linear feedback shift register) 20-bit counter emulates a 20bit random number generator, from which r is obtained. Once a collision is detected, the number of the current retry of the current frame is used to obtain K (eq.2). This value of K translates into the number of bits to use from the LFSR counter. If the value of K is 3, the MAC takes the value in the first three bits of the LFSR counter and uses it to count down to zero on every slot-time. This effectively causes the MAC to wait eight slot-times. To give the user more flexibility, the BOLMT value forces the number of bits to be used from the LFSR counter to a predetermined value as in the table below. Thus, if the value of K = 10, the MAC will look at the BOLMT if it is 00, then use the lower ten bits of the LFSR counter for the wait countdown. If the BOLMT is 10, then it will only use the value in the first four bits for the wait countdown, etc. **Slot-time = 512 bit times. (See IEEE 802.3 Spec., Secs. 4.2.3.25 and 4.4.2.1) Bits Description BOLMT Value # Bits Used from LFSR Counter 2โb00 10 2โb01 8 2โb10 4 2โb11 1 |
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