Automatic driver slew rate control for LAN adapters
Original Publication Date: 2000-Aug-01
Included in the Prior Art Database: 2003-Jun-19
In some Local Area Network (LAN) the cabling is made up of Unshielded Twisted Pairs (UTP) for cost reasons. The cabling of this LANcan reach up to kilometers . Of course, this configuration from Electro Magnetic Interference (EMI) standpoint is bad. The main problem is coming from the sharpness of the rising time of signals conveyed into this large antenna. Based on the capability of a LAN to have a selectable frequency, the goal of the invention is to tune this rising edge (called slew rate) in function of the speed of the data sent on the LAN. While an adapter is able to be selected from a variable speed from 1Mega hertz to 100 Mega hertz, it would be not appropriated to have always the same rising edge for all these speeds. For example at 100 Mega hertz , due to the sampling problem , the rising time should be comprised between 0 and 2 nano seconds, but at 1 Mega hertz, it is 0 and 200 nano seconds. As we can see , the LAN configured at 1 Megahertz, with a slew rate of 200 nano seconds will be very less noisy than at 2 nano seconds. As described on figure 1, the microcontroller has the possibility to select the speed of the data xmit framer thru the clock divider. The slew rate of the line driver is adjustable by setting a resistor on it’s SL input. The appropriate resistor will be automatically chosen by the logic block in function of the clock set by the microcontroller.