Hi team,
I had consulted the TMDXRM46HDK schematics. In the page 19, I have been seen that in the pins RX and TX of DP83640 there are resistences on serie but I can't find where they are specified.
I had consulted on MCU datasheet and the transceptor datasheet. Could someone tell me what they are due to?
Hi Aaron,
It is to match the rise/fall and setup/hold times by matching the characteristic impedance. I'm not sure exactly how we arrived to 22 Ohms specifically.
Regards,
Alvaro
Hi Aaron,
The RX Signals are outputs from the PHY and generally speaking: an output pin (driver) of an IC has a lower impedance than the trace/transmission line + the high impedance of an input pin (receiver). Without a series termination resistor reflections will come back from the Receiver and cause ringing across the line, leading to misinterpreted bits. It is important to also place them as close as possible to the driver, this is why you see them on the RX lines and not on the TX lines. TX/RX lines are named relevant to the MAC. So RX are what the MAC receives, hence output from PHY, and vice versa. You'll probably notice that TX_CLK has a series termination resistor too, that is because in MII mode, both TX & RX clock are supplied by the PHY. In RMII, the PHY only supplies RX, TX is supplied by the MAC.
The exact value was probably taken from a simulation tool, like HyperLynx, before building the board.
Regards,
Alvaro
