RM46L852: Questions about RMII interface for RM46L852.

Tool/software:

Hi,

I have some questions regarding RMII interface implementations for the RM46L852 using the DP83822 10/100BASE-TX ETHERNET PHY transceiver. Texas Instrument provides a lot of documentation to help developers: crystal or CMOS oscillator selection, RMII interface implementation in MASTER mode, design and layout guide, TMDXRM46HDK EVB schematics, etc. Therefore, my questions are only due to possible contradictions I see between the documents and a lack of understanding on some points.

My developed schematic is the following:

Questions:

1º The TMDXRM46HDK EVB schematic assigns one Ferrite-Bead for the analog power supply (ANA33VDD) of the DP83640 and another one for the ASEM1-25.000MHZ-LC-T CMOS oscillator. I would like to know if a single Ferrite-Bead used for both devices would be enough or if this implementation would cause any problems.

2º The TMDXRM46HDK EVB schematic assigns a 360ohm resistor to the anode of LED_ACT and a 2.2kohm resistor to the cathode of LED_ACT. It assigns a 360ohm resistor to the anode of LED_ACT and a 2.2kohm resistor to the cathode of LED_LINK. It assigns a 2.2kohm and 2.2kohm pull-up to the anode of LED_ACT. In all cases, this configuration causes the LED pins to be configured as active low.

According to the document called AN-1469 PHYTER Design & Layout Guide, the following is recommended:

According to the document called DP83640 Precision PHYTERTM - IEEE 1588 Precision Time Protocol Transceiver, the following is specified:

Therefore, if we want the option which is marked in red and the default value is 111 for the LEDs pins then it is unnecessary to assign a 2.2 kohm pull-up resistor on the LED_Speed. Is it correct? On the other hand, Why does the schematic of the TMDXRM46HDK EVB  assigns two resistors to the LEDs on the Led_ACT and Led_LINK pins? I am unsure about this since the LED pins to be configured as active low and uA current into the LEDs I would get the same effect by using a 2.2 kohm resistor on the LED anode.

3º According to the document called AN-1794 Using RMII Master Mode and the document called DP83640 Precision PHYTERTM - IEEE 1588 Precision Time Protocol Transceiver, the following is specified:

I can't understand why a 2.2 kohm pull-down resistor is assigned to the TXD_2 pin. This pin has no strap functionality and is not used in the RMII interface, I think it's a mistake. On the other hand, I use a CMOS oscillator to generate the 25 MHz clock signal needed for the MASTER mode of the RMII interface. Therefore, I only use the X1 pin and leave the X2 pin unconnected. This causes the CLK_OUT, RX_CLK and TX_CLK signals can transmit 50 MHz. I use the CLK_OUT signal to transmit the 50 MHz clock signal returned by the transceiver to the RMII_REFCLK signal of the MCU. If I want this situation to occur by default without having to program the transceiver registers, would it be necessary to assign a pull-up resistor to the GPIO1 pin? Or is the strap functionality of this pin for something else and I am misunderstanding it?

Best regards,

Francisco.