ZDS2024 plus oscilloscope MDIO decoding/triggering practice

MDIO is a transmission protocol of an Ethernet controller, which is widely used for data communication between Ethernet controllers and PHY. Next, let us take a closer look at the MDIO protocol.

MDIO (Management Data Input/Output) is an Ethernet industry standard defined by IEEE802.3. It carries the heavy responsibility of communication between the MAC layer and the physical layer. The MDIO interface includes two signal lines: MDC and MDIO. The MAC layer chip (or other control chip) can access the registers of the physical layer chip through them, and control and manage the physical chip through these registers.

MDC: The clock of the management interface is a non-periodic signal with a minimum period of 400ns, a minimum positive level time and a minimum negative level time of 160ns, and no maximum positive and negative level time restrictions.

MDIO: It is a bidirectional data line used to transmit MAC layer control information and physical layer status information. It is synchronized with the MDC clock and is valid on the rising edge of the MDC.

Now let's take a look at the performance of MDIO on the ZDS2024 plus oscilloscope:

Open the Decode menu, we can see that MDIO has been added.

Select the MDIO protocol, and you can see that the MDIO parameter settings are very simple. You only need to set the MDC, MDIO channel, and data transmission mode.

Connect the probe to the MDIO bus to capture data. Because the baud rate of MDIO transmission is high, we need to set the probe attenuation ratio to x10 to reduce waveform distortion. We can see that MDIO decoding has started and the results are accurate.

Here, we can see from the event table that the decoded PHYAD of this frame is 0x12, so can we capture this frame from the data torrent? In fact, we can. The MDIO protocol not only implements the decoding function, but also has a powerful trigger function. We can capture this frame separately through the MDIO protocol trigger.

We first set the trigger parameters, as shown in the figure below, we set the OP type to: read, and the chip address to: 0x12.

The access signal triggers an immediate response, and the trigger point stabilizes after the PHYAD segment with a value of 0x12. The specific frame is captured successfully!

In addition to PHYAD trigger, MDIO trigger can also be triggered according to ST, OP, REGAD and DATA segments, which is very practical.

So far, the demonstration of the MDIO protocol has come to an end. Do you need to use the MDIO bus in your development? Are you eager to try to spy on various problems that may occur on the bus? ZDS2022 plus and ZDS2024 plus currently support the MDIO protocol. Upgrade your oscilloscope and add MDIO to it.

Appendix: MDIO frame structure

A standard MDIO frame includes PRE, ST, OP, PHYAD, PEGAD, TA, and DATA segments, as shown in the following figure.

PRE: Frame prefix field, which is 32 consecutive "1" bits. This frame prefix field is not necessary. The MDIO operation of some physical layer chips does not have this field.

OP: frame operation code, bit "10" indicates that this frame is a read operation frame, and bit "01" indicates that this frame is a write operation frame.

PHYAD: The address of the physical layer chip, 5 bits. Each chip compares its own address with these 5 bits. If they match, it responds to the subsequent operations. If they do not match, it ignores the subsequent operations.

REGAD: Used to select the address of a register among the 32 registers of the physical layer chip.

TA: state transition domain, if it is a read operation, the MDIO is in high impedance state in the first bit, and the physical layer chip sets MDIO to "0" in the second bit. If it is a write operation, the MDIO is still controlled by the MAC layer chip, which continuously outputs two bits of "10".

DATA: The data field of the frame register, 16 bits. If it is a read operation, it is the data sent from the physical layer to the MAC layer. If it is a write operation, it is the data sent from the MAC layer to the physical layer.

IDLE: The idle state after the frame ends. At this time, MDIO is passively driven and in a high-impedance state, but a pull-up resistor is generally used to keep it at a high level, that is, the MDIO pin requires a pull-up resistor.