Reduced Media-Independent Interface: Rmii; Figure 318. Mii Clock Sources; Figure 319. Reduced Media-Independent Interface Signals - ST STM32F205 series Reference Manual

Ethernet (ETH): media access control (MAC) with DMA controller
28.4.3

Reduced media-independent interface: RMII

The reduced media-independent interface (RMII) specification reduces the pin count
between the microcontroller Ethernet peripheral and the external Ethernet in 10/100 Mbit/s.
According to the IEEE 802.3u standard, an MII contains 16 pins for data and control. The
RMII specification is dedicated to reduce the pin count to 7 pins (a 62.5% decrease in pin
count).
The RMII is instantiated between the MAC and the PHY. This helps translation of the MAC's
MII into the RMII. The RMII block has the following characteristics:
•
It supports 10-Mbit/s and 100-Mbit/s operating rates
• The clock reference must be doubled to 50 MHz
•
The same clock reference must be sourced externally to both MAC and external
Ethernet PHY
•
It provides independent 2-bit wide (dibit) transmit and receive data paths
848/1378

Figure 318. MII clock sources

STM32
MCU
25 MHz
HSE
MCO

Figure 319. Reduced media-independent interface signals

STM32
MCU
RM0033 Rev 8
External
TX _CLK
PHY
RX _CLK
25 MHz
TXD[1:0]
TX_EN
RXD[1:0]
External
PHY
CRS_ D V
MDC
MDIO
REF _ CLK
Clock source
RM0033
For 10/100 Mbit/s
ai15623b
ai15624b