Fast Ethernet Controller (FEC)
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
45-25
The MII_SPEED field must be programmed with a value to provide an MDC frequency of less than or
equal to 2.5 MHz to comply with the IEEE MII specification. MII_SPEED must be non-zero to source a
read or write management frame. After the management frame is complete, MII_SPEED may optionally
cleared to turn off the MDC. The MDC generated has a 50% duty cycle except when MII_SPEED is
changed during operation (changes take effect following either a rising or falling edge of MDC).
If the system clock is 25 MHz, programming this register to 0x0000_000A generates an MDC frequency
of 25 MHz * 1/10 = 2.5 MHz.
Table 45-22
shows optimum values for MII_SPEED as a function of system clock frequency.
45.3.2.15 FIFO Receive Bound Register (R_BOUND)
The R_BOUND register,
Figure 45-19
, is a read-only register the user can read to determine the upper
address bound of the FIFO RAM. Drivers can use this value, along with the R_FSTART and X_FSTART
to appropriately divide the available FIFO RAM between the transmit and receive data paths.
25–30
MII_SPEED
MII_SPEED controls the frequency of the MII management interface clock (MDC) relative
to system clock. Clearing MII_SPEED, turns off the MDC and leaves it in low-voltage
state. Any non-zero value generates an MDC frequency of 1/(MII_SPEED*2) of the
system clock frequency.
31
—
Reserved, should be cleared by the host processor.
Table 45-22. Programming Examples for MII_SPEED Register
System Clock Frequency
MII_SPEED[MII_SPEED]
MDC Frequency
25 MHz
0x05
2.5 MHz
33 MHz
0x07
2.36 MHz
40 MHz
0x08
2.5 MHz
50 MHz
0x0A
2.5 MHz
Table 45-21. MII_SPEED Field Descriptions (continued)
Bits
Name
Description
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