Chapter 11. Ethernet Module
11-19
Programming Model
11.5.8 MII Speed Control Register (MII_SPEED)
The MII_SPEED register, Figure 11-11, provides control of the MII clock (E_MDC pin)
frequency, allows dropping the preamble on the MII management frame and provides
observability (intended for manufacturing test) of an internal counter used in generating the
E_MDC clock signal.
Table 11-14. describes the MII_SPEED fields.
The MII_SPEED field must be programmed with a value to provide an E_MDC frequency
of less than or equal to 2.5 MHz to be compliant with the IEEE MII specification. The
MII_SPEED must be set to a non-zero value in order to source a read or write management
frame. After the management frame is complete, the MII_SPEED register may optionally
be set to zero to turn off the E_MDC. The E_MDC generated will have a 50% duty cycle
except when MII_SPEED is changed during operation. Change will take effect following
either a rising or falling edge of E_MDC.
If the system clock is 50 MHz, programming this register to 0x0000_000A results in an
E_MDC frequency of 25 MHz * 1/10 = 2.5 MHz. Table 11-15 shows optimum values for
31
16
Field
—
Reset
0000_0000_0000_0000
R/W
Read/Write
15
8
7
6
1
0
Field
—
DIS_PREAMBLE
MII_SPEED
—
Reset
0000_0000
0
000_000
0
R/W
Read/Write
Addr
MBAR + 0x884
Figure 11-11. MII Speed Control Register (MII_SPEED)
Table 11-14. MII_SPEED Field Descriptions
Bits
Name
Description
31–8
—
Reserved, should be cleared.
7
DIS_PREAMBLE
Disable preamble. Asserting this bit causes the preamble of 32 consecutive 1’s not to
be prepended to the MII management frame. The MII standard allows the preamble
to be dropped if the attached PHY device(s) do not require it.
6–1
MII_SPEED
MII frequency divider. MII_SPEED controls the frequency of the MII management
interface clock (E_MDC) relative to system clock. A value of 0 in this field turns off the
E_MDC and leaves it in low-voltage state. Any non-zero value results in an E_MDC
frequency given by the following formula:
MDC_FREQUENCY = system frequency / (4 * MII_SPEED)
0
—
Reserved, should be cleared.
Содержание DigitalDNA ColdFire MCF5272
Страница 1: ...MCF5272UM D Rev 0 02 2001 MCF5272 ColdFire Integrated Microprocessor User s Manual ...
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Страница 58: ...1 10 MCF5272 User s Manual MCF5272 Specific Features ...
Страница 90: ...2 42 MCF5272 User s Manual Exception Processing Overview ...
Страница 96: ...3 6 MCF5272 User s Manual MAC Instruction Execution Timings ...
Страница 158: ...5 46 MCF5272 User s Manual Motorola Recommended BDM Pinout ...
Страница 184: ...7 12 MCF5272 User s Manual Interrupt Controller Registers ...
Страница 338: ...13 44 MCF5272 User s Manual Application Examples ...
Страница 414: ...18 6 MCF5272 User s Manual PWM Programming Model ...
Страница 452: ...19 38 MCF5272 User s Manual Power Supply Pins ...
Страница 482: ...20 30 MCF5272 User s Manual Reset Operation ...
Страница 492: ...21 10 MCF5272 User s Manual Non IEEE 1149 1 Operation ...
Страница 548: ...INDEX Index 12 MCF5272 User s Manual ...