MPC8540 datasheet(211/1302 Pages) FREESCALE

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Functional Description

MPC8540 PowerQUICC III Integrated Host Processor Reference Manual, Rev. 1

Freescale Semiconductor

4-25

If the MPC8540 is the initial master of the RapidIO clock, then it can derive the RapidIO transmit

clock from either its internal platform (CCB) clock or from a dedicated LVDS clock input,

RIO_TX_CLK_IN / RIO_TX_CLK_IN.

4.4.4.3

Ethernet Clocks

The Ethernet blocks operate asynchronously with respect to the rest of the device. These blocks

use receive and transmit clocks supplied by their respective PHY chips, plus a 125-MHz clock

input for gigabit protocols. Data transfers are synchronized to the CCB clock internally.

4.4.4.4

Real Time Clock

As shown in Figure 4-8, the real time clock (RTC) input can optionally be used to clock the e500

core timer facilities. RTC can also be used (optionally) by the MPC8540 programmable interrupt

controller (PIC) global timer facilities. The RTC is separate from the e500 core clock and is

intended to support relatively low frequency timing applications. The RTC frequency range is

specified in the MPC8540 Integrated Processor Hardware Specifications, but the maximum value

should not exceed one-eighth of the core frequency.

Before being distributed to the core time base, RTC is sampled and synchronized with the CCB

clock.

The clock source for the core time base is specified by two fields in HID0: time base enable

(TBEN), and select time base clock (SEL_TBCLK). If the time base is enabled, (HID0[TBEN] is

set), the clock source is determined as follows:

•

HID0[SEL_TBCLK] = 0, the time base is updated every 8 CCB clocks

•

HID0[SEL_TBCLK] = 1, the time base is updated on the rising edge of RTC

The default source of the time base is the CCB clock divided by eight. For more details, see the

section “Hardware Implementation-Dependent Register (HID0),” in the “PowerPC™ e500 Core

Complex Reference Manual.”

Section 10.3.2.6, “Timer Control Register (TCR),” provides additional information on the use of

the RTC signal to clock the global timers in the PIC unit.

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