AM3359 datasheet(93/162 Pages) TI1

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AM3359, AM3358, AM3357

AM3356, AM3354, AM3352

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SPRS717

– OCTOBER 2011

4.2

Clock Specifications

4.2.1

Input Clock Specifications

The AM335x device has two clock inputs. Each clock input passes through an internal oscillator which can

be connected to an external crystal circuit (oscillator mode) or external LVCMOS square-wave digital clock

source (bypass mode). The oscillators automatically operate in bypass mode when their input is

connected to an external LVCMOS square-wave digital clock source. The oscillator associated with a

specific clock input must be enabled when the clock input is being used in either oscillator mode or bypass

mode.

The OSC1 oscillator provides a 32.768-kHz reference clock to the real-time clock (RTC) and is connected

to the RTC_XTALIN and RTC_XTALOUT terminals. This clock source is referred to as the 32K oscillator

(CLK_32K_RTC) in the AM335x ARM Cortex-A8 Technical Reference Manual (literature number

SPRUH73). OSC1 is disabled by default after power is applied. This clock input is optional and may not

be required if the RTC is configured to receive a clock from the internal 32k RC oscillator (CLK_RC32K) or

peripheral PLL (CLK_32KHZ) which receives a reference clock from the OSC0 input.

The OSC0 oscillator provides a 19.2-MHz, 24-MHz, 25-MHz, or 26-MHz reference clock which is used to

clock all non-RTC functions and is connected to the XTALIN and XTALOUT terminals. This clock source is

referred to as the master oscillator (CLK_M_OSC) in the AM335x ARM Cortex-A8 Technical Reference

Manual (literature number SPRUH73). OSC0 is enabled by default after power is applied.

For more information related to recommended circuit topologies and crystal oscillator circuit requirements

for these clock inputs, see Section 4.2.2.

4.2.2

Input Clock Requirements

4.2.2.1

OSC0 Internal Oscillator Clock Source

Figure 4-6 shows the recommended crystal circuit. It is recommended that pre-production printed circuit

board (PCB) designs include the two optional resistors Rbias and Rd in case they are required for proper

oscillator operation when combined with production crystal circuit components. In most cases, Rbias will

not be required and Rd will be a zero ohm resistor. These resistors may be removed from production PCB

designs after evaluating oscillator performance with production crystal circuit components installed on

pre-production PCBs.

The XTALIN terminal has a 15 - 40 k

Ω internal pull-down resistor which is enabled when OSC0 is

disabled. This internal resistor prevents the XTALIN terminal from floating to an invalid logic level which

may increase leakage current through the oscillator input buffer.

Power and Clocking

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