AM3359, AM3358, AM3357
AM3356, AM3354, AM3352
SPRS717F –OCTOBER 2011–REVISED APRIL 2013
4.2 Clock Specifications
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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 Microprocessors (MPUs) 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 Microprocessors
(MPUs) 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 4-8 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 is not
required and Rd is a 0-Ω 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 kΩ 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.
106
Power and Clocking
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