Clock Circuitry; Reset Circuitry - Atmel SAM4E-EK User Manual

4.3.3

Clock circuitry

The clock generator of a SAM4E16 microcontroller is composed of:
•
A low-power 32.768Hz Slow Clock Oscillator with bypass mode
•
A 3 to 20MHz Crystal Oscillator, which can be bypassed (12MHz needed in case of USB)
•
A factory programmed fast internal RC Oscillator. Three output frequencies can be selected: 4 (default value),
8 or 12MHz
•
A 80 to 240MHz PLL (PLLB) providing a clock for the USB Full Speed Controller
•
An 80 to 240MHz programmable PLL (PLLA), capable to provide the clock MCK to the processor and to the
peripherals. The input frequency of PLLA is from 3 to 32MHz
The SAM4E-EK board is equipped with one 12MHz crystal, optional Piezoelectric Ceramic Resonator 12MHz (Murata
ref. CSTCE12M0G15L99-R0), one 32.768Hz crystal and an external clock input connector.
Figure 4-3. External clock source.
J2
SMB
1
2 3
4 5
4.3.4

Reset circuitry

On-board NRST button BP1 provides an external reset control of the SAM4E16.
The NRST pin is bidirectional. It is handled by the on-chip reset controller. It can be driven low to provide a reset signal
out to the external components. Conversely, it can be asserted low from the outside to reset the microcontroller Core
and the peripherals. The NRST pin integrates a permanent pull-up resistor of 100kΩ to VDDIO.
On the SAM4E-EK board, the NRST signal is connected to the LCD module and JTAG port.
Note: At power-on, the NRST signal is asserted with default duration of two clock cycles. That duration may not be
sufficient to correctly reset any other system or board devices connected to that signal. First, in your custom
application, you need to check for these device's datasheets about reset duration requirements. Then, you need to
set an appropriate configuration in the NRST Manager. This is done through the ERSTL field in the RSTC_MR
register. The NRST duration is thereby configurable between 60μs and 2s, whether it is subsequently activated by
a software reset or a user reset. Refer to the SAM4E16 datasheet for in-depth information.
C22 18pF R3 0R
Y 1 32.768 kHz
C27 18pF R4 0R
R5 DNP
R6
R7 DNP
49.9R 1%
3
Y 2
R12
PA7
PA8
C28 15pF R8
Y 3
12MHz
C31 15pF R11
DNP
PB9
0R
PB8
0R
SAM4E-EK User Guide
42067B−SAM4E−04/2013
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