UM10413
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© NXP B.V. 2011. All rights reserved.
User manual
Rev. 1 — 16 December 2011
37 of 268
NXP Semiconductors
UM10413
MPT612 User manual
10.4.1 XTAL1 input
The input voltage to the on-chip oscillators is limited to 1.8 V. If a clock drives the oscillator
in slave mode, it is recommended that the input is coupled through a capacitor with C
i
=
100 pF. To limit the input voltage to the specified range, choose an additional capacitor to
ground C
g
which attenuates the input voltage by a factor C
i
/(C
i
+ C
g
). In slave mode, a
minimum of 200 mV
rms
is needed.
10.4.2 XTAL and RTC Printed-Circuit Board (PCB) layout guidelines
Place the crystal on the PCB as close as possible to the chip oscillator input and output
pins. Take care that the load capacitors C
x1
and C
x2
, and C
x3
in case of third overtone
crystal usage, have a common ground plane. Connect the external components to the
ground plain. Make loops as small as possible to reduce the noise coupled in via the PCB
as small as possible. Parasitics must be as small as possible. Start with small values for
C
x1
and C
x2
and, if necessary, increase the value in proportion to the level of parasitics on
the PCB layout.
Fig 9.
f
osc
selection algorithm
aaa-000575
mode a and/or b
mode a
mode b
MIN f
osc
= 10 MHz
MAX f
osc
= 25 MHz
MIN f
osc
= 1 MHz
MAX f
osc
= 50 MHz
external crystal
oscillator used?
MIN f
osc
= 1 MHz
MAX f
osc
= 30 MHz
false
ISP used for initial
code download?
false
on-chip PLL used
in application?
false
f
osc
selection
true
true
true
Fig 10. Slave mode operation of the on-chip oscillator
MPT612
XTAL1
Ci
100 pF
Cg
aaa-001862