Product data
MFRC631 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2014. All rights reserved.
Product data sheet
COMPANY PUBLIC
Rev. 3.3 — 4 February 2014
227433 46 of 120
NXP Semiconductors
MFRC631
High performance ISO/IEC 14443 A/B reader solution
8.8 Clock generation
8.8.1 Crystal oscillator
The clock applied to the MFRC631 acts as time basis for generation of the carrier sent out
at TX and for the quadrature mixer I and Q clock generation as well as for the coder and
decoder of the synchronous system. Therefore stability of the clock frequency is an
important factor for proper performance. To obtain highest performance, clock jitter has to
be as small as possible. This is best achieved by using the internal oscillator buffer with
the recommended circuitry.
8.8.2 IntegerN PLL clock line
The MFRC631 is able to provide a clock with configurable frequency at CLKOUT from
1 MHz to 24 MHz (PLL_Ctrl and PLL_DIV). There it can serve as a clock source to a
microcontroller which avoids the need of a second crystal oscillator in the reader system.
Clock source for the IntegerN-PLL is the 27.12 MHz crystal oscillator.
Two dividers are determining the output frequency. First a feedback integer-N divider
configures the VCO frequency to be N fin/2 (control signal pll_set_divfb). As supported
Feedback Divider Ratios are 23, 27 and 28, VCO frequencies can be
23 fin / 2 (312 MHz), 27 fin / 2 (366 MHz) and 28 fin / 2 (380 MHz).
The VCO frequency is divided by a factor which is defined by the output divider
(pll_set_divout). Table 35 “
Divider values for selected frequencies using the integerN PLL”
shows the accuracy achieved for various frequencies (integer multiples of 1 MHz and
some typical RS232 frequencies) and the divider ratios to be used. The register bit
ClkOutEn enables the clock at CLKOUT pin.
The following formula can be used to calculate the output frequency:
Fig 29. Quartz connection
Table 34. Crystal requirements recommendations
Symbol Parameter Conditions Min Typ max Unit
f
xtal
crystal frequency - 27.12 - MHz
f
xtal
/f
xtal
relative crystal
frequency variation
250 - +250 ppm
ESR equivalent series
resistance
-50100
C
L
load capacitance - 10 - pF
P
xtal
crystal power
dissipation
-50100W
001aam308
27.12 MHz
XTAL1 XTAL2
READER IC