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Design Considerations / Test Results
EMI Considerations
Electro-magnetic interference is a concern in all electronic designs. The effects of EMI are enhanced as designs become more
digital and the digital circuits' speed increases. The N56XX is no exception.
The N56XX does not preclude end product integrations from obtaining regulatory and safety standards. The OEM integrator will
need to verify compliance as implemented in their host system.
The N56XX product is comprised of two major components, each with its own base frequencies.
The Decoder Board
The decoder board is based on a 24KHz crystal in the decoder board section that is used to generate a 400MHz clock for
the core and a 133MHz clock for the memory interface. There are a number of other frequencies that may be generated at
any given time depending on what interface the decoder board is set up to use or various intermittent signatures that occur
in a typical image capture and decode process. There are three switching power supplies on the decoder boards that oper-
ate between 1MHz and 1.6MHz.
The Image Engine
The imager runs based on a 48MHz pixel clock frequency. To reduce EMI, the pixel clock spread is determined by the
spread setting of the spread spectrum device. The spread spectrum provides the 48MHz output with a down spread of -2%
(default).
Design Considerations
There are several considerations that must be made when designing a system to utilize the N56XX. When integrating the
N56XX to other components in the system, ensure that a clean power supply is being used and that there is good signal ground
integrity (the quieter the better). The other major consideration in any system is interconnects. The N56XX uses flex strips/flex
circuits for its interconnect to the host system and between the decoder board and image engine. Proper flex strip design is crit-
ical to achieving adequate EMI results. The length, impedance, shape, and routing path of the flex can play big roles in the EMI
signature of a product. A short list of considerations when designing with flexes follows:
Impedance - flexes have specifications for impedance and resistance per unit length. Try to make sure your impedance is
matched to the typical 50 ohms of a CMOS circuit and keep the resistance as low as possible.
Grounding - Keep the ground traces on the flex strip as low resistance as possible.
Length - shorter is better. Flexes tend to act like antennas; the longer they are, the more EMI transmission and reception can
occur.
Routing - keep the flex from passing over other high frequency components or input/output paths. This helps to reduce coupling
in or out of the flex. Also, as a rule, avoid loops in the flex. Loops can add to the antenna effect.
Test Results
The N56XX is designed to meet EN55022 B emission levels. The N56XX has been tested for compliance using representative
models.
Model 1
(page
A-2) is based on a cabled platform (RS232):
• The N56XX is mounted on the Honeywell demo board.
• The demo board is connected to the host via an 8 foot long, coiled, TTL level 232 cable (42206422-01E).
• The N56XX is operating in TTL serial-232 mode.
Model 2
(page
A-6) is an alternate cabled platform (USB Full Speed):
• The N56XX is mounted on the Honeywell demo board
• The demo board is connected to the host via a Honeywell 8 foot long, straight, USB cable (42206161-01E). A clamp-on ferrite
(Fair-Rite p/n 0444164281) was added to USB cable on this configuration.
• The N56XX is operating in USB mode (USBSPD0).
Model 3
(page
A-10) is an alternate cabled platform (USB High Speed):
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