SPRADN4B October 2024 – September 2025 AM62P , AM62P-Q1
There is no firm rule or requirement for external pull unless the pull requirements are defined in an industry standard. Industry standard definition for pulls is the main reason we can make firm recommendations for external pulls on the eMMC and SD card signals. For the other peripherals, the recommendation is for customers to evaluate the function of the attached devices connected to every processor signal on the custom board and apply appropriate technical/engineering judgment to determine the need to have external pulls that prevent any input from floating when attached device input buffer is turned on. The recommendations provided in the design guide are generic and customer is expected to review the design requirements and the availability of pulls internal to the attached device before implementing. Be sure to not provide an external pull in contention with an internal pull. Example: An example is adding an external pull that is in contention with the internal pull (internal to the attached device), such that the contention creates a mid-supply potential on the signal (input).
The recommendation is to provide provision for parallel pulls to the processor IOs that has a trace connected and not being driven actively or for the IOs connected to the attached device inputs that can float (to prevent the attached device inputs from floating until the host software configures the IO). Parallel pull polarity and pull value depends on the specific peripheral connectivity recommendations, recommendations for improved processor performance and reliability, and relevant interface or interface standards requirements. The recommendations for pullups are provided.
Pull values used in processor-specific SK can be used as a starting point and custom board designer can select the appropriate pull values based on the recommendations for the processor and attached device, or specific board design requirements.10kΩ or 47kΩ (choice of pullup allowed to standardize the component selection and BOM) pull value is recommended for IOs or interfaces that do not have specific recommendations. The pull value can be chosen based on the board design to optimize the use of components or reduce current or improve noise performance.
When a trace is connected to the processor pins (IO pads) and the IOs are not being actively driven (floating), a parallel pull (47kΩ) is recommended. Processor IO buffers are (TX (Output) and RX (Input) are disabled during and after reset and internal pulls (pullup and pulldown)) turned off during reset and after reset. The IOs are in a high impedance state, effectively behaving as an antenna that can pick up noise. Without a parallel pull, the IOs are in high impedance state. High impedance makes noise to couple energy easily on the floating signal trace and develop a potential that can exceed the IO recommended operating conditions. The potential creates an electrical over-stress (EOS) on the IOs. Electrostatic discharge (ESD) protection circuits internal to the processor were designed to only protect the device from ESD during handling before being installed on a PCB.