Cypress Perform

Answer: The PSoC® 5 device IMO clock accuracy is ±5%. The user cannot use IMO as a clock source when USB, CAN, and UART communication is required because these communication interfaces require a higher accuracy clock source (see the following examples):

• USB Clock for Full-Speed operation – 48 MHz (+0.25% tolerance).
• UART with 8x oversampling, voting enabled, the tolerance for tclock: ±3.9%
• UART with 16x oversampling, voting enabled, he tolerance for tclock: ±4.6%
• The accuracy of CAN CLK_BUS must be at least 1.58% for 125 Kbps and slower bit rates

  For bit rates faster than 125 Kbps, the accuracy of CAN CLK_BUS must be 0.5% or better.

Therefore, the project that uses IMO as clock source with USB/UART/CAN does not work in PSoC Creator 2.0 or later version. You must use an external crystal-base for clocking the device when using USB, CAN, and UART communication. For USB communication, you must use a 24-Mhz external crystal.

Refer to AN54439 - PSoC® 3 and PSoC 5 External Crystal Oscillators for more details on how to configure hardware and firmware for PSoC 3 or PSoC 5 using the integrated oscillator subsystems and external resonators.