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I have made my own prototype PCB using the CYW20735 and using the eval schematic as a reference. The problem is as follows on a reset the 735 only occasionally 1 in 10 accesses the serial flash where the firmware resides. On the majority of occasions the flash isn't read and I'm not sure what the M4 is doing!? Powers and grounds all look good and are decoupled. The 24MHz XTAL looks good and have tried adding a 32k XTAL. I have tried various RC combinations on the reset and this makes no improvement. I have used the reset IC from the eval board and this actually made things worse! I read the documentation several times and nothing seems to be critical about the reset pin. Finally in desperation I have added an MCU to the reset line so I could control the exact timings and have found that the following works:
After POR the reset pulse needs to be 50ms. After this with the board already powered the reset period needs to be very short 1ms other wise the flash isn't read. Obviously I don't want to add an additional MCU to my design. Please anyone help?!
The POR has to asserted for longer than 50ms after VDDIO has been attained so as to prevent a brownout. For this reason, the reference design of this chip includes an explicit voltage detector to ensure this. This is in particular critical for battery powered products. The voltage detector we recommend is Torex XC6119N16A7R-G. We also recommend TXC 7V24070013 as the 24MHz crystal with 10pF as the loading capacitors.