Slow set up with nRF8001 on custom board

Question

We have recently moved to a new PCB which puts the nRF8001 on the layout instead of plugging into a nRF2741 development module. The nRF8001 connects to a TI Cortex-M4 Tiva MCU (previously a TI Cortex-M3). 
 Two issues have now arose. In the setup stage the RDYN line is held high for 1s after the nRF8001 sends a Command Response Event to the Setup system command (0x06). Setup completes successfully, but takes a lot longer than it did with the nRF2741 module. 
 The second issue is that the 16mhz clock often momentarily gets pulled low every few ms. Similar phenomenon was seen on the development kit, but the effect is now far more pronounced. 
 The code used on both boards is functionally the same and is adapted from the HID mouse example. I've attached an screenshot of the ACI lines for the transmission of one setup packet and response (i.e. what happens every 1s).

run_ar · Accepted Answer

Hi, 
 There is a design limitation in the firmware of the nRF8001. The limitation will cause the nRF8001 to delay servicing the REQN line when it goes low. This will cause the RDYN line to be delayed by up to 1 second. It is also possible for the delay to be lowered by the connection or advertising interval when the radio is running. Example: If the service of the REQN line is delayed when in a connection with a connection interval of 25 ms. The RDYN can be delayed by 25ms. 
 Issue: 
 
 After clocking out, the data REQN is set high to indicate end of data. 
 The RDYN line will follow and will be high 
 Setting the REQN line low 110-125 us after the REQN line goes high will cause the nRF8001 to delay servicing the REQN line. 
 
 Workaround:
The workaround is to set the REQN line low outside this 110-125 us window. 
 As Tie Fighter suggests, you can change the driver to avoid this problem as an interrupt based system can be more deterministic on the width of the REQN line. 
 Best regards,
Runar