What's the min delay from BOOT+RESET before sending '?' to start the bootloader?

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What's the min delay from BOOT+RESET before sending '?' to start the bootloader?

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funbotix
Contributor I

I'm writing an test app for use in our PCB back shop for an LPC4074 based board.  Part of this app must load test code, then test the board, then load released production code.  I've done several bootloaders over the years, but there seems to be an oddity with this one.  Specifically, if I assert the BOOTLOAD pin (drive it low), then temporarily assert RESET, then send a '?' character at 9600,n,8,1 I get no Synchronized response.  However, if I wait at least 200 msecs after negating RESET before sending the '?' I do get the Synchronized response.  If I wait 150 msecs or less before sending the '?' I do not get the Synchronized response.  My question is, is this delay defined anywhere?  If not, how long should one wait after negating RESET before sending the '?' to start the bootloader?  Should one just keep sending the '?' and then wait a while, and repeat this some number of times before giving up (if no response is generated)?

Thanks,

Dave

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jeremyzhou
NXP TechSupport
NXP TechSupport

Hi Dave S,

Thank you for your interest in NXP Semiconductor products and for the opportunity to serve you.
Whether you mean that MCU wouldn't response 'Synchronized ' to '?' in 150 mS after forcing the LPC4074 to enter UART ISP mode, however, it can recover response ability when exceeds 200 mS.
If yes, I'd like to say this non-response time is not the delay time, during this time, ROM bootload initializes the clock, peripheral, etc.
So please wait for a while before sending '?', just keep in mind.
Hope this is clear.
Have a great day,
TIC

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