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(This message was transferred over from our old forum)
Posted August 17, 2015
By Todd DeBoer
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On 9/27/11 Gizmo asked, “I would like to use the USB-ICP-LPC9XX for development with 89LPC922 and 89LPC933. I understand the required 5 pin connections. But, do I need to use jumpers on my PCB to disconnect other circuitry from these lines while programming? I would really prefer not to have jumpers on the PCB for critical power and signal lines.”
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(Follow up question)Answered:
For reliability, we recommend to disconnect user circuitry from the MCU during programming to ensure proper programming.
If it is not feasible, then the following must be considered;
For the programming clock and data, it depends on the circuitry attached to these signals – if the signals are all outputs from the MCU, lightly loaded, and your circuit doesn’t mind these signals ‘wiggling’ during programming, then it should be ok.
The power supply and reset signal are a different issue – entry into ICP programming mode depends on the USB-ICP-LPC9XX being able to switch power off/on to the target MCU AND toggling the reset input (to the MCU) with specific timing. If you’ve got a circuit driving reset, then it will conflict with the USB-ICP-LPC9XX. If you’ve got a heavy load on the power supply (either capacitive or circuitry), then the USB-ICP-LPC9XX can’t switch it properly. Either one of these cases will cause the ICP entry to fail, and programming failure. Hence, we strongly recommend isolating these two connections during programming.
ICP programming is a synchronous interface, any circuit loading will affect this timing and generally cause programming failure.
The ICP Programming Specification is available for download in the support area of the USB-ICP-LPC9XX
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