Просто внимательно почитаем даташит на LPC 1768
8.30.3 Code security (Code Read Protection - CRP)
This feature of the LPC17xx allows user to enable different levels of security in the system so that access to the on-chip flash and use of the JTAG and ISP can be restricted. When needed, CRP is invoked by programming a specific pattern into a dedicated flash location. IAP commands are not affected by the CRP.
There are three levels of the Code Read Protection.
CRP1 disables access to chip via the JTAG and allows partial flash update (excluding flash sector 0) using a limited set of the ISP commands. This mode is useful when CRP is required and flash field updates are needed but all sectors can not be erased.
CRP2 disables access to chip via the JTAG and only allows full flash erase and update using a reduced set of the ISP commands.
Running an application with level CRP3 selected fully disables any access to chip via the JTAG pins and the ISP. This mode effectively disables ISP override using P2[10] pin, too. It is up to the user’s application to provide (if needed) flash update mechanism using IAP calls or call reinvoke ISP command to enable flash update via UART0.
CAUTION : If level three Code Read Protection (CRP3) is selected, no future factory testing can be performed on the device.