Entering ISP mode using ISP RXD PIN

[AlfTeleco]

Hello everyone and thanks in advance again for this great community of wisdom.

I am interested in ISP programming in the LPC81xM.

Several questions: 

1. Are 6 pins the minimum quantity to be able to flash the MCU? As far as I can see the needed pins are: 

- VDD

- VSS

- ISP pin mode

- RESET

- TXD

- RXD

2. What would happen if we try to enter in ISP mode without controlling the RESET pin, so to say, just keeping low the ISP pin mode when powering the LPC? Would it enter in ISP mode?

3. This is an interesting question. Would we be able to program the LPC through ISP with the configuration below?

- Al ISP programming through a connector of 4 pins: 
              - VDD

              - VSS

              - RXD internally wired to ISP pin mode: the intent is to hold RXD low when powering the LPC.

              - TXD

*The question is what happens with ISP pin after the LPC enters in ISP mode. If it becomes neutral it might be used as the the RXD as well. 

Thanks in advance. 

Regards

[Alice_Yang]

Hello AlfTeleco,

Enter ISP mode only need the ISP entry pin is pulled LOW on reset.

After enter ISP mode, the pin functions are fixed, 

BR

Alice

[AlfTeleco]

Hello and thanks for the answer.

We have tested flashing the LPC with 5 pins:

1. VDD
2. VSS
3. TXD
4. RXD
5. ISP Pin

Reset pin is not mandatory as you can rely on power up reset. Keeping ISP Pin low when power up is enough to make LPC enter in ISP mode

A solution wiring RXD and ISP Pin mode has been tested but does not seem to be reliable.

Thanks.

[Alice_Yang]

Hello,

About design of hardware, you can refer to schematic of lpcxpresso812-MAX:

https://www.nxp.com/design/software/development-software/mcuxpresso-software-and-tools-/lpcxpresso81...  

BR

Alice