MC9S12XEP100 Reset and Configuration mode pin sharing

[danielflor]

Hello, 

I'm migrating from the MC9S12DP512 microcontroller to the MC9S12XEP100 microcontroller. And I have the following questions:

1)  For the MC9S12DP512, the MC34064D05 CI was used as a RESET circuit, which is a circuit for low voltage detection. However, in the MC9S12XEP100 microcontroller, there is an LVR (Low-voltage reset) module, so, I wonder if the LVR module would eliminate the need to use an external low voltage detector circuit with the MC34064D05 IC.

2)  RE and MODA are shared, pin 50 of the microcontroller MC9S12XEP100 . MODA is a microcontroller configuration pin and is used only during the reset phase, RE is required to access external memory. I will need both features and I believe that the pull-down resistor at the MODA pin will not cause any problems during use of RE, but I'm not sure. I mean, can I change the pin 50 to be used as RE during normal operation after the reset ?

With my best regards,

Daniel.

[lama]

Hi,

1) Low voltage module runs only in full performance mode when the mcu is supplied by 5V. The question is how your external reset circuit is used. There are two possibilities because the mcu is able to work also at lower voltages – of course voltage levels at pins are also lower.

1. Inform mcu about voltage drop (usually at higher voltage side by voltage divider..before 5V stabilizer) to be mcu able to close all necessary processes and reset itself (for example using COP = https://community.nxp.com/docs/DOC-103737 ) or wait for LVR.
2. standard LVR reset, processes are interrupted and reset is launched.

Of course, item 1 can be solved by LVI interrupt (senses internal voltage). But it necessary to ensure time for ending processes … time period depends on the falling slope of voltage (LVIA to LVRXA). An additional capacitor is suitable to be used to extend fall time. (I am sorry, I write very briefly ad hope you understand what I mean)

So, the decision is up to you and your requirements what and how the LVI/LVR is used.

2) MODA, if the pin is connected to a device which input does not influence reset status. Of course there are also other solutions. If the device is required to work in expanded mode then it can be entered into this mode by reset status at the MODx pins or later by writing mode register. For debugging purpose I have usually used the second option which does not change anything if you start the mcu in expanded mode by reset. (only init data moment in external memory must by considered if we init any variable there. Expanded mode must be set sooner than init() function is called in start12.c file in this case.)

I suppose you do not want to want to do your design more complex so you will use the same approach and start the device in the normal or special (when you debug) single chip mode and then you will switch to the expanded mode.

Some notes to exbus I attached also here https://community.nxp.com/docs/DOC-93594

Best regards,

Ladislav