Electronic – Should I define default register values in start up code

armmicrocontrollerprogramming

It has come to my attention that some ARM programmers set some CPU registers with values that are the same default values of the register on microcontroller reset.

For example: the default value of IOCON_PIO2_9, parameter MODE on LCP1114 is 0x02 (pull-up resistor enabled). In Olimex's example code this value is set to 0x02 in software initialization but this is already the default value.

This behavior occurs with many other registers.

Why would someone do that? In my opinion, this extra code just pollutes everything and make code readability worse. Is this a good programming practice?

Best Answer

To make code modular. If your A/D routine, for example, always sets up the A/D completely for whatever mode it uses it in, then that routine can be called at any time whether immediately after powerup or after the A/D was used in some other way.
Code may jump back to the startup location to do a software reset. In that case, the settings may not be at the powerup default values.
The powerup defaults may be different on different processors, so setting them in code makes the code more portable.
It documents what the settings are. If these registers are explicitly set on startup, then someone looking at the code doesn't have to go digging in the datasheet to see what they are set to.
It documents that you care. Let the hardware default the settings you aren't using, but explicitly set the ones you rely on. This may alert some future maintainer to the importance of these particular settings.

Related Solutions

Electronic – How to configure an ARM GPIO port (STR9) to an alternate function

The STR91x family of microcontrollers has very flexible peripherals. Unfortunately, this flexibility makes them somewhat complicated to set up. My suggestion is to start by downloading the STR91xFA Firmware Library from ST.com. Once you have the library linked into your project, you need to do something similar to the following:

void InitGPIO7( void )
{
   GPIO_InitTypeDef GPIO_InitStructure;

   /* Enable the GPIO7 clock */
   SCU_APBPeriphClockConfig(__GPIO7, ENABLE);

   /* Initialize the GPIO port */
   GPIO_DeInit(GPIO7);

   /* Configure pin GPIO7.3 */
   GPIO_InitStructure.GPIO_Direction = GPIO_PinOutput;
   GPIO_InitStructure.GPIO_Pin = GPIO_Pin_3;
   GPIO_InitStructure.GPIO_Type = GPIO_Type_PushPull;
   GPIO_InitStructure.GPIO_IPConnected = GPIO_IPConnected_Disable;
   GPIO_InitStructure.GPIO_Alternate = GPIO_OutputAlt1;
   GPIO_Init (GPIO7, &GPIO_InitStructure);
}

Note that this is untested but it should be pretty close.

Electronic – On Freescale iMX31, how can I translate SDRAM address to CPU address

To answer your base question, to get to the address of pin A10 of the memory you need to look at the memory map for the ARM device...

In this case it looking at the memory map:

0x8000 0000 -> 0x8FFF FFFF is mapped to CSD0 (SDRAM/DDR)

and

0x9000 0000 -> 0x9FFF FFFF is mapped to CSD1 (SDRAM/DDR)

You'd need to know what chip select was used in the PCB design to determine your answer as to which bank your DDR is attached to.

This implies that A10 is located as either 0x8000 0400 (as you mentioned) or 0x9000 0400.

As to why 0x8000 0F00 was used in place of 0x8000 0400...reading the datasheet for that memory implies, but does not state that the other address pins, A(n) are don't cares for this operation so the coder probably just tossed in an F there instead of figuring out that it was only a 4 that was needed.

I also don't find those 2 sections of the datasheet to contradict each other. The first is basically just saying that you need to reference the device's memory map to locate the real address where the memory is mapped so you can use that address as a base.

The second quotes tells you that bit 0 of the address corresponds to address pin 0 on the memory in this mode, which may not always be the case in normal operation. It may depend on the data/address width of the memory combined with alignment issues for the core.

Best Answer

Related Solutions

Electronic – How to configure an ARM GPIO port (STR9) to an alternate function

Electronic – On Freescale iMX31, how can I translate SDRAM address to CPU address

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