topic Re: Clear a GPIO Pin in ColdFire/68K Microcontrollers and Processors

Clear a GPIO Pin

blakehodder — Mon, 11 Feb 2013 15:12:14 GMT

I am using CW 10.3 for the MCF52255. When I attempt

SETTJ_SETTJ5_BITMASK=1;

CLRTJ_CLRTJ5_BITMASK=0;

I get the error "not an lvalue".

How do set and clear this pin without using MQX.

Thanks,

Blake

Re: Clear a GPIO Pin

blakehodder — Mon, 11 Feb 2013 17:51:38 GMT

I need to add, I can set or clear the entire port if I use SETTJ=1 or CLRTJ=0 but I cannot set a specific bit within that port.

Any hints on how to accomplish this or as to why it does not work?

Thanks,

Blake

Re: Clear a GPIO Pin

TomE — Mon, 11 Feb 2013 22:44:27 GMT

This is a CodeWarrior question, so you could ask the question in that forum. You should also look for example code that uses those registers so you can copy working code.

Maybe you're familiar with 8-bit CPUs that have bit-set and bit-clear instructions that work on I/O pins, and are looking for an equivalent instruction in this one? The ColdFire CPU does have BSET and BCLR instructions, but that is a slow way to change an I/O pin. The Port Set and Clear registers are a faster way to set and clear pins, that's why they're there.

The next thing you should when you get this sort of error from the compiler do is to search in the provided header files for the definition of "SETTJ_SETTJ5_BITMASK" to find out what actual code the preprocessor is generation. My guess (I don't have CodeWarrior) is that it will be defined to something like "(1 << 5)" or 0x20. And because you have to write zeros to the bits in CLRTJ to clear bits (and ones to every other bit) I'd guess CLRTJ_CLRTJ5_BITMASK is defined as "~(1 << 5)" or 0xdf.

Read "15.6.3 Port Pin Data/Set Data Registers (PORTnP/SETn)" in the Reference Manual. In there it tells you to write an 8-bit value to the SETTJ port, and that bits written as "1" will set the port bits. Likewise 0's written to CLRTJ will clear a bit.

So writing "1" to it should set the bottom bit, and not all 8. I'd guess the way you're meant to use those definitions is to write:

SETTJ = SETTJ_SETTJ5_BITMASK;

CLRTJ = CLRTJ_CLRTJ5_BITMASK;

Tom

Re: Clear a GPIO Pin

JimDon — Tue, 12 Feb 2013 01:16:45 GMT

This is the code I used in an actual CW project:

MCF_GPIO_SETTJ = MCF_GPIO_PORTTJ_PORTTJ5 ; // Set a bit

MCF_GPIO_CLRTJ &= ~MCF_GPIO_PORTTJ_PORTTJ5 ; // Clear the bit.

But that was 10.2, not sure where your defines are coming from. These are from MCF52255_GPIO.h.

Also not sure why you would use a define like MCF_GPIO_PORTTJ_PORTTJ5 as it really has no logical meaning and it not any better than 0x20.

I would use somethhing like DATA_STROBE or what ever that line is doing in the design.

Re: Clear a GPIO Pin

blakehodder — Tue, 12 Feb 2013 18:10:21 GMT

Thanks Tom,

That did work for setting a bit. For clearing a bit I had to change the header file as CLRTJ_CLRTJ5_BITMASK was defined as 0x20 instead of 0xdf.

Blake