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Port vs Pin

Posted: Thu May 29, 2014 4:33 pm
by nsoatw
I have searched but found nothing on how to access ports as a byte rather than as pins only.
What I'd like to do is connect an LCD, which has an 8-bit data port and 4 bits worth of control pins.
I was hoping I could access it something like this:
data_port = Port (PA0-PA7, output)

Re: Port vs Pin

Posted: Thu May 29, 2014 5:12 pm
by dhylands
On the STM32F4xx, the GPIO ports are 16-bits wide.

The description of the various registers can be found in the STM32F4xx Reference Manual (Note: This is a 1713 page document - about 21 Mb)

The GPIO section starts on page 265 and the register descriptions start on page 278.

You'll most likely bee interested in the GPIOx_ODR, and/or GPIOx_BSRRx registers.

Using GPIOx_ODR will allow multiple bits to be set and/or cleared simultaneously, but you need to do it as a read-modify-write sequence which can have consequences if you also have ISRs on the system which are trying to modify bits which your code isn't using.

If you use the GPIOx_BSRRx register, then you can set or clear specified bits and not affect any other code (since using the GPIOx_BSRRx registers is purely a write operation, and only the bits set to 1 in the register are affected). From my perspective, this would be the preferred register to use.

There is an stm module (I don't see any docs for it yet) which allows arbitrary registers to be read/written from MicroPython. Here's an example for manipulating some of the LEDs simultaneously. The 4 LEDs are mapped as follows:
PA13 - RED

So Red, Green, and Yellow are all on Port A, so we'll use those. To turn on all 3 LEDs at the same time:

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>>> import stm
>>> stm.mem16[stm.GPIOA + stm.GPIO_BSRRL] = 0xE000
and to turn all 3 off at the same time:

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>>> import stm
>>> stm.mem16[stm.GPIOA + stm.GPIO_BSRRH] = 0xE000
To do on and off at the same time, you could do:

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>>> stm.mem32[stm.GPIOA + stm.GPIO_BSRRL] = 0x40002000 # Red on Green off
>>> stm.mem32[stm.GPIOA + stm.GPIO_BSRRL] = 0x20004000 # Red off, Green on
Due to limitations in the way ints work on MicroPython, you can only specify 31 of the 32 bits, so if you try to manipulate bit 31, you'll get an error:

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>>> stm.mem32[stm.GPIOA + stm.GPIO_BSRRL] = 0x80000000
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: overflow converting long int to machine word
You can use

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>>> import stm
>>> dir(stm)
to see which register offsets have been defined.

Re: Port vs Pin

Posted: Fri May 30, 2014 1:48 pm
by nsoatw
Many thanks Dave, precisely what I nedded!
I have read parts of the ref manual, and just wanted to know if there were any specific stm-commands available, rather than writing assembler.