theller at ctypes
May 2, 2012, 1:05 PM
Post #2 of 3
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Re: mmap and bit wise twiddling - Raspberry Pi
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Am 02.05.2012 19:40, schrieb Petr Jakes:
> Hi,
> I am trying to work with HW peripherals on Raspberry Pi
> To achieve this, it is necessary read/write some values from/to the
> memory directly.
>
> I am looking for some wise way how to organize the bit twiddling.
>
> To set some specific bit, for example, it is necessary:
> - read 4 bytes string representation (I am using mmap)
> - transform it to the corresponding integer (I am using numpy)
> - do some bit masking over this integer
> - transport integer to the string representation (numpy again)
> - write it back to the memory
>
> In other words I mean: is there wise way to create an instrument/
> machinery to define Class and then simply define all necessary objects
> and set the bit values over object attributes so the whole bit-
> twiddling remains under the hood.
>
> say:
> LED01 = GPIO(4) # gpio PIN number 4 is assigned to the LED01 name
> (attribute)
> LED01.on()
> LED01.off()
> or
>
> gpio = GPIO()
> LED01 = gpio.pin04
> LED01 = 1 # led diode is shining (or gpio pin 4 is set)
> LED01 = 0 # led diode is off
I have an abstract BitVector base-class that allows to get/set single
bits or several bits in a convenient way. You must define concrete
subclasses which define a _value get/set property that actually
updates the byte or word in the hardware. I use it to access bits
or groups of bits of I2C devices.
You would basically code like this, assuming an 8-bit GPIO port:
class GPIO(BitVector):
def __init__(self, address, value=0xFF, nbits=8):
self.address = address
super(GPIO, self).__init__(value, nbits)
def _get_value(self):
"read an 8-bit value from the hardware as 8-bit integer"
...
def _set_value(self, v):
"write the 8-bit value 'v' to the hardware"
...
then you can do:
gpio = GPIO(0x12345678)
led0 = gpio[4] # bit 4
led0.value = 1 # switch led on
print led0.value # get led status
For multiple bits use this (note that different from standard Python
practices, indexing works inclusive and uses [high_bitnum:low_bitnum]:
port = GPIO(0x12345678)
high_nibble = port[7:4]
print high_nibble.value
low_nibble = port[3:0]
low_nibble.value = 0xF
Thomas
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bitvector.py
