Arduino FORTH: Register Access

Edit after more research: the approach below might not be best possible approach when working with GPIO pins, as it might not be taking advantage of special opcodes for this purpose. Also, due to the two byte FVM cell size, I think there might be some potential for strange effects in nearby registers of a different type (like, the PINC register being after the PORTB register). Heading back to the data sheets…

Edit 2: It looks like FORTH (including Arduino-FVM) has the C@ and C! commands which are single-byte (char) versions of @ and !.

The Atmel specific keywords that come with Arduino-FVM are rather limited – basically just PINMODE, DIGITALREAD, and DIGITALWRITE. So, to do much of interest you would want to start playing around with the atmel microprocessor registers. This is mostly straightforward, however, as the registers are accessible simply as locations in ram, meaning you just write or read bits to ram to get what you want. Doing this in FORTH, you need a few tools

  • @ word: read from a memory location
  • ! word: write to a memory location
  • HEX word: switch to hexidecimal interpretation of the stack input/output (DECIMAL to switch back).
  • Pinout for your arduino board (available on the internet)
  • Data sheet for your specific microcontroller (available on the internet)

Here is a quick example of lighting up the LED through a register write. First, we need to know our pin number and also which register we are dealing with, available from the pinout.

Arduino Nano Pinout

There we have pin number 13 (decimal) for the LED, and register PB5. First, to keep this post a bit shorter, we we simply use the PINMODE keyword to set pin 13 to output mode:

true 13 pinmode

Now we look at the datasheet to get the memory location for register PB5.

A page from 328P Datasheet

So, PORTB register is at 0x25 (hex) and bit five is what we want. Now, let’s see what is already in that memory location. On my chip, I got:

hex
0x25 @ . 0

Just keep in mind however, as you can see using the CELL command, that you have actually read in two bytes. So, if you get something like 0x2000, you are only interested in the first byte (the right-most 00).

As a general practice, I only want to write to the bit of interest, preserving other bit states. So, I drop a 1 for that bit on the stack, and use an OR to light up just that bit. A 1 in bit 5 is hex 0x20.

0x25 @ 0x20 or 0x25 ! 

And the LED lights up. We can see now what we have in register memory:

0x25 @ . 20

That is a trivial example, but I think you would need to go into this memory “peeking” and “poking” in order to do more advanced things like control the interrupt system or special purpose pins. At least, I find the idea more appealing than having to write C interface functions on the backend. Of course, you would want to hide the details inside nice forth functions.

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