Hardware versions

[CamDavidsonPilon]

We can use the hardware, I2C channels, and EEPROM to know what version of the HAT was are using. How should the software change?

[CamDavidsonPilon]

This is now an immediate issue.

List of current changes needed between dev hardware versions:

1. pin 1 in PWM_TO_PIN
2. ADS11115 vs ADS1015
3. Heating PWM is fixed (should be removed from config)
4. eeprom exists

---

Let's look at a simple example: PWM pin 1 changed from 6 to 17 between versions 0.0 and 0.1. How should we be able to support both? (Yes, we must support all versions of boards)

(master, as of writing)

PWM_TO_PIN = {
    # map between PCB labels and GPIO pins
    1: 6,  # TODO: 17
    2: 13,  # hardware PWM1 available
    3: 16,
    4: 12,  # hardware PWM0 available
    5: 18,
}

---

Have some mapping in the code?

from pioreactor.version import update_for_version

PWM_TO_PIN = {
    # map between PCB labels and GPIO pins
    1: update_for_version['pwm_to_pin1']
    2: 13,  # hardware PWM1 available
    3: 16,
    4: 12,  # hardware PWM0 available
    5: 18,
}

### version.py

def get_hardware_version():
   ... logic for hardware version here

if get_hardware_version() == 0.0:
   update_for_version = {
     pwm_to_pin1: 6,
     ...
   }
elif get_hardware_version() == 0.1:
   update_for_version = {
     pwm_to_pin1: 17,
     ...
   }
...

It's very simple, which I like, but will this cover all required cases? A problem may be that over time, as new versions come out, this update_for_version gets large and starts to permeate more and more of the code base. OTOH, YAGNI.

---

Instead of dicts, where an update of a hardware thing needs to be applied to every dict, another idea is a hierarchy of classes, each modifying / replacing the previous one.

class _Base:
    # defaults here
    pwm_to_pin1 = 6
    ads = "ads1115"
    heater_fixed = False
    ...
    def __getitem__(self, key):
        return getattr(self, key)

class Version0P0(_Base):
    pass

class Version0P1(Version0P0):
    pwm_to_pin1 = 17
    ads = "ads1015"
    heater_fixed = True

class Version0P2(Version0P1):
    ads = "ads1115"

....

This is a bit more abstracted. It's easier to reason about "diffs" between versions (good), but seeing the current state is harder. The if statement is still needed.

[CamDavidsonPilon]

Another option is to have different code bases. This seems really complicated, and limits swapping HATs between RPis (since the hard disk and the HAT are on different devices).