Yes, they are…and that's an acknowledged failure risk. A number of divers mitigate that by having a separate independent fourth O2 cell, monitored by a separate computer (from another manufacturer). Divers are trained to watch for any sign that the computer isn't doing its job (and there are certain typical indicators, although they don't cover every possible scenario), and if there's any doubt, switch to an alternative (either bailout to open-circuit, or switch to a fully-manual mode of operation) - either way, the computer is then out of the picture. Of course, fully-manual mode just isn't possible for, say, a passenger jet.