I was wondering if anyone had any experience with half-stepping a bipolar stepper motor using the l297/l298 drivers. In SGS-Thomson's AN1679, figure 18 gives a proposed hardware solution for boosting the motor current during times when it is in a half-step position. This is accomplished by multiplying your normally set current by the squareroot of 2, and should theoretically boost your torque to near-full-step levels.
So, I ordered a nand gate to try this technique out, as per figure 18. Unfortunately, SGS failed to mention that while the inhibit lines do indeed drop to ground when the motor is in a half-step position, when it is not, the lines are bouncing all over place between 5V and ground. This caused the logic gate to be very unhappy.
I next tried to accomplish this in software using a check for an odd step number during the motor commutation interrupt. This has placed a lot of load on my TWI bus, however, which is used to control a DAC which supplies the reference voltage to the l297. The motor will function, but only up to about 450rpm, at which point I believe the bus communication throws an error.
Does anybody have any clever ideas for handling this torque boost? Basically, I need to be able to detect when a line drops to ground in hardware, or come up with a clever way of handling current boosts every other step in software (which seems unlikely given that the interrupt frequency increases as motor speed increases)