SLLU335A August 2021 – January 2022 MCF8316A
MCF8316A device provides information about the motor speed through the frequency generator (FG) pin, which is also known as a TACH (tachometer) out. In MCF8316A, the FG pin behavior is configured through FG_CONFIG. Configure FG_SEL to output FG signal only in closed loop, both open loop and closed loop, or only in open loop for the first try. Configure FG_DIV to number of motor poles so that the FG output matches the motor mechanical speed in Hz.
When FG_CONFIG is configured to 0 (FG active as long as motor is driven), the FG output is active as long as MCF8316A is driving the motor. FG will not be active during a motor stop and coasting condition. In this mode, FG is released high when MCF8316A enters sleep or standby mode. This mode is useful in applications that require real time motor speed information as long as MCF is driving the motor.
When FG_CONFIG is configured to 1 (FG active till BEMF drops below FG_BEMF_THR), MCF8316A provides FG output until BEMF falls below FG_BEMF_THR. The FG output will continue to indicate motor speed even if the motor is not being actively driven. This mode is useful in applications that require motor speed information above a certain speed, and rotor motion information during the coast and braking conditions.
For example, if the motor Ke is 5 mV/Hz and the application require measuring motor speed above 4 Hz, then the user can configure FG_BEMF_THR to 20 mV. Once when the motor speed reaches 4 Hz, the device will output FG as the BEMF voltage will be 20 mV at 4 Hz.