MC68HC908EY16 Datasheet, PDF (55/278 Pages) Motorola, Inc – Microcontrollers

Chapter 4
BEMF Counter Module (BEMF)
4.1 Introduction
This section describes the BEMF module. The BEMF counter integrates over time, while the
PTD0/TACH0 pin is active. This function is useful for measuring recirculation currents in motors occurring
on switching of inductive loads.
BEMF is the abbreviation for Back ElectroMagnetic Force.
4.2 Functional Description
The 8-bit BEMF counter runs at the internal bus frequency divided by 64. Whenever PTD0/TACH0 is a
logic 1, the counter increments by 1 with each period.
4.3 BEMF Register
The BEMF register contains the eight read-only bits of the BEMF counter, showing its actual value. A read
access to the BEMF register resets all counter bits to 0.
Address:
Read:
Write:
Reset:
$000B
Bit 7
BEMF7
0
6
BEMF6
5
BEMF5
4
BEMF4
3
BEMF3
2
BEMF2
0
0
0
0
0
= Unimplemented
Figure 4-1. BEMF Register (BEMF)
1
BEMF1
0
Bit 0
BEMF0
0
4.4 Input Signal
Port D shares the PTD0/TACH0 pin with the BEMF module. To measure an external signal with the BEMF
module, PTD0/TACH0 must be configured as an input (DDRD0 = 0).
4.5 Low Power Modes
The WAIT and STOP instructions put the MCU in low power-consumption standby modes.
4.5.1 Wait Mode
The BEMF module remains active after execution of the WAIT instruction. In WAIT mode the BEMF
register is not accessible by the CPU.
MC68HC908EY16 • MC68HC908EY8 Data Sheet, Rev. 10
Freescale Semiconductor
55