mc68hc916y3 Freescale Semiconductor, Inc, mc68hc916y3 Datasheet - Page 301

• Manufacturer Part Number: mc68hc916y3
• Description: Mc68hc16y3 16 Bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

MC68HC16Y3/916Y3
USER’S MANUAL
Edge-detection logic consists of control bits that enable edge detection and select a
transition to detect. The EDGExA/B bits in timer control register 2 (TCTL2) determine
whether the input capture functions detect rising edges only, falling edges only, or both
rising and falling edges. Clearing both bits disables the input capture function. Input
capture functions operate independently of each other and can capture the same
TCNT value if individual input edges are detected within the same timer count cycle.
Input capture interrupt logic includes a status flag, that indicates that an edge has been
detected, and an interrupt enable bit. An input capture event sets the ICxF bit in the
timer interrupt flag register 1 (TFLG1) and causes the GPT to make an interrupt re-
quest if the corresponding ICxI bit is set in the timer interrupt mask register 1 (TMSK1).
If the ICxI bit is cleared, software must poll the status flag to determine that an event
has occurred. Refer to13.4 Polled and Interrupt-Driven Operation for more informa-
tion.
Input capture events are generally asynchronous to the timer counter. Because of this,
input capture signals are conditioned by a synchronizer and digital filter. Events are
synchronized with the system clock and digital filter. Events are synchronized with the
system clock so that latching of TCNT content and counter incrementation occur on
opposite half-cycles of the system clock. Inputs have hysteresis. Capture of any tran-
sition longer than two system clocks is guaranteed; any transition shorter than one
system clock has no effect.
Figure 13-4 shows the relationship of system clock to synchronizer output. The value
latched into the capture register is the value of the counter several system clock cycles
after the transition that triggers the edge detection logic. There can be up to one clock
cycle of uncertainty in latching of the input transition. Maximum time is determined by
the system clock frequency.
The input capture register is a 16-bit register. A word access is required to ensure
coherency. If coherency is not required, byte accesses can be used to read the
register. Input capture registers can be read at any time without affecting their values.
Freescale Semiconductor, Inc.
For More Information On This Product,
GENERAL-PURPOSE TIMER
Go to: www.freescale.com
MOTOROLA
13-13