MC68HC908GT16 Datasheet, PDF (113/412 Pages) Motorola, Inc – Microcontrollers

Internal Clock Generator (ICG) Module
Functional Description
7.4.1 Clock Enable Circuit
The clock enable circuit is used to enable the internal clock (ICLK) or
external clock (ECLK) and the port logic which is shared with the
oscillator pins (OSC1 and OSC2). The clock enable circuit generates an
ICG stop (ICGSTOP) signal which stops all clocks (ICLK, ECLK, and the
low-frequency base clock, IBASE). ICGSTOP is set and the ICG is
disabled in stop mode if the oscillator enable stop bit (OSCENINSTOP)
in the CONFIG2 register is clear. The ICG clocks will be enabled in stop
mode if OSCENINSTOP is high.
The internal clock enable signal (ICGEN) turns on the internal clock
generator which generates ICLK. ICGEN is set (active) whenever the
ICGON bit is set and the ICGSTOP signal is clear. When ICGEN is clear,
ICLK and IBASE are both low.
The external clock enable signal (ECGEN) turns on the external clock
generator which generates ECLK. ECGEN is set (active) whenever the
ECGON bit is set and the ICGSTOP signal is clear. ECGON cannot be
set unless the external clock enable (EXTCLKEN) bit in the CONFIG2
register is set. when ECGEN is clear, ECLK is low.
The port E4 enable signal (PE4EN) turns on the port E4 logic. Since port
E4 is on the same pin as OSC1, this signal is only active (set) when the
external clock function is not desired. Therefore, PE4EN is clear when
ECGON is set. PE4EN is not gated with ICGSTOP, which means that if
the ECGON bit is set, the port E4 logic will remain disabled in stop mode.
The port E3 enable signal (PE3EN) turns on the port E3 logic. Since port
E3 is on the same pin as OSC2, this signal is only active (set) when
2-pin oscillator function is not desired. Therefore, PE3EN is clear when
ECGON and the external crystal enable (EXTXTALEN) bit in the
CONFIG2 register are both set. PE3EN is not gated with ICGSTOP,
which means that if ECGON and EXTXTALEN are set, the port E3 logic
will remain disabled in stop mode.
MC68HC908GT16 • MC68HC908GT8 — Rev. 2
MOTOROLA
Internal Clock Generator (ICG) Module
Technical Data
113