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MPC555
/
MPC556
CLOCKS AND POWER CONTROL
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
8-12
Figure 8-7 Clocks Timing For DFNH = 1 (or DFNL = 0)
8.6.2 CLKOUT
CLKOUT has the same frequency as the general system clock (GCLK2_50). Unlike
the main system clock GCLK1/GCLK2 however, CLKOUT (and GCLK2_50) repre-
sents the external bus clock, and thus will be one-half of the main system clock if the
external bus is running at half speed (EBDF = 0b01). The CLKOUT frequency defaults
to VCO/2. CLKOUT can drive full- or half-strength or be disabled. The drive strength
is controlled in the system clock and reset-control register (SCCR). Disabling or de-
creasing the strength of CLKOUT can reduce power consumption, noise, and electro-
magnetic interference on the printed circuit board.
When the PLL is acquiring lock, the CLKOUT signal is disabled and remains in the low
state (provided that BUCS = 0).
8.6.3 Engineering Clock
ENGCLK is an output clock with a 50% duty cycle. Its frequency defaults to VCO/128
1
,
which is one-sixtyfourth of the main system frequency. ENGCLK frequency can be pro-
grammed to the main system frequency divided by a factor from one to 64, as con-
trolled by the ENGDIV[0:5] bits in the SCCR. ENGCLK can drive full- or half-strength
or be disabled (remaining in the high state). The drive strength is controlled by the EE-
CLK[0:1] bits in the SCCR. Disabling ENGCLK can reduce power consumption, noise,
and electromagnetic interference on the printed circuit board.
1.
Mask sets prior to K62N default to VCO/4.
GCLK1
GCLK2
GCLK1_50
GCLK2_50
CLKOUT
GCLK1_50
GCLK2_50
(EBDF = 00)
(EBDF = 00)
(EBDF = 01)
(EBDF = 01)
CLKOUT
(EBDF = 00)
(EBDF = 01)
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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