![Digital Equipment Alpha 21164PC Скачать руководство пользователя страница 95](http://html.mh-extra.com/html/digital-equipment/alpha-21164pc/alpha-21164pc_hardware-reference-manual_2498508095.webp)
29 September 1997 – Subject To Change
Clocks, Cache, and External Interface
4–7
Clocks
4.2.1 CPU Clock
The 21164PC uses the differential input clock lines osc_clk_ in_h,l as a source to
generate its CPU clock. The input signals clk_mode_h<1:0> control generation of
the CPU clock, as listed in Table 4–1 and as shown in Figure 4–4.
The 21164PC uses clk_mode_h<0> to provide onchip capability to equalize the
duty cycle of the input clock (eliminating the need for a 2× oscillator). When
clk_mode_h<0> is asserted, the equalizing circuitry, called a symmetrator, is
enabled.
The 21164PC uses clk_mode_h<1> to reset the CPU clock. When clk_mode_h<1>
is set, the internal CPU clock is reset to a known state. When it is clear, the CPU
clock is driven at the same frequency as the osc_clk_h,l differential input.
Caution:
A clock source should always be provided on osc_clk_ in_h,l when sig-
nal dc_ok_h is asserted.
Table 4–1 CPU Clock Generation Control
Mode
clk_mode_h<1:0> Description
Normal
0
0
CPU clock frequency is the same as the input clock
frequency; symmetrator is disabled.
Normal
0
1
CPU clock frequency is the same as the input clock
frequency; symmetrator is enabled. Also used to
accommodate chip testers.
Reset
1
0
Initializes CPU clock, allowing system clock to be
synchronized to a stable reference clock; symmetrator
is disabled.
Reset
1
1
Initializes CPU clock, allowing system clock to be
synchronized to a stable reference clock; symmetrator
is enabled.