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Lucent Technologies Inc.
B900
Advance Data Sheet
Baseband Signal Processor
July 1999
4 Hardware Architecture (continued)
4.5
Clock Generation (continued)
4.5.2 Core Clock Switching
When any of the smooth MUX control bits are changed, because of synchronization issues, a delay is incurred
before the clock switch takes place. Five conditional flags can be checked via software to determine when the clock
change actually takes place. These five conditionals are pllon, plloff, slowon, slowoff, stopclk. These five flags indi-
cate that:
s
CLKPLL currently is or is not the output of the smooth MUX.
s
CLKLOW currently is or is not the output of the smooth MUX.
s
STOPCLK main clock for B900 core turned off.
When the SELCLK control bits (clkc[5:4]) are changed, resulting in the smooth MUX switching the source for the
core clock, there is a maximum time for the clock switch to occur (see
Table 15). The latencies are defined in terms
of input clock cycles (CLKIN period) and/or low-frequency clock cycles (CLKLOW period). In addition, a stabilization
time is required for the entire clock path of the new source clock before that clock is selected via the SELCLK con-
Table 15. Clock Switch Latencies
Source Clock
Before Switch
Source Clock
After Switch
Maximum Clock Cycles for Switch
CLKIN
CLKPLL
[((N + 1)
2.5) + ((K + 1)/(M + 2)) + 1.5]CLKIN Period
CLKIN
CLKLOW
1.5 CLKLOW Period
CLKIN
CLKCORE2X stopped
1.5 CLKIN Period
CLKLOW
CLKIN
1.5 CLKLOW Period + 1.5 CLKIN Period
CLKLOW
CLKPLL
[((N + 1)
2.5) + ((K + 1)/(M + 2)) + 1.5]CLKIN Period +
1.5 CLKLOW Period
CLKLOW
CLKCORE2X stopped
1.5 CLKLOW Period
CLKPLL
CLKIN
[((N + 1)
3) + 0.5](CLKIN Period)
CLKPLL
CLKLOW
1.5 CLKLOW Period
CLKPLL
CLKCORE2X stopped
[((N + 1)
2.5) + ((K + 1)/(M + 2)) + 1.5]CLKIN Period
CLKCORE2X stopped
CLKIN
1.5 CLKIN Period
CLKCORE2X stopped
CLKLOW
1.5 CLKLOW Period
CLKCORE2X stopped
CLKPLL
[((N + 1)
2.5) + ((K + 1)/(M + 2)) + 1.5]CLKIN Period
Table 16. Core Clock Stabilization Requirements
Source Clock to Be Selected
Condition
Required Stabilization Time Prior
to Selecting Source Clock
CLKIN*
* Assumes that OSCBYP = 0.
Oscillator just enabled
10 ms (preliminary)
CLKRING
Ring oscillator just enabled
100
s (preliminary)
CLKPLL
PLL just enabled
50
s (preliminary)
CLKPLL
Any change to pllc register bits
50
s (preliminary)
CLKLOW
Any change to SLOWMUX[1:0] bits
1 instruction cycle