MPC7455 RISC Microprocessor Hardware Specifications, Rev. 4.1
20
Freescale Semiconductor
Electrical and Thermal Characteristics
5.2.3 L3 Clock AC Specifications
The L3_CLK frequency is programmed by the L3 configuration register (L3CR[6:8]) core-to-L3 divisor ratio. See
Table 18
for example core and L3 frequencies at various divisors.
Table 10
provides the potential range of L3_CLK
output AC timing specifications as defined in
Figure 7
.
The maximum L3_CLK frequency is the core frequency divided by two. Given the high core frequencies available
in the MPC7455, however, most SRAM designs will be not be able to operate in this mode using current technology
and, as a result, will select a greater core-to-L3 divisor to provide a longer L3_CLK period for read and write access
to the L3 SRAMs. Therefore, the typical L3_CLK frequency shown in
Table 10
is considered to be the practical
maximum in a typical system. The maximum L3_CLK frequency for any application of the MPC7455 will be a
function of the AC timings of the MPC7455, the AC timings for the SRAM, bus loading, and printed-circuit board
trace length, and may be greater or less than the value given in
Table 10
.
Freescale is similarly limited by system constraints and cannot perform tests of the L3 interface on a socketed part
on a functional tester at the maximum frequencies of
Table 10
. Therefore, functional operation and AC timing
information are tested at core-to-L3 divisors which result in L3 frequencies at 200 MHz or less.
Table 10. L3_CLK Output AC Timing Specifications
At recommended operating conditions. See
Table 4
.
Parameter
Symbol
All Speed Grades
Unit
Notes
Min
Typ
Max
L3 clock frequency
f
L3_CLK
75
250
—
MHz
1
L3 clock cycle time
t
L3_CLK
—
4.0
13.3
ns
L3 clock duty cycle
t
CHCL
/t
L3_CLK
50
%
2
L3 clock output-to-output skew (L1_CLK0 to
L1_CLK1)
t
L3CSKW1
—
—
200
ps
3
L3 clock output-to-output skew (L1_CLK[0:1]
to L1_ECHO_CLK[2:3])
t
L3CSKW2
—
—
100
ps
4
L3 clock jitter
—
—
±50
ps
5
Notes
:
1. The maximum L3 clock frequency will be system dependent. See
Section 5.2.3, “L3 Clock AC Specifications,”
for
an explanation that this maximum frequency is not functionally tested at speed by Freescale.
2. The nominal duty cycle of the L3 output clocks is 50% measured at midpoint voltage.
3. Maximum possible skew between L3_CLK0 and L3_CLK1. This parameter is critical to the address and control
signals which are common to both SRAM chips in the L3.
4. Maximum possible skew between L3_CLK0 and L3_ECHO_CLK1 or between L3_CLK1 and L3_ECHO_CLK3 for
PB2 or late write SRAM. This parameter is critical to the write data signals which are separately latched onto each
SRAM part by these pairs of signals.
5. Guaranteed by design and not tested. The input jitter on SYSCLK affects L3 output clocks and the L3
address/data/control signals equally and, therefore, is already comprehended in the AC timing and does not have
to be considered in the L3 timing analysis. The clock-to-clock jitter shown here is uncertainty in the internal clock
period caused by supply voltage noise or thermal effects. This must be accounted for, along with clock skew, in
any L3 timing analysis.