Released
Datasheet
PM73487 QRT
PMC-Sierra, Inc.
PMC-980618
Issue 3
622 Mbps ATM Traffic Management Device
11.4.4 Relationship Between ATM_CLK and SYSCLK
The maximum frequency allowed at the UTOPIA interface per the UTOPIA Level 2 specification
is 50 MHz., However, the QRT is designed to operate at up to 55 MHz if the ATM layer devices
or PHY devices permit it.
The maximum bandwidth of the QRT is 720 Mbit/s when SYSCLK is at 100 MHz, ATM_CLK is
at 50 MHz, and SECLK is at 66 MHz. Theoretically, at these clock rates, throughput is a function
of SYSCLK, SE_CLK, and the switch topology. Speeding up the UTOPIA interface can improve
the Cell Transfer Delay (CTD), but it cannot improve the throughput.
The relationship between the ATM_CLK and SYSCLK is defined by the following equation for
maximum throughput:
ATM_CLK ≤ (52 ÷ 100) × SYSCLK
If ATM_CLK operates below this ratio, the throughput of the device drops linearly in the same
proportion. When ATM_CLK is operated above this ratio, the QRT UTOPIA interface throttles
back, disallowing bandwidth greater than what the device could handle.
11.4.5 Relationship Between the PCLK and the SYSCLK
The upper bound on the PCLK is defined by the following equation:
PCLK ≤ (50 ÷ 100) × SYSCLK
If PCLK exceeds this bound, the handshake protocol between the processor interface and the rest
of the device could be violated.
The lower bound on the PCLK is defined by the following equation:
PCLK > (1 ÷ 8) × SYSCLK
If this constraint is not met, incorrect data could be registered during write operations.
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PMC [ PMC-SIERRA, INC ]
