MSC8144EC Datasheet, PDF (31/80 Pages) Freescale Semiconductor, Inc – Quad Core Digital Signal Processor

2.6.2 Serial RapidIO DC Electrical Characteristics
DC receiver logic levels are not defined since the receiver is AC-coupled.
Electrical Characteristics
2.6.2.1 DC Requirements for SerDes Reference Clocks
The SerDes reference clocks SRIO_REF_CLK and SRIO_REF_CLK are AC-coupled differential inputs. Each differential
clock input has an internal 50 Ω termination to GND_SXC. The reference clock must be able to drive this termination. The
recommended minimum operating voltage is –0.4 V; the recommended maximum operating voltage is 1.32 V; and the
maximum absolute voltage is 1.72 V.
The maximum average current allowed in each input is 8 mA. This current limitation sets the maximum common mode input
voltage to be less than 0.4 V (0.4 V/50 Ω = 8 mA) while the minimum common mode input level is GND_SXC. For example, a
clock with a 50/50 duty cycle can be driven by a current source output that ranges from 0 mA to 16 mA (0–0.8 V). The input is
AC-coupled internally, so, therefore, the exact common mode input voltage is not critical.
Note: This internal AC-couple network does not function correctly with reference clock frequencies below 90 MHz.
If the device driving the SRIO_REF_CLK inputs cannot drive 50 Ω to GND_SXC, or if it exceeds the maximum input current
limitations, then it must use external AC-coupling. The minimum differential peak-to-peak amplitude of the input clock is 0.4 V
(0.2 V peak-to-peak per phase). The maximum differential peak-to-peak amplitude of the input clock is 1.6 V peak-to-peak (see
Figure 5. The termination to GND_SXC allows compatibility with HCSL type reference clocks specified for PCI-Express
applications. Many other low voltage differential type outputs can be used but will probably need to be AC-coupled due to the
limited common mode input range. LVPECL outputs can produce too large an amplitude and may need to be source terminated
with a divider network to reduce the amplitude. The amplitude of the clock must be at least a 400 mV differential peak-peak for
single-ended clock. If driven differentially, each signal wire needs to drive 100 mV around common mode voltage. The
differential reference clock (SRIO_REF_CLK/ SRIO_REF_CLK) input is HCSL-compatible DC coupled or LVDS-compatible
with AC-coupling.
SRIO_REF_CLK
50 Ω
50 Ω
SRIO_REF_CLK
GND_SXC
Figure 5. SerDes Reference Clocks Input Stage
2.6.2.2 Spread Spectrum Clock
SRIO_REF_CLK/ SRIO_REF_CLK is designed to work with a spread spectrum clock (0 to 0.5% spreading at 3033 kHz rate
is allowed), assuming both ends have same reference clock. For better results use a source without significant unintended
modulation.
MSC8144EC Quad Core Digital Signal Processor Data Sheet, Rev. 6
Freescale Semiconductor
31