Electrical Characteristics
MSC8144E Quad Core Digital Signal Processor Data Sheet, Rev. 14
Freescale Semiconductor
31
2.5.2
Serial RapidIO DC Electrical Characteristics
DC receiver logic levels are not defined since the receiver is AC-coupled.
2.5.2.1
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 GNDSXC 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.
Figure 5. SerDes Reference Clocks Input Stage
SRIO_REF_CLK
50
Ω
50
Ω
GNDSXC