Fujitsu MBC2073RC, MBC2036RC, MBA3300RC, MBA3147RC Clock skew management, Idle physical link

1.4 Link layer

1.4.5 Clock skew management

The internal clock for a device is typically based on a PLL with its own clock generator and is used when transmitting dwords on the physical link. When receiving, however, dwords need to be latched based on a clock derived from the input bit stream itself. Although the input clock is nominally a fixed frequency, it may differ slightly from the internal clock frequency up to the physical link rate tolerance. Over time, if the input clock is faster than the internal clock, the device may receive a dword and not be able to forward it to an internal buffer; this is called an overrun. If the input clock is slower than the internal clock, the device may not have a dword when needed in an internal buffer; this is called an underrun.

To solve this problem, transmitting devices insert ALIGNs or NOTIFYs in the dword stream.

A phy that is the original source for the dword stream (i.e., a phy that is not an expander phy forwarding dwords from another expander phy) shall insert one ALIGN or NOTIFY for clock skew management as described in Table 1.16.

Table 1.16 Clock skew management ALIGN insertion requirement

Physical link rate	requirement
1.5 Gbps	One ALIGN or NOTIFY within every 2 048 dwords
3.0 Gbps	Two ALIGNs or NOTIFYs within every 4 096 dwords

In the clock skew management of a drive, one ALIGN primitive is always inserted every 1024 dwords regardless of the physical link rate.

1.4.6 Idle physical link

In any of the conditions below, when there is no dword to be sent, an idle dword is sent. This idle dword is scrambled.

1)In an outside connection

2)The SSP or SMP connection is an inside connection.

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