Fujitsu MBA3147RC Manuel D’Utilisation

 

C141-C013 

57 

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. 

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. 

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.