SBPH400-3 データシートの表示（PDF） - STMicroelectronics
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SBPH400-3 Datasheet PDF : 43 Pages
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SBPH400-3
2.8.4 Link device request (LREQ)
The link layer device requests the bus or accesses a register in the SBPH400 by sending a
short serial stream on the LREQ pin. The information sent includes the type of bus request,
the speed at which the packet is to be sent, or a read or write command. The length of the
request stream depends on whether it is a bus request (8 bits), a read register request (9 bits),
a write register request (17 bits), or an accelerate control request (6 bits).
The request stream always commences with a start bit (value 1b) and terminates with a stop
bit (0b). Bit 0 is transmitted first on the serial request stream.
The LREQ timing and the definition of the bits in the transfer are shown in Figure 2.10
Figure 2.10 LREQ timing
LR0
LR1
LR2
LR3
LR(n-2) LR(n-1)
The Bus Request is 8 bits long and is specified in Table 2.7. (Note that this is backwards
compatible with the 7-bit format as specified in IEEE 1394-1995, which is also supported by
the SBPH400.)
Table 2.7 Bus request format
Bit(s)
0
1-3
4-6
7
Name
Start_Bit
Request_Type
Request_Speed
Stop_Bit
Description
Indicates start of transfer. Always 1.
Indicates the type of bus request being performed. See Table 2.12 for the
encoding of this field.
Indicates the speed at which the SBPH400 will be sending the packet for
this request. This field has the same encoding as the speed code from the
first symbol of the receive packet. See Table 2.11 for the encoding of this
field.
Indicates end of transfer. Always 0.
The Read Request is used to read a register in the device. It is 9 bits long and is specified in
Table 2.8.
Table 2.8 Read register request format
Bit(s)
0
1-3
4-7
8
Name
Start_Bit
Request_Type
Address
Stop_Bit
Description
Indicates the start of transfer. Always 1.
100b (Register read)
Indicates the address of the physical register to be read.
Indicates end of transfer. Always 0.
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