SBPH400-3 datasheet(17/43 Pages) STMICROELECTRONICS

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SBPH400-3

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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

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.)

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 2.7

Bus request format

Bit(s)

Name

Description

Start_Bit

Indicates start of transfer. Always 1.

1-3

Request_Type

Indicates the type of bus request being performed. See Table 2.12 for the

encoding of this field.

4-6

Request_Speed

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.

Stop_Bit

Indicates end of transfer. Always 0.

Table 2.8

Read register request format

Bit(s)

Name

Description

Start_Bit

Indicates the start of transfer. Always 1.

1-3

Request_Type

100b (Register read)

4-7

Address

Indicates the address of the physical register to be read.

Stop_Bit

Indicates end of transfer. Always 0.

LR0

LR1

LR2

LR3

LR(n-2)

LR(n-1)

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