AM79C975VCW datasheet(248/304 Pages) AMD

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248

Am79C973/Am79C975

P R E L I M INARY

Key:

Figure 72. Write Byte Command

Note that the Am79C975 controller does not validate

the register address specified in the Write Byte com-

mand. A Write Byte command to a non-existing regis-

ter, or to register that is read-only, or to one of the

registers that require a Block Read or Write command

may cause unexpected reprogramming of an SMIU

Read Byte Command

The Read Byte command is used to read 1 byte of data

from an SMIU register. This command is more complex

compared to the Write Byte command, since it involves

a change in the direction of the data transfer. The com-

mand starts with the START condition (S). The next 7

bits are the slave address of the Am79C975 controller,

followed by a 0 bit to indicate that the data transfer

starts with a write operation from the master to the

Am79C975 controller (W). The Am79C975 controller

must acknowledge the first byte by driving MDATA LOW

for 1 bit time (A). The next byte specifies the address of

the SMIU register (MReg) that is accessed, followed by

another ACK from the Am79C975 controller. The mas-

ter initiates the turn-around of the transfer direction by

asserting a repeated START condition, followed by the

repeated 7-bit slave address. This time, the Read/Write

bit is set to 1 to indicate that the next byte of data is

dr i ven b y t he Am79C975 c ont r o ller ( R ) . T h e

Am79C975 controller acknowledges the transfer and

then drives the one byte of register data onto the

MDATA line. The acknowledge for the register data is

generated by the master, since he is the receiver of the

data. The master generates a NACK (N) to force the

Am79C975 controller to stop driving the MDATA line

since the data transfer consists of only one byte. The

Read Byte command terminates with the assertion of

the STOP condition (P) by the master.

Key:

Figure 73. Read Byte Command

Note: The Am79C975 controller does not validate the

A Read Byte command to a non-existing register, or to

require a Block Read or Write command will yield un-

defined data.

If the second slave address in the Read Byte Com-

mand is not the one of the Am79C975 controller, the

device will release the MDATA line to generate a NACK.

The master, receiving the NACK, must abort the cycle

by generating a STOP condition.

Block Write Command

The Block Write command is used to write data to the

SMIU Transmit Data memory or to the Receive Pattern

RAM. The command starts with the START condition

(S). The next 7 bits are the slave address of the

Am79C975 controller, followed by a 0 bit to indicate a

write operation (W). The next byte specifies the ad-

dress of the SMIU register (MReg) that is accessed.

The address of the Receive Pattern RAM Data port

(34) or the address of the Transmit Data port (36) is the

only valid values for the MReg address. The third byte

Slave Address

MReg Address

MReg Data

Master to Am79C975 controller

Am79C975 controller to Master

1 1

1 17

1 1

Slave Address

W A

MReg Address

A S

Slave Address

R A

MReg Data

Master to Am79C975 controller

Am79C975 controller to Master

AM79C975VCW Datenblatt(PDF) 248 Page - Advanced Micro Devices

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