DS125BR800A Datasheet, PDF (21/53 Pages) Texas Instruments – Low-Power 12.5-Gbps 8-Channel (Unidirectional) Repeater With Input Equalization

DS125BR800A
www.ti.com
SNLS467 – NOVEMBER 2013
SYSTEM MANAGEMENT BUS (SMBUS) AND CONFIGURATION REGISTERS
The System Management Bus interface is compatible to SMBus 2.0 physical layer specification. ENSMB = 1kΩ
to VDD to enable SMBus slave mode and allow access to the configuration registers.
The DS125BR800A has the AD[3:0] inputs in SMBus mode. These pins are the user set SMBUS slave address
inputs. The AD[3:0] pins have internal pull-down. When left floating or pulled low the AD[3:0] = 0000'b, the device
default address byte is B0'h. Based on the SMBus 2.0 specification, the DS125BR800A has a 7-bit slave
address. The LSB is set to 0'b (for a WRITE). The device supports up to 16 address byte, which can be set with
the AD[3:0] inputs. Below are the 16 addresses.
Table 10. Device Slave Address Bytes
AD[3:0] Settings
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
Address Bytes (HEX)
B0
B2
B4
B6
B8
BA
BC
BE
C0
C2
C4
C6
C8
CA
CC
CE
The SDA, SCL pins are 3.3V tolerant, but are not 5V tolerant. External pull-up resistor is required on the SDA.
The resistor value can be from 1 kΩ to 5 kΩ depending on the voltage, loading and speed. The SCL may also
require an external pull-up resistor and it depends on the Host that drives the bus.
TRANSFER OF DATA VIA THE SMBus
During normal operation the data on SDA must be stable during the time when SCL is High.
There are three unique states for the SMBus:
START: A High-to-Low transition on SDA while SCL is High indicates a message START condition.
STOP: A Low-to-High transition on SDA while SCL is High indicates a message STOP condition.
IDLE: If SCL and SDA are both High for a time exceeding t_BUF from the last detected STOP condition or if they
are High for a total exceeding the maximum specification for t_HIGH then the bus will transfer to the IDLE state.
SMBus TRANSACTIONS
The device supports WRITE and READ transactions. See Table 11 for register address, type (Read/Write, Read
Only), default value and function information.
WRITING A REGISTER
To write a register, the following protocol is used (see SMBus 2.0 specification).
1. The Host drives a START condition, the 7-bit SMBus address, and a "0" indicating a WRITE.
2. The Device (Slave) drives the ACK bit ("0").
3. The Host drives the 8-bit Register Address.
4. The Device drives an ACK bit ("0").
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