Datasheet
Host
(e.g. FPGA)
LVCMOS GPIO
SMBus Interface
LMH0307
#1
GPO
SCL
SDA
RSTI RSTO
SCL
SDA
LMH0307
#2
RSTI RSTO
SCL
SDA
LMH0307
#N
RSTI RSTO
SCL
SDA
3.3V
LMH0307
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SNLS286I –APRIL 2008–REVISED APRIL 2013
COMMUNICATING WITH MULTIPLE LMH0307 CABLE DRIVERS VIA THE SMBus
A common application for the LMH0307 will utilize multiple cable driver devices. Even though the LMH0307
devices all have the same default SMBus device ID (address), it is still possible for them share the SMBus
signals as shown in Figure 5. A third signal is required from the host to the first device. This signal acts as a
“Enable / Reset” signal. Additional LMH0307s are controlled from the upstream device. In this control scheme,
multiple LMH0307s may be controlled via the two-wire SMBus and the use of one GPO (General Purpose
Output) signal. Other SMBus devices may also be connected to the two wires, assuming they have their own
unique SMBus addresses.
Figure 5. SMBus Configuration for Multiple LMH0307 Cable Drivers
The RSTI pin of the first device is controlled by the system with a GPO pin from the host. The first LMH0307
RSTO pin is then daisy chained to the next device's RSTI pin. That device’s RSTO pin is connected to the next
device and so on.
The procedure at initialization is to:
1. Hold the host GPO pin Low in RESET, to the first device. RSTO output default is also Low which holds the
next device in RESET in the chain.
2. Raise the host GPO signal to LMH0307 #1 RSTI input pin.
3. Write to Address 8’h2E (7’h17) Register 0 with the new address value (e.g. 8’h2C (7’h16).
4. Upon writing Register 0 in LMH0307 #1, its RSTO signal will switch High. Its new address is 8’h2C (7’h16),
and the next LMH0307 in the chain will now respond to the default address of 8’h2E (7’h17).
5. The process is repeated until all LMH0307 devices have a unique address loaded.
6. Direct SMBus writes and reads may now take place between the host and any addressed device.
The 7-bit address field allows for 128 unique addresses. The above procedure allows for the reprogramming of
the LMH0307 devices such that multiple devices may share the two-wire SMBus. Make sure all devices on the
bus have unique device IDs.
If power is toggled to the system, the SMBus address routine needs to be repeated.
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