Q
I would like to use several ADT7460 devices on one board. therefore, I would
like to assign different addresses to these devices. The datasheet says that to
assign an address, the ADDRESS_ENABLE pin must be pulled low on power-up. How
long must this pin be pulled low before it can be released again?
A
After power-on reset, the ADT7463 enters Address Select Mode. In Address Select
Mode, /ADDR_EN and ADDR_SEL pins directly drive the SMBus Address Register as a
transparent latch.
The SMBus address is latched on the 1st. valid SMBus transaction which
addresses the ADT7463. More precisely, the SMBus address is latched on the low
to high transition at the beginning of the 8th SCL pulse, when the serial bus
address byte matches with the selected slave address set via the /ADDR_EN and
ADDR_SEL pins. When the SMBus address is latched, the ADT7463 then exits
Address Select Mode and any changes made to /ADD_EN or ADDR_SEL will have no
effect.
So in fact there are two points where the state /ADDR_EN and ADDR_SEL pins is
critical. The pins must be stable during the serial bus address byte and on the
8th SCL pulse.
We recommend that the state of the /ADDR_EN and ADDR_SEL pins are held constant
during all SMBus transactions until after the first valid read/write to the
ADT7463.
Of course, holding the ADDR_EN and ADDR_SEL pins constant after power up
becomes more important if you have multiple devices on the SMBus and the first
write to the ADT7463 after power up occurs after SMBus reads and writes to
other devices. The SMBus address of the ADT7463 is not latched until after the
first read/write to the part, so it is important that the ADDR_EN and ADDR_SEL
pins are correctly driven during bus activity which addresses other devices.