LED1202 Address Pins

[Archadious]

I notice that in the schematics for the STEVAL-LLL007M1 STEVAL-LLL007V1, part of the evaluation kit for the LED1202, several of the A0 pins for the attached LED1202s are pulled to ground with 10K resistors.  However, in the datasheet for the LED1202 the example schematics do not show such an arrangement.

What is the purpose of the 10K pull down resistor?  And is it necessary in other designs using the LED1202?

-A

[Peter BENSCH · Accepted solution]

The data sheet of the LED1202 mentions several possible configurations, including, in section 4.9.1, the common clock domain configuration, which also mentions daisy chain distribution via clock output (A0) to the clock input (A1) of the next LED1202. This is precisely the configuration used in the case of the STEVAL-LLL007V1.

As far as pull resistors are concerned, only U3 has a pull-down, while U1 and U2 have pull-ups. You can also find information on this in the data sheet in table 5 and the sentences below it:

Local address is acquired and latched inside the device at first acknowledge. After that, any modification of A0
and/or A1 connection has not effect on the device I²C local address.

The addresses are therefore only read and stored during each power-up, after which A0 and A1 are available as clock output/input.

Hope that answers your questions?

Regards
/Peter

[Archadious]

But why are either a pull-up or pull-down used if the A0 and A1 pin are only used for I2C address determination?  I would like to understand the purpose of resistor in the design.  Is this best practice?  Under what conditions is this resistor necessary?  

-A

[Peter BENSCH]

Well, a pull-up or pull-down sets the address line to logical 0 or 1 during power-up, so that, according to table 5, the corresponding I2C address results, under which the LED1202 can be addressed. 10k has been selected because this value does not interfere when the pin is later switched as an output (A0 = Output). You can use a different value instead of 10k as long as the pull-up does not overload the output on this line – for example, the LED1202 is tested with 5mA, which also allows 2.2K or 1k, maybe even 560ohms (the smaller the value, the greater the power dissipation).

The pull resistors are necessary if you want to set individual I2C addresses and use A0 later as clock output. Imagine what would happen if you connected A0 directly to GND or VCC in such a constellation?

Regards
/Peter

[Archadious]

OK, thank you.  The pull-up or pull-down is only useful if the pin will be used as clock output after setting the address for I2C.  That explains the schematics and clears up my confusion.  Thank you!

-A