I2C Master and multiple slaves using Freescale Demo boards

mad_dad — Tue, 08 Jul 2008 18:37:00 GMT

I have a number of Freescale demo boards DEMO9S08AW60 and I am trying to implemet a Master to multiple slaves interface using the I2C (a bit like a 1553 bus). Although I can connect a master to a slave directly with bus pull up resistors and it works fine, if I connect another slave to the bus the whole bus locks up. Holding all the slaves bar one in reset allows the bus to work, i assume the SDA and CLK lines are tri-state on reset.

Is it OK to connect the MCU I2C directly together or do I need some sort of bi-directional buffer for each slave?

Re: I2C Master and multiple slaves using Freescale Demo boards

mad_dad — Thu, 10 Jul 2008 15:51:38 GMT

Reply received from Freescale support:-

In reply to your message regarding Service Request:

The only know issue with IIC module in the AW60 is the following:

"IIC Does Not Function When High Drive Strength Is Enabled on the IIC PinsSE127-IIC

Description

IIC operation requires open drain configuration but when the PTxDS bit associated with the IIC SDA or SCL pin is set for high drive strength (PTxDS = 1) the pin will be in complementary drive mode instead of open drain mode. This will cause IIC communication conflicts that lead to IIC malfunction.

Workaround

The PTxDS bits associated with the IIC SDA and SCL pins should be cleared for low drive strength operation. This configuration will ensure that the corresponding pin will enter open drain mode and IIC communications will function correctly. The IIC pull-up resistor in the final application may need to be modified to account for the lower drive strength of the SDA and SCL pins."

This fixed the problem along with changing the pull up resistors to 2K4.

Processor Expert defaults all the ports to high drive.

I now have one master and four slaves with a MILSTD1553 type protocol using DEMO9S08AW60 boards. The slaves being hot pluggable.

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I2C Master and multiple slaves using Freescale Demo boards

Re: I2C Master and multiple slaves using Freescale Demo boards