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Problem In SPI Communication

hi friends,

I am Using 5 (C8051F120) MicroControllers in My Project with the following settings.

SPI Mode: Multi Master Mode - 4 wire
Clock rate : 2.4 Mhz
clock phase bit: 0
Clock polarity bit: 0
Weak pull up: Enabled

4 MCUs uses its own GPIO pins to select the 5th MCUs's NSS.

The GPIO Pin of the 4 MCUs are Shorted to the NSS pin of the 5th MCU.At this time i Face the Following Problem

1 among the 4 MCUs select the 5th MCU by pulling its GPIO pin low. But at the same time, other 3 MCUs pull the same NSS pin pin high. At this time , the NSS pin of the 5th MCU is not getting low. The GPIO pins are configured as open drain.

Any one know how to solve the problem?

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