I have a overclock on lpc2378 up to 116MHZ but lpc23xx datasheet talk about CCLKmax=72MHZ.
<span style="font-family: Roboto, sans-serif;"><span style="font-size: 14px;">what are the risks? the lpc2378 is a little warm in this clock but work just fine for a long time .</span></span>
The problem with mixing the clock speeds on the bus is that the you don't really know what's going to happen to the 400kHz capable devices when they see the 1 MHz signals on the bus. You are probably violating setup times and other I2C bus specifications for the 400kHz devices, so you don't really know how they're going to react to these out-of-spec conditions.
Even though the probability for encountering a problem may be low, I would not recommend it for a production system.
Clock stretching will not occur with device like the PCA9952 or the LM75B since they do not have the ability to do so. For example, if you look at the block diagram of the LM75B, it shows the SDA signal as bidirectional, while the SCL signal is an input only. I cannot comment on the MAX device.
Generally, you won't experience any problems, but there are a couple of things to consider:
1. The I2C devices that are fast mode compliant are only tested at 400kHz, so operation at 1 MHz is not guaranteed. My experience has been that a well designed device will just not acknowledge its address when the clock speed is too high, but again, there are no guarantees.
2. The I2C fast mode plus spec allows using smaller pull-up resistors so that the rise time requirements of the FM+ spec can be met. The devices that are not FM+ compliant will not be able to sink the additional current. If the resistor values are not lowered, you may be violating the 120ns max rise time spec (this will depend upon your bus capacitance).
My suggestion is that you do not violate the I2C bus specifications, since the behavior of the I2C devices is not guaranteed. You can do a few things:
1. Run the bus at 400 kHz
2. Run the I2C clock to the PCA9952 at 1MHz, and the other devices at 400kHz. Use a device like the PCA9646 to isolate the FM+ from the FM I2C devices.