EOS DSLRs: In-camera HDR and shutter speed

Several EOS model feature an in-camera HDR mode to allow you to capture three images at different exposures and have them combined into one image directly in the camera giving a much-expanded dynamic range. When adjusting the exposure for images such as this, the camera will always change the shutter speed. This is true even in Tv and M modes where you may have set a shutter speed. If you wish to keep both the shutter speed and aperture the same, but instead use ISO to adjust the exposure setting, you should capture your three images manually and composite them later on the computer using the Canon Digital Photo Professional (DPP) software.
Camera settings
EOS 5D Mark IV