Converting raw video to mp4 on beaglebone with ffmpeg

Question

I'd really appreciate help with this one. I am recording video via a webcam with my beaglebone black. The formats supported by the camera are shown below.

    v4l2-ctl --list-formats
ioctl: VIDIOC_ENUM_FMT
    Index       : 0
    Type        : Video Capture
    Pixel Format: 'YUYV'
    Name        : YUV 4:2:2 (YUYV)

    Index       : 1
    Type        : Video Capture
    Pixel Format: 'MJPG' (compressed)
    Name        : MJPEG

When I record a raw 4:2:2 YUV video clip, there is no way to play it back to view, so I am trying to convert it to mp4 with ffmpeg.

It states that yuyv422 is an incompatible pixel format, so I try to first convert the video to 420p with the below command, output_422.yuv being the video that I recorded from the camera.

ffmpeg -y -pix_fmt yuyv422 -s 640x480 -r 30 -i output_422.yuv -f rawvideo -pix_fmt yuv420p -s 640x480 -r 30 output_420.yuv

The result is :

    Input #0, rawvideo, from 'output_422.yuv':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0.0: Video: rawvideo, yuyv422, 640x480, 30 tbr, 30 tbn, 30 tbc
[buffer @ 0x41520] w:640 h:480 pixfmt:yuyv422
[avsink @ 0x412c0] auto-inserting filter 'auto-inserted scaler 0' between the 
filter 'src' and the filter 'out'
[scale @ 0x44700] w:640 h:480 fmt:yuyv422 -> w:640 h:480 fmt:yuv420p 
flags:0x4
Output #0, rawvideo, to 'output_420.yuv':
  Metadata:
    encoder         : Lavf53.21.1
    Stream #0.0: Video: rawvideo, yuv420p, 640x480, q=2-31, 200 kb/s, 90k 
tbn, 30 tbc
Stream mapping:
  Stream #0.0 -> #0.0
Press ctrl-c to stop encoding
Error while decoding stream #0.0
frame=   11 fps=  0 q=0.0 Lsize=    4950kB time=0.37 bitrate=110592.0kbits/s
video:4950kB audio:0kB global headers:0kB muxing overhead 0.000000%

I then try to convert this to mp4 like this :

ffmpeg -f rawvideo -pix_fmt yuv420p -s 640x480 -r 30 -i output_420.yuv -vcodec libx264 output.mp4

It looks like the conversion is successful, however when I try to play this back via vlc player, I have no video, just flickering for a few seconds. I have tried changing resolutions, frame rates etc. but no joy. Something is wrong somewhere but not sure where.

Would anyone have some idea as to what could be wrong? Thanks