Dickinson, 26, joins the club as a free agent after being released by the Hornets during the summer.
The formerStoke man has most recently enjoyed loan spells with Portsmouth and Coventry.
"Carl has performed at a higher level than League One and that experience will be invaluable for us, especially as he can perform in two positions," manager Micky Adams told the club's official website.
"I am certain that he can make a positive contribution to the squad and look forward to working with him this season."
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