Cosby's lawyers sought to discredit her by pointing to discrepancies between her testimony and statements to law enforcement in 2005, when she first reported the incident a year after it allegedly occurred. He says it was consensual.
The jurors have deliberated for 40 hours already and it is unclear exactly what the deadlock is about.
On Thursday, jurors told the court that they could not come to a unanimous decision beyond a reasonable doubt, which is required in criminal cases.
The third count accuses Cosby of impairing Constand "by administering drugs, intoxicants or other means for the goal of preventing resistance".
Count 2 alleges she was unconscious or semi-conscious at the time and could not give consent.
The panel outside Philadelphia has deliberated more than 45 hours without reaching a verdict on charges that Cosby drugged and molested a woman in 2004.
Just before lunch Friday, the jury sent yet another question looking to rehear testimony.
But even the solicitous judge had his limits, putting his foot down late Friday afternoon when the jurors asked to hear a sliver of testimony they'd just had read back to them.
He instructed them to rely on their collective recollection.
O'Neill on Friday noted the factors that could precipitate a mistrial - a jury request to halt deliberations followed by his own agreement that the impasse couldn't be broken.More news: Death toll from London tower block blaze rises to 17
Outside the courthouse in Norristown, Cosby supporters gathered, chanting "free Bill Cosby", while several women who have accused the celebrity of sexual assault hoped for a verdict favoring the prosecution. Constand says Cosby gave her pills that made her woozy, then violated her. Cosby also declined to tell her what pills he had given Andrea Constand, but he did say they were from a prescription bottle, she testified.
The jury is still out on Bill Cosby's sexual assault trial but on Friday, as deliberations continued for a fifth day, the actor and comedian took to Twitter to thank his supporters.
Specifically, the jurors asked to hear a portion where the concerned mom described how she felt after speaking with Cosby. "Let me make sure you understand" what's at stake, he said to Cosby.
Cosby lawyer Brian McMonagle argued the court was "being asked to review the entire trial" with the jury's repeated requests to rehear testimony.
"Question: When you got the Quaaludes, was it in your mind that you would use these Quaaludes to give to women you wanted to have sex with?" He maintains that their sexual encounter was consensual.
Constand, who was an employee at Temple University, said the majority of those calls to Cosby, a powerful Temple trustee, were made as part of her job responsibilities.
Quaaludes was a highly popular party drug in the 1970s that was banned in the U.S.in 1982.
Count 3 alleges all this happened after he gave her an intoxicant that substantially impaired her and stopped her from resisting. Montgomery County District Attorney Kevin Steele, right, Montgomery County Assistant District Attorney Kristen, center, and Bill Cosby's defense attorney Angela Agrusa enter the courtroom during jury deliberat.
Passed along since early Thursday, the word that the panel was at an impasse had teased the possibility of a mistrial for tired lawyers, court staff and onlookers, and the crowd of Cosby supporters outside the Montgomery County Courthouse in Norristown.