The principles in section 1367 are implemented by section 1368, which provides: "(a) If, during the pendency of an action and prior to judgment, a doubt arises in the mind of the judge as to the mental competence of the defendant, he or she shall state that doubt in the record and inquire of the attorney for the defendant whether, in the opinion of the attorney, the defendant is mentally competent. If the defendant is not represented by counsel, the court shall appoint counsel. at the request of the defendant or his or her counsel or upon its own motion, the court shall recess the proceedings for as long as may be reasonably necessary to permit counsel to confer with the defendant and to form an opinion as to the mental competence of the defendant at that point in time. "(b) If counsel informs the court that he or she believes the defendant is or may be mentally incompetent, the court shall order that the question of the defendant's mental competence is to be determined in a hearing which is held pursuant to Sections 1368.1 and 1369. If counsel informs the court that he or she believes the defendant is mentally competent, the court may nevertheless order a hearing. Any hearing shall be held in the superior court. "(c) Except as provided in Section 1368.1, when an order for a hearing into the present mental competence of the defendant has been issued, all proceedings in the criminal prosecution shall be suspended until the question of the present mental competence of the defendant has been determined. "If a jury has been impaneled and sworn to try the defendant, the jury shall be discharged only if it appears to the court that undue hardship to the jurors would result if the jury is retained on call. "If the defendant is declared mentally incompetent, the jury shall be discharged." Although section 1368, subdivision (a), refers to a doubt that arises "in the mind of the judge as to the mental competence of the defendant," case law interpreting this subdivision establishes that when the court becomes aware of substantial evidence which objectively generates a doubt about whether the defendant is competent to stand trial, the trial court must on its own motion declare a doubt and suspend proceedings even if the trial judge's personal observations lead the judge to a belief the defendant is competent. (People v. Pennington, supra, 66 Cal. 2d at p. 518; People v. Jones (1991) 53 Cal. 3d 1115, 1153 [282 Cal. Rptr. 465, 811 P.2d 757].) Due process requirements are not satisfied if the court merely takes the " 'evidence to guide him in determining if he should declare the existence of a "doubt " as to the defendant's competency; the trial court has no discretion on whether or not to order a competency hearing once there exists substantial evidence giving rise to a doubt regarding competency. (People v. Superior Court (Marks) (1991) 1 Cal. 4th 56, 69 [2 Cal. Rptr. 2d 389, 820 P.2d 613] (Marks).) If a trial court proceeds without holding a competency hearing, the defendant has been deprived of his or her due process right to a fair trial, the trial court has acted in excess of its jurisdiction, and the judgment is a nullity. ( Id. at pp. 70-71; People v. Hale (1988) 44 Cal. 3d 531, 541 [244 Cal. Rptr. 114, 749 P.2d 769].)