Does a defendant have to have a statement of certificate or certificate of probable cause to appeal against the validity of an admission of violation of probation?

People v. Billetts (1979) 89 Cal.App.3d 302, 307 ["[w]here a defendant is attacking the validity of [an admission of violation of probation], a certificate of probable cause is required under the express terms of section 1237.5"].) With that said, section 1237.5 contains an exception: The defendant may take an appeal without a statement of certificate grounds or a certificate of probable cause if he does so solely on noncertificate grounds, which go to postplea matters not challenging his plea's or admission of probation violation's validity and/or matters involving a search or seizure whose lawfulness was contested pursuant to section 1538.5. (Mendez, at p. 1096.) Absent a certificate of probable cause, then, an appellate court may decide noncertificate issues but must decline to address certificate issues. (Id. at p. 1099.)