Types of Retention Cells

Depending on how the retained value is stored and retrieved/restored , there are at least two flavors of retention registers, as follows:

1) Balloon-style retention: In a balloon-style retention register, the retained value is held in an

additional latch, often called the balloon latch. In this case, the balloon element is not in the

functional data-path of the register. 

2) Master/slave-alive retention: In a master/slave-alive retention register, the retained value is held in the master or slave latch. In this case, the retention element is in the functional data-path of the

register.

• Ballon-style retention can be Dual-Pin Retention cell or Single-Pin Retention cell. 
• Master/slave-alive retention is same as Zero-Pin Retention cell.

Dual-Pin Retention :

• Dual-Pin Retention is the one which has two separate control signal for save and restore operation. 
• Save operation can be level-sensitive or edge-sensitive.

Single-Pin Retention:

• Single-Pin Retention is the one which has single control signal for both save and restore operation. 
• Save operation can be level-sensitive or edge-sensitive.
• Save operation and Restore operation will be on opposite level/edge of control signal. i.e If save is performed on level high than restore will be performed on level low of control signal.

Zero-Pin Retention:

• Zero-Pin Retention is the one which does not have any control signal .
• Save operation will be performed when the power domain in which cell is sitting goes from NORMAL to CORRUPT state. 
• Restore operation will be performed when the power domain in which cell is sitting goes from CORRUPT to NORMAL state.