11.4.3 Registers

The XL, DX, MX, SX and ACT3 families have dedicated asynchronous reset registers in the sequential modules(SMOD). In each SMOD is a 4:1 multiplexor with some gating logic on the select lines. Implementing a simple register or an asynchronous reset register allows the gating logic in front of the register to be pulled into the SMOD, reducing the path delay by one level. This is called full combinability. Full combinability offers improved speed, increasing a 50MHz operation to 75MHz in some designs. The following examples show how to use registers for combinability and discuss any speed or area penalty associated with using the register.