TimeQuest Example—Clock Enable Multicycle
With the synopsys design constraint (SDC) set_multicycle_path and the get_fanouts commands, you can create a multicycle exception based on an enable register.
Figure 1 shows a simple circuit where register enable_reg is used to create a registered enabled signal for registers din_a_reg[7..0], din_b_reg[7..0], din_x_reg[7..0], din_y_reg[7..0], a_times_b, and x_times_y.
Figure 1. Simple Registered Enable Design
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The enable register enable_reg generates an enable pulse that is two times the register’s clock period and, because of this, a multicycle exception must be applied for the correct analysis. A multicycle setup of 2 and a multicycle hold of 1 must be applied to the enable-driven register fed by the register enable_reg. The multicycle exception is applied only to register-to-register paths where the destination register is controlled by enable_reg.
To accomplish this you can apply the set_multicycle_path exception to all enable-driven registers. Typically this can be tedious, because all enable-driven registers must be specified. Alternatively, the combination of set_multicycle_path and get_fanouts can be used as follows:
#Setup multicycle of 2 to enabled driven destination registers
set_multicycle_path 2 -to [get_fanouts [get_pins enable_reg|q*] \
-through [get_pins -hierarchical *|*ena*]] -end -setup
#Hold multicycle of 1 to enabled driven destination registers
set_multicycle_path 1 -to [get_fanouts [get_pins enable_reg|q*] \
-through [get_pins -hierarchical *|*ena*]] -end –hold
The target of the set_multicycle_path exception is limited to all fanouts of the register reg_en that feed the enable port of a register. This is done with the following option:
[get_fanouts [get_pins enable_reg|q*] -through [get_pins -hierarchical *|*ena*]]
Table 1 shows the new set-up and hold relationships of all enable-driven register-to-register paths in the design after the multicycle exceptions have been applied.
| Table 1. Set-Up and Hold Relationships for Enable-Driven Registers |
| Source Register |
Destination Register |
Set-Up Relationship |
Hold Relationship |
din_a[*] |
din_a_reg[*] |
2x (latch edge time) |
1x (latch edge time) |
din_x[*] |
din_x_reg[*] |
2x (latch edge time) |
1x (latch edge time) |
din_b[*] |
din_b_reg[*] |
2x (latch edge time) |
1x (latch edge time) |
din_y[*] |
din_y_reg[*] |
2x (latch edge time) |
1x (latch edge time) |
fast_mult:mult|* |
a_times_b[*] |
2x (latch edge time) |
1x (latch edge time) |
fast_mult:mult|* |
x_times_y[*] |
2x (latch edge time) |
1x (latch edge time) |
enable_reg |
din_a_reg[*],din_b_reg[*], a_times_b[*],x_times_y[*] |
2x (latch edge time) |
1x (latch edge time) |
From Table 1, notice that the set-up and hold relationships that start at register enable_reg and end at any enable-driven register are 2 and 1, respectively. If these paths do not require the setup and hold relationship to be modified, you can apply the following multicycle exceptions to apply the original relationships:
set_multicycle_path 1 -from [get_pins enable_reg|q*] –end -setup
set_multicycle_path 0 -from [get_pins enable_reg|q*] –end –hold
Download example circuit clock_enable_multicycle.qar.
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