Low power design isolation cell

To prevent shutdown power domain The output signal is in metastable state , So let's turn it off power domain Output signal plus isolation cell, Clamp the signal to a stable 0 or 1 state .

1、UPF Definition

set_isolation

        -power_domain power domain name # The goal is power domain.

        -isolation_supply_set supply_set_name # Decided here isolation cell The power supply mode of the system .

        -supply_to input/output/both # The clamp signal is aimed at the target power domain Is it an input signal or an output signal .

        -clamp_value 0/1/latch # The clamping signal is always 0（ Use and gate to realize ）, Constant for the 1（ Use or gate to realize ）, Decide according to the actual signal （latch）.

        -source/-sink/-diff_supply_set # What kind of power supply is connected to the source or output of the clamping signal , If specified -diff_supply_set true Means isolation The input of is from different supply set

        -isolation_sense high/low # Specify the high and low levels of the control signal

        -isolation_signal control signal # Set the control signal

        -location parent/self # Set up iso cell Of hier level , Relative to the target power domain Of -element for ,parent module or self module.

2、 type

According to the state of the clamping signal, it can be divided into three categories ,0 or 1 Or keep the state .

2.1 clamp the signal to “0”

It is necessary to clamp the output signal to 0 You can use AND-style Of isolation cell, Pictured 1, Just put the control signal ISOLN Keep it low .

chart 1 

2.2 clamp the signal to “1”

It is necessary to clamp the output to 1 You can use OR-style Of isolation cell, Pictured 2, take ISOL Just keep it high .

 

chart 2 

2.3 clamp the signal to the last value

Use latch Keep the output signal state . Pictured 3.

chart 3 

3、 Placement position

according to isolation cell The placement position can be divided into output isolation and input isolation. take isolation Place it in the shutoff power domain, Control signals can share one , If a signal needs to be transmitted to multiple power domain, Use output isolation Just switch off power domain Add one more isolation cell that will do , Instead, use input isolation, The signal is transmitted to every power domain You need one isolation cell. It should be noted that in order to power domain Keep when turning off isolation The working state of needs to be used always on The power supply , So common isolation cell Mostly for input isolation, Put in always on power domain.

4、 common problem

Most common problems are in isolation cell Input pin forward buffer Caused by the , If isolation cell Add in alway on power domain, So in always on power domain in isolation cell input pin Front cannot be inserted buffer, because buffer There is no effect of clamping the signal , But at this time, it can be turned off power domain Insert buffer. If isolation cell Add to the shutoff power domain, Its input pin Unable to insert buffer. So you'd better give it to me isolation cell input pin Set up dont touch.