How are power-switch cells daisy-chained, and why?

Short Answer

In a daisy chain, the switch enable enters the first switch cell, and each cell passes a buffered copy to the next, so the switches turn on one after another instead of all at once. That spreads the in-rush current that charges the virtual rail over time and keeps the peak within what the grid can deliver. The output of the last cell comes back as the acknowledge, telling the power controller that every switch is on.

Technical Explanation

• Chain: each switch cell has an enable input and a buffered enable output; output feeds the next cell's input.
• Why: turning all switches on at once draws a huge in-rush spike that droops the always-on supply and can upset neighbours.
• Acknowledge: the last cell's output is the ack; the controller waits for it before releasing isolation.
• Chain length sets wake time: more cells or slower buffers mean a gentler ramp but a longer wake-up.
• connect_power_switch -mode daisy (ICC2) connects placed switch cells; connect_power_switch -ack_out -ack_port_name (ICC2) also routes the ack.
• Order is sorted from placement unless you use connect_power_switch -object_list -keep_order (ICC2); -direction and -start_point guide the inferred order.
• A broken link leaves later cells off and the ack never rises, so the domain hangs in wake-up.

# [ICC2]  icc2_shell
connect_power_switch -source U_PC/PSE -port_name pse_va_cop -mode daisy -direction horizontal -start_point lower_left -ack_out U_PC/PSE_ACK -ack_port_name pse_ack_va_cop -voltage_area PD_COP
check_mv_design -power_switch

What To Check

• Every switch cell in the voltage area is on the chain, with no floating enable inputs.
• The ack comes from the last cell in the chain, not from an intermediate one.
• The chain order matches the placement so neighbouring cells turn on in sequence without long routes.
• Wake-up time from enable to ack meets the power-controller budget.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): All 60 PD_COP switch cells are chained, check_mv_design -power_switch (ICC2) is clean, and wake time is 1.8 us against a 2 us budget.
• Suspicious (illustrative): Chain is complete but wake time is 1.95 us, leaving little margin if the enable buffers slow at the low-voltage corner.
• Hard stop: One cell's enable input is unconnected, so the chain stops there and PSE_ACK never rises.

What The Interviewer Is Testing

• Links the daisy chain to in-rush current limiting, not just to wiring convenience.
• Knows the ack must come from the end of the chain.
• Knows how ICC2 orders and connects the chain.

Follow-up Question & Model Response

"How does a mother-daughter switch scheme relate to the daisy chain?"

Candidate Model Response: It adds a second stage. A first set of small, weak switches turns on through one chain and charge the virtual rail slowly with limited current. When the rail is close to VDD, a second chain turns on the large switches that carry the full load current. The two chains keep the in-rush peak low while still giving a low-resistance path once the domain is running.