*-------------------------------------------------------------------
* Simple POR circuit for Caravel
*-------------------------------------------------------------------
*
* Architecture:
*
*    Resistive divider sets mvnfet transistor gate voltage to ??V
*    mvnfet current is 240nA nominal
*    mvnfet drives current mirror at 1/400x to 600pA through mvpfet
*    current feeds 1.84pF capacitor (double MiM at 30um x 30um)
*    voltage across capacitor is input to chain of two schmitt trigger
*    inverters.
*
* 	Q = C * V = I * dt
* 
*	t = C * V / I = 1.84pF * 3.3V / 600pA = 10ms
* 
*   ~400x step-down done by mirroring 1:8, 1:7, 1:7 (= 392)
*
* From DC sweep test result, V = 0.7575 on the transtor gate at vin
* Resistor divider at fraction 0.23.
* This yields resistor lengths of 500 on top, 149 on the bottom
*
* Actual response of this circuit by ngspice simulation is 15ms.
*-------------------------------------------------------------------

.subckt simple_por vdda vccd vss por_h por_l porb_l

Xcap1 vcap vss sky130_fd_pr__cap_mim_m3_1 l=30 w=30
Xcap2 vcap vss sky130_fd_pr__cap_mim_m3_2 l=30 w=30

* Note: 20 resistors of length 25um connected in series
Xres1 vdda vin vss sky130_fd_pr__res_xhigh_po_0p69 l=500
Xres2 vin vss vss sky130_fd_pr__res_xhigh_po_0p69 l=149

* Triple current mirror, ratios 8:1, 7:1, and 7:1, with p-cascodes
*   D     G     S     B
Xm1 casc1 vin   vss   vss  sky130_fd_pr__nfet_g5v0d10v5 w=2 l=0.8 m=1
Xc1 mir1  casc1 casc1 vdda sky130_fd_pr__pfet_g5v0d10v5 w=2 l=0.8 m=1
Xm2 mir1  mir1  vdda  vdda sky130_fd_pr__pfet_g5v0d10v5 w=2 l=0.8 m=8
Xm3 mir2  mir1  vdda  vdda sky130_fd_pr__pfet_g5v0d10v5 w=2 l=0.8 m=1
Xc2 casc2 casc1 mir2  vdda sky130_fd_pr__pfet_g5v0d10v5 w=2 l=0.8 m=1 
Xm4 casc2 casc2 vss   vss  sky130_fd_pr__nfet_g5v0d10v5 w=2 l=0.8 m=7
Xm5 casc3 casc2 vss   vss  sky130_fd_pr__nfet_g5v0d10v5 w=2 l=0.8 m=1
Xc3 mir3  casc3 casc3 vdda sky130_fd_pr__pfet_g5v0d10v5 w=2 l=0.8 m=1  
Xm6 mir3  mir3  vdda  vdda sky130_fd_pr__pfet_g5v0d10v5 w=2 l=0.8 m=7
Xm7 mir4  mir3  vdda  vdda sky130_fd_pr__pfet_g5v0d10v5 w=2 l=0.8 m=1
Xc4 vcap  casc3 mir4  vdda sky130_fd_pr__pfet_g5v0d10v5 w=2 l=0.8 m=1 

* Buffered with schmitt trigger buffer
Xtrig vcap vss vss vdda vdda out sky130_fd_sc_hvl__schmittbuf_1

* High voltage output (buffer)
Xbuf out vss vss vdda vdda por_h sky130_fd_sc_hvl__buf_8

* Level shift down (buffer)
Xlv1 out vss vss vccd vccd por_l sky130_fd_sc_hvl__buf_8

* Level shift down (inverter)
Xlv2 out vss vss vccd vccd porb_l sky130_fd_sc_hvl__inv_8

* No tap cell in library?
* Xtap vdda vss sky130_fd_sc_hvl__tapvpwrvgnd_1

.ends

.end