Added a simple power-on-reset circuit with schmitt trigger output, and
decoupled the reset pin from the porb/porb_h. The reset for the
housekeeping SPI remains connected to porb and not the reset pin, so
that the processor can be put in reset but the housekeeping SPI can
be accessed in that state. That prevents the user from bricking the
system by having a program override the housekeeping SPI and then get
into an erroneous state.
diff --git a/verilog/rtl/caravel.v b/verilog/rtl/caravel.v
index c51bef1..58dd692 100644
--- a/verilog/rtl/caravel.v
+++ b/verilog/rtl/caravel.v
@@ -47,6 +47,7 @@
`include "chip_io.v"
`include "user_id_programming.v"
`include "gpio_control_block.v"
+`include "simple_por.v"
`ifdef USE_OPENRAM
`include "sram_1rw1r_32_8192_8_sky130.v"
@@ -175,6 +176,9 @@
wire porb_h;
wire porb_l;
+ wire rstb_h;
+ wire rstb_l;
+
// To be considered: Master hold signal on all user pads (?)
// For now, set holdh_n to 1 (NOTE: This is in the 3.3V domain)
// and setting enh to porb_h.
@@ -196,6 +200,7 @@
.flash_io1(flash_io1),
// SoC Core Interface
.porb_h(porb_h),
+ .resetb_core_h(rstb_h),
.clock_core(clock_core),
.gpio_out_core(gpio_out_core),
.gpio_in_core(gpio_in_core),
@@ -297,7 +302,8 @@
.flash_io1_do(flash_io1_do_core),
.flash_io0_di(flash_io0_di_core),
.flash_io1_di(flash_io1_di_core),
- // Power-on Reset
+ // Master Reset
+ .resetb(rstb_l),
.porb(porb_l),
// Clocks and reset
.clock(clock_core),
@@ -452,7 +458,7 @@
.pad_gpio_in(mprj_io_in[(`MPRJ_IO_PADS-1):2])
);
- sky130_fd_sc_hvl__lsbufhv2lv levelshift (
+ sky130_fd_sc_hvl__lsbufhv2lv porb_level (
`ifdef LVS
.vpwr(vdd3v3),
.vpb(vdd3v3),
@@ -470,4 +476,24 @@
.mask_rev(mask_rev)
);
+ // Power-on-reset circuit
+ simple_por por (
+ .vdd3v3(vdd3v3),
+ .vss(vss),
+ .porb_h(porb_h)
+ );
+
+ // XRES (chip input pin reset) reset level converter
+ sky130_fd_sc_hvl__lsbufhv2lv rstb_level (
+ `ifdef LVS
+ .vpwr(vdd3v3),
+ .vpb(vdd3v3),
+ .lvpwr(vdd1v8),
+ .vnb(vss),
+ .vgnd(vss),
+ `endif
+ .A(rstb_h),
+ .X(rstb_l)
+ );
+
endmodule
diff --git a/verilog/rtl/chip_io.v b/verilog/rtl/chip_io.v
index dc46ebf..d34cfec 100644
--- a/verilog/rtl/chip_io.v
+++ b/verilog/rtl/chip_io.v
@@ -11,7 +11,8 @@
inout flash_io0,
inout flash_io1,
// Chip Core Interface
- output porb_h,
+ input porb_h,
+ output resetb_core_h,
output clock_core,
input gpio_out_core,
output gpio_in_core,
@@ -169,7 +170,7 @@
.tie_hi_esd(),
.tie_lo_esd(),
.pad_a_esd_h(xresloop),
- .xres_h_n(porb_h),
+ .xres_h_n(resetb_core_h),
.disable_pullup_h(vss), // 0 = enable pull-up on reset pad
.enable_h(vdd3v3), // Power-on-reset to the power-on-reset input??
.en_vddio_sig_h(vss), // No idea.
diff --git a/verilog/rtl/mgmt_core.v b/verilog/rtl/mgmt_core.v
index bd52027..e6b837b 100644
--- a/verilog/rtl/mgmt_core.v
+++ b/verilog/rtl/mgmt_core.v
@@ -25,6 +25,7 @@
input flash_io0_di,
input flash_io1_di,
// Master reset
+ input resetb,
input porb,
// Clocking
input clock,
@@ -79,7 +80,7 @@
.ext_clk_sel(ext_clk_sel),
.ext_clk(clock), // Should be better handled. . .
.pll_clk(pll_clk),
- .resetb(porb),
+ .resetb(resetb),
.ext_reset(ext_reset),
.core_clk(core_clk),
.resetb_sync(core_rstn)
@@ -203,7 +204,7 @@
.vdd(vdd1v8),
.vss(vss),
`endif
- .resetb(porb),
+ .resetb(resetb),
.extclk_sel(ext_clk_sel),
.osc(clock),
.clockc(pll_clk),
diff --git a/verilog/rtl/simple_por.v b/verilog/rtl/simple_por.v
new file mode 100644
index 0000000..4b92a55
--- /dev/null
+++ b/verilog/rtl/simple_por.v
@@ -0,0 +1,52 @@
+module simple_por(
+ input vdd3v3,
+ input vss,
+ output porb_h
+);
+
+ wire mid, porb_h;
+ reg inode;
+
+ // This is a behavioral model! Actual circuit is a resitor dumping
+ // current (slowly) from vdd3v3 onto a capacitor, and this fed into
+ // two schmitt triggers for strong hysteresis/glitch tolerance.
+
+ initial begin
+ inode <= 1'b0;
+ end
+
+ // Emulate current source on capacitor as a 500ns delay either up or
+ // down.
+
+ always @(posedge vdd3v3) begin
+ #500 inode <= 1'b1;
+ end
+ always @(negedge vdd3v3) begin
+ #500 inode <= 1'b0;
+ end
+
+ // Instantiate two shmitt trigger buffers in series
+
+ sky130_fd_sc_hvl__schmittbuf hystbuf1 (
+`ifdef LVS
+ .VPWR(vdd3v3),
+ .VGND(vss),
+ .VPB(vdd3v3),
+ .VNB(vss),
+`endif
+ .A(inode),
+ .X(mid)
+ );
+
+ sky130_fd_sc_hvl__schmittbuf hystbuf2 (
+`ifdef LVS
+ .VPWR(vdd3v3),
+ .VGND(vss),
+ .VPB(vdd3v3),
+ .VNB(vss),
+`endif
+ .A(mid),
+ .X(porb_h)
+ );
+
+endmodule
