Reduce DTR/RTS-low time for reset generation to 100 us

Standard AVR reset circuits deploy a connection from the DTR/RTS line of a
USB-to-serial chip to the AVR via a small capacitor, see

  https://onlinedocs.microchip.com/pr/GUID-F626284A-58F0-4C25-A6F3-0EA5054F3E2B-en-US-6/index.html?GUID-B80B25FF-E9D7-4766-B562-DA197B8B938C

Setting DTR/RTS low once will issue a reset. Setting this signal high
again after a short time ensures that a direct connection DTR/RTS to reset
also works. The duration of pulling DTR/RTS low must be relatively short,
say RC/10, in order to avoid a reset spike above Vcc. If Vcc exceeds 5.5 V
then a full 2 Vcc reset spike can potentially trigger HV programming.

See also
 - https://github.com/avrdudes/avrdude/pull/1504#issuecomment-1735295710
 - https://github.com/avrdudes/avrdude/pull/1505#issuecomment-1738053365

src/arduino.c

@@ -97,10 +97,12 @@ static int arduino_open(PROGRAMMER *pgm, const char *port) {
   usleep(250 * 1000);
   // Pull the RTS/DTR line low to reset AVR
   serial_set_dtr_rts(&pgm->fd, 1);
-  usleep(50 * 1000);
-  // Set the RTS/DTR line back to high
+  // Max 100 us: charging a cap longer creates a high reset spike above Vcc
+  usleep(100);
+  // Set the RTS/DTR line back to high, so direct connection to reset works
   serial_set_dtr_rts(&pgm->fd, 0);
-  usleep(50 * 1000);
+
+  usleep(100 * 1000);
 
   /*
    * drain any extraneous input

src/avrdude.1

@@ -1666,7 +1666,7 @@ query on flash prepared with -xnometadata yields useful results.
 .It Ar delay=<n>
 Add a <n> ms delay after reset. This can be useful if a board takes a
 particularly long time to exit from external reset. <n> can be negative,
-in which case the default 100 ms delay after issuing reset will be
+in which case the default 120 ms delay after issuing reset will be
 shortened accordingly.
 .It Ar strict
 Urclock has a faster, but slightly different strategy than -c arduino to
@@ -1795,7 +1795,7 @@ is greater than 0.
 Add a <n> milliseconds delay after resetting the part through toggling the
 DTR/RTS lines. This can be useful if a board takes a particularly long
 time to exit from external reset. <n> can be negative, in which case the
-default 80 ms delay after issuing reset will be shortened accordingly.
+default 100 ms delay after issuing reset will be shortened accordingly.
 .It Ar help
 Show help menu and exit.
 .El

src/doc/avrdude.texi

@@ -1304,7 +1304,7 @@ yields useful results.
 @item @samp{delay=<n>}
 Add a <n> ms delay after reset. This can be useful if a board takes a
 particularly long time to exit from external reset. <n> can be negative,
-in which case the default 100 ms delay after issuing reset will be
+in which case the default 120 ms delay after issuing reset will be
 shortened accordingly.
 @item @samp{strict}
 Urclock has a faster, but slightly different strategy than -c arduino to
@@ -1447,7 +1447,7 @@ No toggling of DTR/RTS is performed if @var{snooze} > 0.
 @item @samp{delay=<n>}
 Add a <n> milliseconds delay after reset. This can be useful if a board
 takes a particularly long time to exit from external reset. <n> can be
-negative, in which case the default 80 ms delay after issuing reset will
+negative, in which case the default 100 ms delay after issuing reset will
 be shortened accordingly.
 @item @samp{help}
 Show help menu and exit.

src/stk500.c

@@ -112,9 +112,11 @@ int stk500_getsync(const PROGRAMMER *pgm) {
       // This code assumes a negative-logic USB to TTL serial adapter
       // Pull the RTS/DTR line low to reset AVR: it is still high from open()/last attempt
       serial_set_dtr_rts(&pgm->fd, 1);
-      usleep(20*1000);
-      // Set the RTS/DTR line back to high
+      // Max 100 us: charging a cap longer creates a high reset spike above Vcc
+      usleep(100);
+      // Set the RTS/DTR line back to high, so direct connection to reset works
       serial_set_dtr_rts(&pgm->fd, 0);
+      usleep(20*1000);
       stk500_drain(pgm, 0);
     }
 

src/urclock.c

@@ -2220,12 +2220,13 @@ static int urclock_open(PROGRAMMER *pgm, const char *port) {
   usleep(20*1000);
   // Pull the RTS/DTR line low to reset AVR
   serial_set_dtr_rts(&pgm->fd, 1);
-  usleep(20*1000);
-  // Set the RTS/DTR line back to high
+  // Max 100 us: charging a cap longer creates a high reset spike above Vcc
+  usleep(100);
+  // Set the RTS/DTR line back to high, so direct connection to reset works
   serial_set_dtr_rts(&pgm->fd, 0);
 
-  if((100+ur.delay) > 0)
-    usleep((100+ur.delay)*1000); // Wait until board comes out of reset
+  if((120+ur.delay) > 0)
+    usleep((120+ur.delay)*1000); // Wait until board comes out of reset
 
   pmsg_debug("%4ld ms: enter urclock_getsync()\n", avr_mstimestamp());
   if(urclock_getsync(pgm) < 0)

src/wiring.c

@@ -146,14 +146,15 @@ static int wiring_open(PROGRAMMER *pgm, const char *port) {
     // Pull the RTS/DTR line low to reset AVR
     pmsg_notice2("wiring_open(): asserting DTR/RTS\n");
     serial_set_dtr_rts(&pgm->fd, 1);
-    usleep(20*1000);
 
+    // Max 100 us: charging a cap longer creates a high reset spike above Vcc
+    usleep(100);
     // Set the RTS/DTR line back to high, so direct connection to reset works
     serial_set_dtr_rts(&pgm->fd, 0);
 
     int delay = WIRINGPDATA(pgm)->delay;
-    if((80+delay) > 0)
-      usleep((80+delay)*1000); // Wait until board comes out of reset
+    if((100+delay) > 0)
+      usleep((100+delay)*1000); // Wait until board comes out of reset
   }
 
   // Drain any extraneous input