Add slow mode for the JTAG3 ISP programmer

This uses a slower bitclock, allowing programming of parts < 4Mhz (like
fresh 328 MCU's, which run at 1Mhz by default).

For the JTAG version, this is not needed, since the JTAG clock
completely drives the chip, regardless of the configured CPU clock.

Author: matthijskooijman

hardware/arduino/avr/programmers.txt

@@ -81,7 +81,7 @@ stk500.program.extra_params=-P{serial.port}
 # bootloader does not work.
 # As a workaround, you can add -V to disable verification and modify
 # boards.txt to set all unused lock bits to 1 instead of 0.
-jtag3isp.name=Atmel JTAGICE3 (ISP mode)
+jtag3isp.name=Atmel JTAGICE3 (ISP mode, for chips at or above 4Mhz)
 jtag3isp.communication=usb
 jtag3isp.protocol=jtag3isp
 jtag3isp.program.protocol=jtag3isp
@@ -90,6 +90,20 @@ jtag3isp.program.tool=avrdude
 # microcontrollers running at 4Mhz and above.
 jtag3isp.program.extra_params=-B1
 
+# Note that until https://savannah.nongnu.org/bugs/?21954 and
+# https://savannah.nongnu.org/bugs/?42267 are fixed, burning a
+# bootloader does not work.
+# As a workaround, you can add -V to disable verification and modify
+# boards.txt to set all unused lock bits to 1 instead of 0.
+jtag3ispslow.name=Atmel JTAGICE3 (ISP mode, for chips below 4Mhz)
+jtag3ispslow.communication=usb
+jtag3ispslow.protocol=jtag3isp
+jtag3ispslow.program.protocol=jtag3isp
+jtag3ispslow.program.tool=avrdude
+# Set a bitclock of 125us (the slowest supported value). This works for
+# microcontrollers running at 8kHz and above.
+jtag3ispslow.program.extra_params=-B125
+
 # Note that until https://savannah.nongnu.org/bugs/?21954 is fixed,
 # burning a bootloader does not work.
 # As a workaround, you can modify boards.txt to set all unused lock bits