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).

Author: matthijskooijman

hardware/arduino/avr/programmers.txt

@@ -62,7 +62,7 @@ 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
@@ -71,6 +71,20 @@ 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 than 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