Terminal mode improvements

[MCUdude]

Extends #1574

• Fix bug that makes Avrdude segfault when trying to set a new clock speed using the sck command using a JTAG3 compatible programmer (bug was introduced when we added SIB read support
• Make it possible to use the sck command to set the clock speed in Hz, kHz, and MHz.
• running sck without a second argument now prints the clock speed in kHz as well as us.
• jtag3_get_vtarget and jtag3_get_sck_period added to jtag3.c

TODO:
Test all JTAG2, stk500v2, and stk600 compatible programmers I have to make sure all of them can read the target voltage in terminal mode using vtarg, and clock speed using sck. When this is done (and missing programmers added), I'll mark this PR as "ready to review".

Resolves issue #1329

[mcuee]

It seems to work well for STK500v2, tested under Linux.

mcuee@UbuntuSwift3 ~/build/avr/avrdude_bin $ ./avrdude_pr1587 -C ./avrdude_pr1587.conf -c stk500v2 -P ch340 -p m32a -qqt
avrdude> vtarg
Vtarget = 5.1 V
avrdude> sck
SCK period = 8.7 us
SCK freq   = 115 kHz
avrdude> sck 230 kHz
Syntax: sck <value>
Function: set the SCK period in us or frequency in [kM]Hz
avrdude> sck 8.7
avrdude> sck
SCK period = 17.4 us
SCK freq   = 57 kHz
avrdude> sck 3
avrdude> sck
SCK period = 8.7 us
SCK freq   = 115 kHz
avrdude> sck 20
avrdude> sck
SCK period = 22.2 us
SCK freq   = 44 kHz
avrdude> sck 33
avrdude> sck
SCK period = 35.3 us
SCK freq   = 28 kHz
avrdude> quit

[MCUdude]

Sorry for the slow pace, but this is a quite busy week for me with very little spare time. I'll pick this up again as soon as I have more time.

[MCUdude]

I think this PR is ready for review. Tested with a Curiosity Nano (UPDI), an AVR Dragon (ISP, JTAG), and STK600 (ISP, HVPP).

[stefanrueger]

Maybe parse as above? Consider introducing a function so the same code doesn't need to be there twice

[MCUdude]

I replaced the entire implementation with the one you suggested. Would be great if you could have a look at it again.

[stefanrueger]

Does it still work as intended?

[MCUdude]

Forgot about this. Will check this evening.

[MCUdude]

Appears to work perfectly fine!

$ ./avrdude -cstk600 -patmega2560 -t
avrdude: AVR device initialized and ready to accept instructions
avrdude: device signature = 0x1e9801 (probably m2560)

avrdude: processing -t interactive terminal
avrdude> sck
SCK period = 3.4 us
SCK freq   = 296 kHz
avrdude> sck 100
avrdude> sck
SCK period = 100.0 us
SCK freq   = 10 kHz
avrdude> sck 120k
avrdude> sck
SCK period = 8.4 us
SCK freq   = 119 kHz
avrdude> sck 1MHz
avrdude> sck
SCK period = 1.0 us
SCK freq   = 1000 kHz
avrdude> sck 8us
avrdude> sck
SCK period = 8.0 us
SCK freq   = 125 kHz
avrdude> sck 6u
avrdude error: (sck) invalid bit clock unit u
avrdude> sck
SCK period = 8.0 us
SCK freq   = 125 kHz
avrdude> sck 32
avrdude> sck
SCK period = 32.0 us
SCK freq   = 31 kHz

[mcuee]

I should be able to give this PR some simple tests over the weekend when I have access to my HW.

[mcuee]

Looks good to me.

PS>.\avrdude_pr1587v1 -C .\avrdude_pr1587v1.conf -c avrispmkii -p m328pb -qqt
avrdude> sck
SCK period = 8.0 us
SCK freq   = 125 kHz
avrdude> sck 100
avrdude> sck
SCK period = 100.4 us
SCK freq   = 9 kHz
avrdude> sck 120k
avrdude> sck
SCK period = 10.4 us
SCK freq   = 96 kHz
avrdude> sck 1MHz
avrdude> sck
SCK period = 1.0 us
SCK freq   = 1000 kHz
avrdude> sck 8us
avrdude> sck
SCK period = 8.0 us
SCK freq   = 125 kHz
avrdude> sck 6u
avrdude_pr1587v1 error: (sck) invalid bit clock unit u
avrdude> sck
SCK period = 8.0 us
SCK freq   = 125 kHz
avrdude> sck 32
avrdude> sck
SCK period = 32.1 us
SCK freq   = 31 kHz
avrdude> quit