Touch up SGB packet documentation (#537)

Turn the diagram into a SVG, and reformat/improve the doc

Author: ISSOtm

src/SGB_Command_Packet.md

@@ -1,66 +1,58 @@
 # Command Packet Transfers
 
-Command packets (aka Register Files) are transferred from the Game Boy to
-the SNES by using P14 and P15 output lines of [the JOYPAD register](<#FF00 — P1/JOYP: Joypad>) (FF00).  These same lines are also used to select the two rows in the
-Game Boy keyboard matrix (which still works).
+Command packets are transferred from the Game Boy to the SNES by using bits 6 and 7 of of [the `JOYP` register][JOYP].
 
 ## Transferring Bits
 
-A command packet transfer must be initiated by setting both P14 and P15
-to LOW; this will reset and start the ICD2 packet receiving circuit.
-Data is then transferred (LSB first), setting P14=LOW will indicate a
-"0" bit, and setting P15=LOW will indicate a "1" bit. For example:
-
-```
-    RESET  0   0   1   1   0   1   0
-P14  --_---_---_-----------_-------_--...
-P15  --_-----------_---_-------_------...
-       ↑         ↑         ↑         ↑
-Time   0         50       100       150
-```
-
-[The boot ROM](<#Super Game Boy (SGB, SGB2)>) and licensed software keep data and reset pulses LOW for at least 5 μs and leave P14 and P15 HIGH for at least 15 μs after each pulse.
-Though the hardware is capable of receiving pulses and spaces as short as 2 μs (as tested using [sgb-speedtest]),
-following the common practice of 5-cycle pulses and 15-cycle spaces may improve reliability in some corner case that the community has not yet discovered.
-
-Obviously, it'd be no good idea to access [the joypad register](<#FF00 — P1/JOYP: Joypad>) during the transfer,
-for example, in case that your VBlank interrupt procedure reads-out
-joypad states each frame, so be sure to disable that interrupt during the
-transfer (or disable only the joypad procedure by using a software
-flag).
+A command packet transfer must be initiated by setting [`JOYP`][JOYP] bits 6 and 7 both to 0; this will reset and start the ICD2 packet receiving circuit.
+Data is then transferred (LSB first), setting bit 6 to 0 will indicate a `0` bit, and setting bit 7 to 0 will indicate a `1` bit.
+For example:
 
-[sgb-speedtest]: https://github.com/zlago/sgb-speedtest/
+{{#include imgs/sgb_bits.svg}}
+
+[The boot ROM](<#Super Game Boy (SGB, SGB2)>) and licensed software keep data and reset pulses LOW for at least 5 M-cycles and leave bit 6 and 7 both to 1 for at least 15 M-cycles after each pulse.
+Though the hardware is capable of receiving pulses and spaces as short as 2 M-cycles (as tested using [sgb-speedtest]), following the common practice of 5 M-cycle pulses and 15 M-cycle spaces may improve reliability in some corner case that the community has not yet discovered.
+
+:::tip
+
+Obviously, it'd be no good idea to modify [the joypad register][JOYP] in the middle of a transfer.
+For example, if your VBlank interrupt procedure normally reads out button states each frame, you should disable that behavior using a variable (or disable the interrupt handler entirely).
+
+:::
 
-## Transferring Packets
+:::warning
 
-Each packet is invoked by a RESET pulse, then 128 bits of data are
-transferred (16 bytes, LSB of first byte first), and finally, a
-"0" bit must be transferred as a stop bit.
-These 130 bit periods correspond to at least 2600 cycles at the recommended rate.
+The GB program should wait 60 ms (4 frames) between each packet transfer and the next, as the "bomb" tool to erase a user-drawn border can cause the SGB system software not to check for packets for 4 frames.
+
+:::
+
+## Packet format
+
+Each packet transfer is started by a "Start" pulse, then 16 bytes (128 bits) of data are transferred, the LSB of each byte first; and finally, a `0` bit must be transferred as a stop bit.
+These 130 bit periods correspond to at least 2600 M-cycles at the recommended rate.
 
 The structure of the first packet in a transmission is:
 
 1. 1 pulse: Start signal
-2. 1 byte: Header byte (Command Code \* 8 + Length)
-3. 15 bytes: Parameter Data
-4. 1 bit: Stop Bit (0)
+2. 1 byte: Header byte (<var>Command Code</var> × 8 + <var>Length</var>)
+3. 15 bytes: <var>Data</var>
+4. 1 bit: Stop Bit (`0`)
 
-The above "Length" indicates the total number of packets (1-7,
-including the first packet) which will be sent.  If more than 15
-parameter bytes are used, then further packet(s) will follow, as such:
+The above <var>Length</var> indicates the total number of packets (1-7, including the first packet) which will be sent.
+If more than 15 data bytes are used, then further packet(s) will follow, as such:
 
 1. 1 pulse: Start signal
-2. 16 bytes: Parameter Data
-3. 1 bit: Stop Bit (0)
+2. 16 bytes: <var>Data</var>
+3. 1 bit: Stop Bit (`0`)
 
-By using all 7 packets, up to 111 data bytes (15+16\*6) may be sent.
-Unused bytes at the end of the last packet don't matter.
-The GB program should wait 60 ms (4 frames) between each packet transfer and the next,
-as the "bomb" tool to erase a user-drawn border can cause the SGB system software not to check for packets for 4 frames.
+By using all 7 packets, up to 111 data bytes (15 + 16 × 6) may be sent.
 
 :::tip
 
 Bytes with no indicated purpose are simply ignored by the SGB BIOS.
 They can be set to any value (but they must still be transferred).
 
 :::
+
+[JOYP]: <#FF00 — P1/JOYP: Joypad>
+[sgb-speedtest]: https://github.com/zlago/sgb-speedtest/