Correct SGB packet delay durations (#460)

Co-authored-by: Eldred Habert <[email protected]>

Author: pinobatch

src/SGB_Command_Packet.md

@@ -8,30 +8,37 @@ Game Boy keyboard matrix (which still works).
 ## Transferring Bits
 
 A command packet transfer must be initiated by setting both P14 and P15
-to LOW, this will reset and start the SNES packet receiving program.
+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
 ```
 
-Data and reset pulses must be kept LOW for at least 5us. P14 and P15
-must be kept both HIGH for at least 15us between any pulses. Obviously,
-it'd be no good idea to access the JOYPAD register during the transfer,
+[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, be sure to disable that interrupt during the
+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).
 
+[sgb-speedtest]: https://github.com/zlago/sgb-speedtest/
+
 ## Transferring Packets
 
 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 stop bit. The structure of normal
-packets thus is:
+"0" bit must be transferred as a stop bit.
+These 130 bit periods correspond to at least 2600 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)
@@ -47,5 +54,6 @@ parameter bytes are used, then further packet(s) will follow, as such:
 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. A 60ms (4
-frames) delay should be invoked between each packet transfer.
+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.