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Fix SGB output lines documentation #547

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Mar 20, 2024
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9 changes: 5 additions & 4 deletions src/SGB_Command_Packet.md
Original file line number Diff line number Diff line change
@@ -1,16 +1,17 @@
# Command Packet Transfers

Command packets are transferred from the Game Boy to the SNES by using bits 6 and 7 of of [the `JOYP` register][JOYP].
Command packets are transferred from the Game Boy to the SNES by using bits 4 and 5 of of [the `JOYP` register][JOYP].
These lines are normally used to select one of the two button groups (which [still works as usual][JOYP]).

## Transferring Bits

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.
A command packet transfer must be initiated by setting [`JOYP`][JOYP] bits 4 and 5 both to 0; this will reset and start the ICD2 packet receiving circuit.
Data is then transferred (LSB first), setting bit 4 to 0 will indicate a `0` bit, and setting bit 5 to 0 will indicate a `1` bit.
For example:

{{#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.
[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 4 and 5 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
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4 changes: 2 additions & 2 deletions src/imgs/sgb_bits.svg
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