Changeset 39e523d in avrstuff

Timestamp:

Jul 24, 2014, 1:12:42 PM (10 years ago)

Author:

Adrien Destugues <pulkomandy@…>

Branches:

main

Children:

b83dc2a

Parents:

c2c5ac9

Message:

Update docs now that I have things working.

git-svn-id: ​svn://pulkomandy.tk/avrstuff@102 c6672c3c-f6b6-47f9-9001-1fd6b12fecbe

Location:

kbd/xtk/doc

Files:

• hardware.txt (added)
• signalling.txt (modified) (3 diffs)

kbd/xtk/doc/signalling.txt

@@ -5 +5 @@
 serial link.
 
-When the link is idle, the CLK line is high (floating), and the DATA line is
-low (pulled by the keyboard).
+Both the data and clock line are open collector. When the system is idle, both
+are floating high. It is unclear wether it's up to the keyboard or PC to have
+pull-up resistors, but it is safe to have both.
 
 When the keyboard wants to send a byte:
-- It checks that the CLK line is not pulled low (the PC/XT does this when the
-receiving buffer is full).
+- It waits for the CLK and DAT lines to be high, meaning the PC is ready to
+receive a byte.
 - It sends the byte for the keycode. There is a start bit (DATA=1), followed by
 the 7-bit keycode (LSb first), and the make/break bit (set for release, clear
@@ -18 +19 @@
 read the received byte and cleared the receiving register.
 
+Timing is not critical as this is handled on the PC side using a shift register.
+The only limitation is that the clock signal is gated with the CPU clock, and
+should not go faster than 2.3MHz.
+
 The PC stores the data on the falling edge of the clock. It can also reset the
-keyboard by pulling the clock line low for too long (unsure what the exact
-timing is).
+keyboard by pulling the clock line low for too long (10ms or more). At boot and
+upon reset, the keyboard is supposed to send 0xAA when it is ready to work.
+However, I'm not sure my XT clone BIOS actually checks for that.
 
 It is up to the keyboard to manage the states of the LEDs. At power on, the LEDs
@@ -27 +33 @@
 the keyboard to handle typematic (key repeat) and a keyboard buffer, if needed.
 
-At boot, the keyboard is supposed to send 0xAA when it is ready to work.
-However, I'm not sure my XT clone BIOS actually checks for that.
+The scancodes are the same as PS/2 keyboards scan code set 1. Unfortunately, a
+lot of keyboards won't support this properly, so we use the default scan code
+set 2 (used by Windows, so it should work everywhere) and translate it back
+using a lookup table.