;;********************************************************************************
;; @module ps2_keyboard 
;; @type driver
;; @details:
;;  Support for a PS2 Keyboard using the shift register of a 6522 VIA
;;  @section Reading a scancode/command answer
;;   Pressing a key causes 11 bits to be sent: 1 start - 8 scancode - 1 parity - 1 stop 
;;   The VIA is set up to interrupt after 8 bits have been shifted into the shift register
;;   from the external clock pulses of the keyboard (additional hardware required to 
;;   address the hardware bug of the VIA, where bit get lost when the external clock 
;;   transition happens during falling edge of PHI2). After reading the shift register,
;;   the VIAs T2 timer is set to interrupt after the last 3 bits have been shifted in,
;;   which takes about ~230ms.
;;	 T2 is used because it leaves to more versatile T1 available for something else

;;  @section Processing a scancode
;;	 The scancode may be processed by storing the address of a handler subroutine in
;;	 ps2kb::scancode_handler. This handler can load the scancode byte from ps2kb::scancode.
;;
;;  
;;  To use a different layout, change the enum K and the CHARS_NOMOD and CHARS_MODSHIFT
;; @warning
;;  The value with which the timer is loaded must depends on the clock frequency
;;	of the keyboard and the computer
;;  
;;********************************************************************************
.ifndef INCLUDE_PS2_KEYBOARD
INCLUDE_PS2_KEYBOARD = 1
.include "system.h65"

.scope ps2kb
Import ps2kb,init,begin_receive,send_command,scancode,status,scancode_handler
Import ps2kb, _receive_irq_shift_reg_handler, _receive_irq_timer_handler, _send_byte, _send_irq_shift_reg_handler, _send_irq_timer_handler,
Import ps2kb, send_last_bits, send_cmd, send_data, key_read, prev_status

VIA = IO1
; Enough time must pass for 3 bits to be shifted in.
TIMER_RECV = 230  ; 230 ms (@1MHz)
; Enough time must pass for one bit must be shifted out (parity),
; but at most two (parity+stop).
; After that, the interrupt must have happened because the keyboard will pull data low.
; At that point, the SR needs to be set to input again
TIMER_SEND = 230  ; 180 ms (@1MHz)
; use RA4 to pull the clock low
PULL_REG = IO::RANH
PULL_DDR = IO::DDRA

PULL_MASK_CLK = %00010000
; PULL_MASK_DAT = %00001000
NO_DATA = $ff   ; indicates that no data byte should be send

.enum STATUS
    RECEIVE_KEYS	= %10000000
    RECEIVE_ANSWER	= %01000000
    SEND_CMD   		= %00100000
    SEND_DATA   	= %00010000
    SEND_CMD_WAIT   = %00001000
    NONE			= %00000000
.endenum
.endscope

.endif  ; guard