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diff --git a/tmk_core/protocol/ps2_usart.c b/tmk_core/protocol/ps2_usart.c
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+/*
+Copyright 2010,2011,2012,2013 Jun WAKO <wakojun@gmail.com>
+
+This software is licensed with a Modified BSD License.
+All of this is supposed to be Free Software, Open Source, DFSG-free,
+GPL-compatible, and OK to use in both free and proprietary applications.
+Additions and corrections to this file are welcome.
+
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+* Redistributions of source code must retain the above copyright
+ notice, this list of conditions and the following disclaimer.
+
+* Redistributions in binary form must reproduce the above copyright
+ notice, this list of conditions and the following disclaimer in
+ the documentation and/or other materials provided with the
+ distribution.
+
+* Neither the name of the copyright holders nor the names of
+ contributors may be used to endorse or promote products derived
+ from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGE.
+*/
+
+/*
+ * PS/2 protocol USART version
+ */
+
+#include <stdbool.h>
+#include <avr/interrupt.h>
+#include <util/delay.h>
+#include "ps2.h"
+#include "ps2_io.h"
+#include "print.h"
+
+
+#define WAIT(stat, us, err) do { \
+ if (!wait_##stat(us)) { \
+ ps2_error = err; \
+ goto ERROR; \
+ } \
+} while (0)
+
+
+uint8_t ps2_error = PS2_ERR_NONE;
+
+
+static inline uint8_t pbuf_dequeue(void);
+static inline void pbuf_enqueue(uint8_t data);
+static inline bool pbuf_has_data(void);
+static inline void pbuf_clear(void);
+
+
+void ps2_host_init(void)
+{
+ idle(); // without this many USART errors occur when cable is disconnected
+ PS2_USART_INIT();
+ PS2_USART_RX_INT_ON();
+ // POR(150-2000ms) plus BAT(300-500ms) may take 2.5sec([3]p.20)
+ //_delay_ms(2500);
+}
+
+uint8_t ps2_host_send(uint8_t data)
+{
+ bool parity = true;
+ ps2_error = PS2_ERR_NONE;
+
+ PS2_USART_OFF();
+
+ /* terminate a transmission if we have */
+ inhibit();
+ _delay_us(100); // [4]p.13
+
+ /* 'Request to Send' and Start bit */
+ data_lo();
+ clock_hi();
+ WAIT(clock_lo, 10000, 10); // 10ms [5]p.50
+
+ /* Data bit[2-9] */
+ for (uint8_t i = 0; i < 8; i++) {
+ _delay_us(15);
+ if (data&(1<<i)) {
+ parity = !parity;
+ data_hi();
+ } else {
+ data_lo();
+ }
+ WAIT(clock_hi, 50, 2);
+ WAIT(clock_lo, 50, 3);
+ }
+
+ /* Parity bit */
+ _delay_us(15);
+ if (parity) { data_hi(); } else { data_lo(); }
+ WAIT(clock_hi, 50, 4);
+ WAIT(clock_lo, 50, 5);
+
+ /* Stop bit */
+ _delay_us(15);
+ data_hi();
+
+ /* Ack */
+ WAIT(data_lo, 50, 6);
+ WAIT(clock_lo, 50, 7);
+
+ /* wait for idle state */
+ WAIT(clock_hi, 50, 8);
+ WAIT(data_hi, 50, 9);
+
+ idle();
+ PS2_USART_INIT();
+ PS2_USART_RX_INT_ON();
+ return ps2_host_recv_response();
+ERROR:
+ idle();
+ PS2_USART_INIT();
+ PS2_USART_RX_INT_ON();
+ return 0;
+}
+
+uint8_t ps2_host_recv_response(void)
+{
+ // Command may take 25ms/20ms at most([5]p.46, [3]p.21)
+ uint8_t retry = 25;
+ while (retry-- && !pbuf_has_data()) {
+ _delay_ms(1);
+ }
+ return pbuf_dequeue();
+}
+
+uint8_t ps2_host_recv(void)
+{
+ if (pbuf_has_data()) {
+ ps2_error = PS2_ERR_NONE;
+ return pbuf_dequeue();
+ } else {
+ ps2_error = PS2_ERR_NODATA;
+ return 0;
+ }
+}
+
+ISR(PS2_USART_RX_VECT)
+{
+ // TODO: request RESEND when error occurs?
+ uint8_t error = PS2_USART_ERROR; // USART error should be read before data
+ uint8_t data = PS2_USART_RX_DATA;
+ if (!error) {
+ pbuf_enqueue(data);
+ } else {
+ xprintf("PS2 USART error: %02X data: %02X\n", error, data);
+ }
+}
+
+/* send LED state to keyboard */
+void ps2_host_set_led(uint8_t led)
+{
+ ps2_host_send(0xED);
+ ps2_host_send(led);
+}
+
+
+/*--------------------------------------------------------------------
+ * Ring buffer to store scan codes from keyboard
+ *------------------------------------------------------------------*/
+#define PBUF_SIZE 32
+static uint8_t pbuf[PBUF_SIZE];
+static uint8_t pbuf_head = 0;
+static uint8_t pbuf_tail = 0;
+static inline void pbuf_enqueue(uint8_t data)
+{
+ uint8_t sreg = SREG;
+ cli();
+ uint8_t next = (pbuf_head + 1) % PBUF_SIZE;
+ if (next != pbuf_tail) {
+ pbuf[pbuf_head] = data;
+ pbuf_head = next;
+ } else {
+ print("pbuf: full\n");
+ }
+ SREG = sreg;
+}
+static inline uint8_t pbuf_dequeue(void)
+{
+ uint8_t val = 0;
+
+ uint8_t sreg = SREG;
+ cli();
+ if (pbuf_head != pbuf_tail) {
+ val = pbuf[pbuf_tail];
+ pbuf_tail = (pbuf_tail + 1) % PBUF_SIZE;
+ }
+ SREG = sreg;
+
+ return val;
+}
+static inline bool pbuf_has_data(void)
+{
+ uint8_t sreg = SREG;
+ cli();
+ bool has_data = (pbuf_head != pbuf_tail);
+ SREG = sreg;
+ return has_data;
+}
+static inline void pbuf_clear(void)
+{
+ uint8_t sreg = SREG;
+ cli();
+ pbuf_head = pbuf_tail = 0;
+ SREG = sreg;
+}