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/* Copyright 2020 Dimitris Papavasiliou <dpapavas@protonmail.ch>
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <https://www.gnu.org/licenses/>.
*/
#include <spi_master.h>
#include "quantum.h"
#include "split_util.h"
#include "transport.h"
#include "timer.h"
#include "lagrange.h"
struct led_context {
led_t led_state;
layer_state_t layer_state;
};
uint8_t transceive(uint8_t b) {
for (SPDR = b ; !(SPSR & _BV(SPIF)) ; );
return SPDR;
}
/* The SPI bus, doesn't have any form of protocol built in, so when
* the other side isn't present, any old noise on the line will appear
* as matrix data. To avoid interpreting data as keystrokes, we do a
* simple n-way (8-way here) handshake before each scan, where each
* side sends a prearranged sequence of bytes. */
bool shake_hands(bool master) {
const uint8_t m = master ? 0xf8 : 0;
const uint8_t a = 0xa8 ^ m, b = 0x50 ^ m;
bool synchronized = true;
uint8_t i;
i = SPSR;
i = SPDR;
do {
/* Cycling the SS pin on each attempt is necessary, as it
* resets the AVR's SPI core and guarantees proper
* alignment. */
if (master) {
writePinLow(SPI_SS_PIN);
}
for (i = 0 ; i < 8 ; i += 1) {
if (transceive(a + i) != b + i) {
synchronized = false;
break;
}
}
if (master) {
writePinHigh(SPI_SS_PIN);
}
} while (i < 8);
return synchronized;
}
bool transport_master(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
const struct led_context context = {
host_keyboard_led_state(),
layer_state
};
uint8_t i;
/* We shake hands both before and after transmitting the matrix.
* Doing it before transmitting is necessary to ensure
* synchronization: Due to the master-slave nature of the SPI bus,
* the master calls the shots. If we just go ahead and start
* clocking bits, the slave side might be otherwise engaged at
* that moment, so we'll initially read zeros, or garbage. Then
* when the slave gets around to transmitting its matrix, we'll
* misinterpret the keys it sends, leading to spurious
* keypresses. */
/* The handshake forces the master to wait for the slave to be
* ready to start transmitting. */
do {
shake_hands(true);
/* Receive the matrix from the other side, while transmitting
* LED and layer states. */
spi_start(SPI_SS_PIN, 0, 0, 4);
for (i = 0 ; i < sizeof(matrix_row_t[MATRIX_ROWS / 2]) ; i += 1) {
spi_status_t x;
x = spi_write(i < sizeof(struct led_context) ?
((uint8_t *)&context)[i] : 0);
if (x == SPI_STATUS_TIMEOUT) {
return false;
}
((uint8_t *)slave_matrix)[i] = (uint8_t)x;
}
spi_stop();
/* In case of errors during the transmission, e.g. if the
* cable was disconnected and since there is no inherent
* error-checking protocol, we would simply interpret noise as
* data. */
/* To avoid this, both sides shake hands after transmitting.
* If synchronization was lost during transmission, the (first)
* handshake will fail. In that case we go around and
* re-transmit. */
} while (!shake_hands(true));
return true;
}
void transport_slave(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
static struct led_context context;
struct led_context new_context;
uint8_t i;
/* Do the reverse of master above. Note that timing is critical,
* so interrupts must be turned off. */
cli();
shake_hands(false);
do {
for (i = 0 ; i < sizeof(matrix_row_t[MATRIX_ROWS / 2]) ; i += 1) {
uint8_t b;
b = transceive(((uint8_t *)slave_matrix)[i]);
if (i < sizeof(struct led_context)) {
((uint8_t *)&new_context)[i] = b;
}
}
} while (!shake_hands(false));
sei();
/* Update the layer and LED state if necessary. */
if (!isLeftHand) {
if (context.led_state.raw != new_context.led_state.raw) {
context.led_state.raw = new_context.led_state.raw;
led_update_kb(context.led_state);
}
if (context.layer_state != new_context.layer_state) {
context.layer_state = new_context.layer_state;
layer_state_set_kb(context.layer_state);
}
}
}
void transport_master_init(void) {
/* We need to set the SS pin as output as the handshake logic
* above depends on it and the SPI master driver won't do it
* before we call spi_start(). */
writePinHigh(SPI_SS_PIN);
setPinOutput(SPI_SS_PIN);
spi_init();
shake_hands(true);
}
void transport_slave_init(void) {
/* The datasheet isn't very clear on whether the internal pull-up
* is selectable when the SS pin is used by the SPI slave, but
* experimentations shows that it is, at least on the ATMega32u4.
* We enable the pull-up to guard against the case where both
* halves end up as slaves. In that case the SS pin would
* otherwise be floating and free to fluctuate due to picked up
* noise, etc. When reading low it would make both halves think
* they're asserted making the MISO pin an output on both ends and
* leading to potential shorts. */
setPinInputHigh(SPI_SS_PIN);
setPinInput(SPI_SCK_PIN);
setPinInput(SPI_MOSI_PIN);
setPinOutput(SPI_MISO_PIN);
SPCR = _BV(SPE);
shake_hands(false);
}
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