[Keyboard] Fix and improve SPI transport in the Lagrange (#12606)
Co-authored-by: Dimitris Papavasiliou <dpapavas@gmail.com>
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1 changed files with 57 additions and 24 deletions
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@ -18,6 +18,7 @@
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#include "quantum.h"
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#include "split_util.h"
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#include "transport.h"
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#include "timer.h"
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#include "lagrange.h"
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@ -32,15 +33,16 @@ uint8_t transceive(uint8_t b) {
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return SPDR;
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}
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/* The SPI bus, doens't have any form of protocol built in, so when
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/* The SPI bus, doesn't have any form of protocol built in, so when
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* the other side isn't present, any old noise on the line will appear
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* as matrix data. To avoid interpreting data as keystrokes, we do a
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* simple n-way (8-way here) handshake before each scan, where each
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* side sends a prearranged sequence of bytes. */
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void shake_hands(bool master) {
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bool shake_hands(bool master) {
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const uint8_t m = master ? 0xf8 : 0;
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const uint8_t a = 0xa8 ^ m, b = 0x50 ^ m;
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bool synchronized = true;
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uint8_t i;
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@ -48,7 +50,7 @@ void shake_hands(bool master) {
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i = SPDR;
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do {
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/* Cylcling the SS pin on each attempt is necessary, as it
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/* Cycling the SS pin on each attempt is necessary, as it
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* resets the AVR's SPI core and guarantees proper
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* alignment. */
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@ -58,6 +60,7 @@ void shake_hands(bool master) {
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for (i = 0 ; i < 8 ; i += 1) {
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if (transceive(a + i) != b + i) {
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synchronized = false;
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break;
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}
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}
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@ -66,9 +69,11 @@ void shake_hands(bool master) {
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writePinHigh(SPI_SS_PIN);
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}
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} while (i < 8);
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return synchronized;
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}
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bool transport_master(matrix_row_t matrix[]) {
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bool transport_master(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
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const struct led_context context = {
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host_keyboard_led_state(),
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layer_state
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@ -76,11 +81,25 @@ bool transport_master(matrix_row_t matrix[]) {
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uint8_t i;
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/* Shake hands and then receive the matrix from the other side,
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* while transmitting LED and layer states. */
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/* We shake hands both before and after transmitting the matrix.
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* Doing it before transmitting is necessary to ensure
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* synchronization: Due to the master-slave nature of the SPI bus,
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* the master calls the shots. If we just go ahead and start
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* clocking bits, the slave side might be otherwise engaged at
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* that moment, so we'll initially read zeros, or garbage. Then
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* when the slave gets around to transmitting its matrix, we'll
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* misinterpret the keys it sends, leading to spurious
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* keypresses. */
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/* The handshake forces the master to wait for the slave to be
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* ready to start transmitting. */
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do {
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shake_hands(true);
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/* Receive the matrix from the other side, while transmitting
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* LED and layer states. */
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spi_start(SPI_SS_PIN, 0, 0, 4);
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for (i = 0 ; i < sizeof(matrix_row_t[MATRIX_ROWS / 2]) ; i += 1) {
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@ -93,15 +112,27 @@ bool transport_master(matrix_row_t matrix[]) {
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return false;
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}
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((uint8_t *)matrix)[i] = (uint8_t)x;
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((uint8_t *)slave_matrix)[i] = (uint8_t)x;
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}
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spi_stop();
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/* In case of errors during the transmission, e.g. if the
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* cable was disconnected and since there is no inherent
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* error-checking protocol, we would simply interpret noise as
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* data. */
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/* To avoid this, both sides shake hands after transmitting.
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* If synchronization was lost during transmission, the (first)
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* handshake will fail. In that case we go around and
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* re-transmit. */
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} while (!shake_hands(true));
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return true;
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}
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void transport_slave(matrix_row_t matrix[]) {
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void transport_slave(matrix_row_t master_matrix[], matrix_row_t slave_matrix[]) {
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static struct led_context context;
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struct led_context new_context;
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@ -113,15 +144,17 @@ void transport_slave(matrix_row_t matrix[]) {
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cli();
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shake_hands(false);
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do {
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for (i = 0 ; i < sizeof(matrix_row_t[MATRIX_ROWS / 2]) ; i += 1) {
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uint8_t b;
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b = transceive(((uint8_t *)matrix)[i]);
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b = transceive(((uint8_t *)slave_matrix)[i]);
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if (i < sizeof(struct led_context)) {
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((uint8_t *)&new_context)[i] = b;
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}
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}
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} while (!shake_hands(false));
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sei();
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