center/fw: Add backchannel infrastructure

Untested!

1 files changed, 53 insertions, 0 deletions

diff --git a/center_fw/transmit.c b/center_fw/transmit.c
new file mode 100644
index 0000000..c31c833
--- /dev/null
+++ b/center_fw/transmit.c
@@ -0,0 +1,53 @@
+
+#include "global.h"
+#include "transmit.h"
+#include "8b10b.h"
+
+struct {
+    uint8_t *buf;
+    size_t pos, len;
+    uint16_t current_symbol;
+    struct state_8b10b_enc enc;
+} tx_state = {0};
+
+volatile uint32_t tx_overflow_cnt = 0;
+
+void tx_init(uint8_t *tx_buf) {
+    tx_state.buf = tx_buf;
+    tx_state.pos = 0;
+    tx_state.current_symbol = 0;
+    xfr_8b10b_encode_reset(&tx_state.enc);
+}
+
+int tx_transmit(size_t len) {
+    if (!tx_state.buf)
+        return TX_ERR_UNINITIALIZED;
+
+    if (tx_state.len) {
+        tx_overflow_cnt++;
+        return TX_ERR_BUSY;
+    }
+
+    tx_state.len = len;
+    tx_state.current_symbol = 1;
+    return 0;
+}
+
+int tx_next_bit() { 
+    if (!tx_state.len)
+        return TX_IDLE;
+
+    int sym = tx_state.current_symbol;
+    if (sym == 1) /* We're transmitting the first bit of a new frame now. */
+        sym = xfr_8b10b_encode(&tx_state.enc, tx_state.buf[tx_state.pos++]) | (1<<10);
+
+    int bit = sym&1;
+    sym >>= 1;
+
+    if (sym == 1 && tx_state.pos == tx_state.len)
+        /* We're transmitting the last bit of a transmission now. Reset state. */
+        tx_state.pos = tx_state.len = sym = 0;
+
+    tx_state.current_symbol = sym;
+    return bit;
+}