can2040: Minor comment updates

Signed-off-by: Kevin O'Connor <[email protected]>

Author: KevinOConnor

docs/Code_Overview.md

@@ -90,7 +90,8 @@ A secondary task of the PIO "tx" state machine is to inject an ack bit
 to acknowledge messages received from other CAN bus nodes.  To perform
 this task, the ARM core fills the tx fifo with a single dominant bit
 and arranges for the "tx" state machine to start after a "matched"
-irq.
+irq.  After the ack bit is transmitted the PIO "tx" state machine will
+raise an "ackdone" signal (PIO irq 3).
 
 # ARM core state tracking
 

pio/can2040.pio

@@ -126,7 +126,7 @@ tx_align:
 .program sm_tx_ack
     // CPU first disables state machine execution
     // CPU clears irq 2 and irq 3
-    // CPU changes instruction at tx_loop from "out x, 1" to:
+    // CPU changes instruction at tx_got_recessive from "out x, 1" to:
     irq wait 3
     // CPU then loads tx fifo with ack (single off bit)
     set pins, 1

src/can2040.c

@@ -942,19 +942,19 @@ report_note_discarding(struct can2040 *cd)
     pio_irq_set(cd, SI_MAYTX | SI_TXPENDING);
 }
 
-// Received PIO rx "ackdone" irq
+// Received PIO "ackdone" irq ("tx" state machine completed ack bit transmit)
 static void
 report_line_ackdone(struct can2040 *cd)
 {
-    // Setup "matched" irq for fast rx callbacks
+    // Setup rx eof "matched" signal for fast rx callbacks
     uint32_t bits = (cd->parse_crc_bits << 8) | 0x7f;
     pio_match_check(cd, pio_match_calc_key(bits, cd->parse_crc_pos + 8));
     // Schedule next transmit (so it is ready for next frame line arbitration)
     uint32_t check_txpending = tx_schedule_transmit(cd);
     pio_irq_set(cd, SI_MAYTX | SI_MATCHED | check_txpending);
 }
 
-// Received PIO "matched" irq
+// Received PIO "matched" irq ("match" state machine detected eof match_key)
 static void
 report_line_matched(struct can2040 *cd)
 {