Without enabling/using UART interrupt, how to keep transmitting data through UART, without data being lost/not sent. When I send the data by following way,
li a0, 0xaa;
li t0, UART0_BASE;
sw a0, UART_TXDATA_OFFSET(t0); // memory mapped addr of txdata register
the byte 0xaa is sent only once and tx pin stays high as long the loops executes. I understand that this is NOT right to send continuously as the tx buffer might not have space and the txfifo full bit might be set.
But I would expect the transmission to continue once the fifo has space instead of sending just once and never again. Coz, this loop though executed for 3-5s, it just sends once and don’t see any activity in tx pin after for more than 3secs.
Hi @bsvtgc There could be other bits of TXDATA which are reserved and not read-only that should not be changed. You might try doing a read-modify-write style of transaction and making sure only to effect change on bits TXDATA[7:0].
In place of your sw …
li t2, 0xff # txdata[7:0]
not t2, t2;
lw t1, UART_TXDATA_OFFSET(t0); # read
and t1, t1, t2; # out with the old ...
or t1, t1, a0; # ... in with the new
sw t1, UART_TXDATA_OFFSET(t0); # write
As you mentioned, you might also try first using a busy loop like wait_full below to stall until ready, before getting into the read-modify-write step.
The UART response did NOT change much with suggested changes. Adjusting txcnt in txctrl register too didn’t alter the response.
A brief about my code:
I run the code from RAM loading at 0x8000_0000 at the very start after setting sp, startup code disables all interrupt.
csrw mstatus, zero;
csrw mie, zero;
Later the m-timer interrupt alone is enabled
li t0, MIE_MTIE;
csrw mie, t0;
li t0, MSTATUS_MIE;
csrw mstatus, t0;
to switch (time multiplex) between led-blink and uart-tx ( without gpio/uart interrupts).
I recently realized that the initial PLIC settings have had an effect. I did NOT pay attention to initial PLIC settings (from reset to until flash boot and followed by opened/gdb loads my program in RAM 0x8000_0000).
When I programmed the same UART code on flash at 0x2000_0000, and boot from here, it works all the time. So all I could conclude is the PLIC settings have had an effect though I could n’t revert and find back exactly what was the cause as I had overwritten the code in flash with my new uart-tx code.