How to use
- Copy the installation command and review the source package before installing it in your own agent environment.
- Check the source document for required tools, credentials and supported hosts.
- Describe your input and desired deliverable; review the output against the source material.
npx skills add https://github.com/mohitmishra786/low-level-dev-skills --skill uart-serial-baremetalBefore you start
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Original skill document
Rendered from the source repository. Headings, examples, code, tables, links, and referenced images are preserved.
UART Serial (Bare-Metal)
Purpose
Implement UART/USART for debug console and device communication: baud rate calculation, 8N1 framing, polling and interrupt-driven I/O, overrun handling, and optional DMA basics.
When to Use
- First
printf/log output on new hardware - Serial protocol to sensor/module
- Replacing blocking HAL_UART with minimal driver
- Fixing garbled or missing characters
Workflow
1. Baud rate (STM32)
BRR = pclk / (16 * baud) /* oversampling by 16 — check RM for USART */c
void usart2_init(uint32_t pclk, uint32_t baud) {
RCC->APB1ENR |= RCC_APB1ENR_USART2EN;
/* GPIO PA2/PA3 AF — see gpio-baremetal */
USART2->BRR = pclk / baud; /* simplified — RM has fractional formula */
USART2->CR1 = USART_CR1_TE | USART_CR1_RE | USART_CR1_UE;
}Verify pclk from actual clock tree (SystemCoreClock, APB prescaler).
2. Polling TX/RX
c
void uart_putc(USART_TypeDef *u, char c) {
while (!(u->SR & USART_SR_TXE))
;
u->DR = (uint8_t)c;
}
char uart_getc(USART_TypeDef *u) {
while (!(u->SR & USART_SR_RXNE))
;
return (uint8_t)u->DR;
}3. Interrupt-driven RX ring buffer
c
void USART2_IRQHandler(void) {
if (USART2->SR & USART_SR_RXNE) {
uint8_t b = USART2->DR;
rb_push(&rx_rb, b);
}
if (USART2->SR & USART_SR_ORE) {
(void)USART2->DR; /* clear overrun — required on STM32 */
}
}4. retarget printf (newlib)
c
int _write(int fd, char *ptr, int len) {
(void)fd;
for (int i = 0; i < len; i++)
uart_putc(USART2, ptr[i]);
return len;
}Link with --specs=nosys.specs or provide full syscalls.
5. Agent usage
/uart-serial-baremetal Calculate USART BRR for 115200 at 84 MHz PCLKCommon Problems
| Symptom | Cause | Fix |
|---|---|---|
| Garbage chars | Wrong baud/PCLK | Recompute BRR; check APB divider |
| Lost bytes | ORE not cleared | Read DR on ORE; use IRQ + ringbuf |
| No output | TX pin not AF | GPIO alternate function |
printf hangs | _write missing | Implement retarget |
Related Skills
skills/baremetal/gpio-baremetal— TX/RX pin muxskills/baremetal/interrupts-and-exceptions-baremetal— USART IRQskills/baremetal/dma-baremetal— UART RX DMAskills/embedded/openocd-jtag— semihosting alternative
