Embedded Systems Tutoring

Embedded systems courses mix hardware and software, so you can be stuck in either one. A program that looks right in C can still fail because a timer was never enabled, a pin was never configured, or a variable changed inside an interrupt without being declared volatile. Often the bug is not in the logic at all. It is in how the code talks to a register. STEM Tutor 360 teaches students to read the datasheet and reference manual, find the right register and bit, and test one peripheral at a time.

Typical topics are C for embedded work, bit manipulation, memory maps, GPIO, timers and PWM, interrupts, ADC, UART, SPI, I2C, low-power modes, and debugging with a serial console or a hardware debugger. Boards vary: Arduino, STM32 Nucleo and Discovery kits, and other ARM Cortex-M boards are all common. Sessions work from your lab handout, your code, your board's documentation, and the project deadline you are actually facing.

A typical timer problem, worked

A timer runs from a 72 MHz clock. It has a 16-bit prescaler (PSC) and a 16-bit auto-reload register (ARR), and the update frequency is $$\dfrac{f_{clk}}{(\text{PSC} + 1) \times (\text{ARR} + 1)}$$ Set it up for a 1 kHz interrupt, then for 1 Hz.

For 1 kHz the total divide ratio is $72{,}000{,}000 / 1000 = 72{,}000$. Choose $\text{PSC} + 1 = 72$, so the counter ticks at 1 MHz, and $\text{ARR} + 1 = 1000$. The register values are $\text{PSC} = 71$ and $\text{ARR} = 999$. Check: $72{,}000{,}000 / (72 \times 1000) = 1000\ \text{Hz}$.

For 1 Hz the ratio is 72,000,000, far too large for one 16-bit counter, which holds at most 65,536 counts. Split it: $\text{PSC} + 1 = 7200$ gives a 10 kHz tick, and $\text{ARR} + 1 = 10{,}000$ gives 1 Hz. The register values are $\text{PSC} = 7199$ and $\text{ARR} = 9999$. Check: $72{,}000{,}000 / (7200 \times 10{,}000) = 1\ \text{Hz}$.

Two habits prevent many timer bugs. A register holds the divisor minus one, and the clock feeding the timer is often not the CPU clock, so confirm it in the clock tree of your chip's reference manual.

Where students usually get stuck

• Off-by-one errors, since a register often holds the divisor minus one
• Writing to a peripheral's registers before enabling its clock
• Shared variables changed in an ISR without volatile, or long work done inside the ISR
• Overwriting a whole register when only one bit should change, instead of using a mask
• UART settings that do not match the other end (baud rate, parity, stop bits), or an I2C address given in the wrong 7-bit or 8-bit form