DIY Oscilloscope

A friend of mine convinced me to buy a kit to build a simple oscilloscope. At first, I thought it would not be very useful because the analog bandwidth is not more than 1 MHz. On the other hand, most of the times I have needed an oscilloscope, it has been to debug simple and slow data lines, such as SPI and RS232, so I decided to give it a chance. It was a real challenge to solder, because there were a lot of really small surface mount components (mostly capacitors and resistors) to solder. After a few evenings of soldering I was ready to try it out.

Here are a few pictures. First of all I measured the oscilloscope's built-in 500 Hz test signal:



Second, I measured the 9600 bps serial output from my HP48SX calculator. You can see that the output is not proper RS232 signal levels. I guess this is a way to save battery power on the calculator:



After a little testing, I am really impressed by the oscilloscope, especially as it was priced just over 500 swedish kronor (less than $80). It was also fun and interesting to solder all the small parts!

Serial port insanity.

After oceans of time I am back again. The kernel now implements the following system calls:

• rtos_yield (yield to another process)
  
• rtos_allocate (allocate a 64 byte message buffer from the buffer collection)
• rtos_transmit (hand over a message to another process)
• rtos_accept (accept a message from another process)
• rtos_dispose (dispose of a buffer)
• rtos_delay (delay a specified number of system ticks)
  

The final names are yet to be decided. I also implemented support for the Cortex-M3 SysTick timer (which is mandatory in all Cortex-M3 based chips). It's exception handler increases the system tick counter and releases processes that have blocked themselves using rtos_delay.

I also started implementing support for USART2 in the STM32. That is because this USART is connected to the RS232 DSUB connector on the STM32-P103 dev board I use. To start with, I hooked the port up to my laptop using a RS232-USB adapter, trying a number of different serial port sniffer/terminal programs to see the dev board output. With little success! After a few hours, I grabbed my old HP 48SX calculator, which has an RS232 port on it. A few simple commands allow me to configure the serial port settings and then everything worked perfectly fine!

To set up USART2 for just using the transmitter, you have to do the following:

  /* Enable PCLK1 (USART2 is on APB1). */
  *STM32_RCC_APB1ENR |= (0x1 << 17); /* USART2EN - Enable USART2 clock. */

  /* Enable PCLK2 (GPIOA is used by USART2). */
  *STM32_RCC_APB2ENR |= (1 << 2); /* IOPAEN - Enable GPIOA clock. */
  
  /* Set up the PA2 pin for alternate function push/pull. This is the
     TX pin for USART2. */
  *STM32_GPIOA_CRL =
     (2 << 10) | /* CNF2  - Alternate function output push/pull. */
     (1 << 8);   /* MODE2 - Output mode, max 10 MHz. */

  /* Set up the Baud Rate Register for 9600 bps. See RM0008, chapter
     24.3.4. The value to be programmed in the BRR should be 234.375.
     DIV_Mantissa=234 DIV_Fraction=6. */
  *STM32_USART2_BRR = (234 << 4) | 6; /* 9600 bps */ 

  /* After-reset default values for other registers are suitable for an
     8-N-1 setup without enabled interrupts and DMA. */
  
  /* Enable the USART and the transmitter. */
  *STM32_USART2_CR1 = (1 << 13) | (1 << 3);

As I mentioned, I did not set up the receiver as this code is just a simple test to output characters over a serial line. Most of the lines are pretty self-explanatory I think, but there are a few thing worth noting. First, you have to enable clocking not only of the USART, but also for the corresponding GPIO port (in this case GPIOA). Second, when setting up the baud rate register, the fraction value is 16 times the fraction (0.375) i.e. 6. This is the value to be used when the clock frequency used for APB1 is 36 MHz (which is the maximum allowed frequency for APB1, and half the system clock). Do not forget to set up the APB1 clock prescaler!