I mentioned that the microcontroller has several pins. For convenience, these pins are grouped in ports of 16 pins. Each port is named with a letter: Port A, Port B, etc. and the pins within each port are named with numbers from 0 to 15.
The first thing we have to find out is which pin is connected to which LED. This information is in the STM32F3DISCOVERY User Manual (You downloaded a copy, right?). In this particular section:
Section 6.4 LEDs - Page 18
The manual says:
LD3, the North LED, is connected to the pin
PE9is the short form of: Pin 9 on Port E.
LD7, the East LED, is connected to the pin
Up to this point, we know that we want to change the state of the pins PE9 and PE11 to turn the
North/East LEDs on/off. These pins are part of Port E so we'll have to deal with the
Each peripheral has a register block associated to it. A register block is a collection of
registers allocated in contiguous memory. The address at which the register block starts is known as
its base address. We need to figure out what's the base address of the
GPIOE peripheral. That
information is in the following section of the microcontroller Reference Manual:
Section 3.2.2 Memory map and register boundary addresses - Page 51
The table says that base address of the
GPIOE register block is
Each peripheral also has its own section in the documentation. Each of these sections ends with a
table of the registers that the peripheral's register block contains. For the
GPIO family of
peripheral, that table is in:
Section 11.4.12 GPIO register map - Page 243
'BSRR' is the register which we will be using to set/reset. Its offset value is '0x18' from the base address of the 'GPIOE'. We can look up BSRR in the reference manual. GPIO Registers -> GPIO port bit set/reset register (GPIOx_BSRR).
Now we need to jump to the documentation of that particular register. It's a few pages above in:
Section 11.4.7 GPIO port bit set/reset register (GPIOx_BSRR) - Page 240
This is the register we were writing to. The documentation says some interesting things. First, this
register is write only ... so let's try reading its value
We'll use GDB's
(gdb) next 16 *(GPIOE_BSRR as *mut u32) = 1 << 9; (gdb) x 0x48001018 0x48001018: 0x00000000 (gdb) # the next command will turn the North LED on (gdb) next 19 *(GPIOE_BSRR as *mut u32) = 1 << 11; (gdb) x 0x48001018 0x48001018: 0x00000000
Reading the register returns
0. That matches what the documentation says.
The other thing that the documentation says is that the bits 0 to 15 can be used to set the corresponding pin. That is bit 0 sets the pin 0. Here, set means outputting a high value on the pin.
The documentation also says that bits 16 to 31 can be used to reset the corresponding pin. In this case, the bit 16 resets the pin number 0. As you may guess, reset means outputting a low value on the pin.
Correlating that information with our program, all seems to be in agreement:
1 << 9(
BS9 = 1) to
PE9high. That turns the North LED on.
1 << 11(
BS11 = 1) to
PE11high. That turns the East LED on.
1 << 25(
BR9 = 1) to
PE9low. That turns the North LED off.
1 << 27(
BR11 = 1) to
PE11low. That turns the East LED off.