README.md

LED examples

This directory contains assembly language examples related to LEDs.

LED background info

There are four LEDs on this board which we can turn on and we can choose one. There are details about these LEDs on page 14 of the above user manual:

                           LED3
                      LED4      LED5
                           LED6

* User LD3: This red user LED is connected to the I/O PC6
* User LD4: This orange user LED is connected to the I/O PC8
* User LD5: This green user LED is connected to the I/O PC9
* User LD6: This blue user LED is connected to the I/O PC7

So we can see that we need to access PORT C and then a PIN on that port. So we can see that all these PINs are connected to PORT C.

The steps we need to go through to write to one of these pins are the following:

• Enable I/O PORT C clock
• Set the Mode (the direction which is output in our case) (GPIOx_MODER)
• Set the bit to be written (GPIOC_ODR)

AHB peripheral clock enable

This register is at an offset from RCC (think of this as if rcc is a struct and we are accessing a member like rcc->ahbenr but instead we are using addition explicitly where the first term is the RCC_BASE and the second term is 0x14)

6.4.6 AHB peripheral clock enable register (RCC_AHBENR)
Address offset: 0x14
...
Bit 19 IOPCEN: I/O port C clock enable
Set and cleared by software.
  0: I/O port C clock disabled
  1: I/O port C clock enabled

Notice that bit 19 is the bit for controlling PORT C clock enabling, and if we write a 1 into that position we are enabling it.

So we also need to find the address of the Reset and Clock Control register (RCC) which can be found in the Memory Map section:

0x4002 1000 - 0x4002 13FF 1 KB RCC
...

So our RCC_AHB1 would be 0x4002100 + 0x14. In the led.s example we use symbolic names for these values.

So we need to find the address of PORT C. To do this we look in the reference manual on page 38 there is a table with a Memory Map (same table as we used previously):

...
0x4800 0800 - 0x4800 0BFF 1KB GPIOC
...

Next we need to set the Mode to output

8.4.1 GPIO port mode register (GPIOx_MODER) (x =A..F)
Address offset:0x00

Bits 2y+1:2y MODERy[1:0]: Port x configuration bits (y = 0..15)
These bits are written by software to configure the I/O mode.
  00: Input mode (reset state)
  01: General purpose output mode
  10: Alternate function mode
  11: Analog mode

This register is at an offset of 0x00from CPIOC. Using this address we can then set the output mode 01 of the pin we are interested in. Notice that there are two bits per PIN and the register is 32 bits, that leaves 15 pins. And we noted above that the pins of the LEDs are 6, 7, 8, and pin 9. So if we want to set PIN 6 to output mode we have to write a 1 in bit position 12.

With the mode set we now need to write the data using Output Data Register (ODR):

8.4.6 GPIO port output data register (GPIOx_ODR) (x = A..F)
Address offset: 0x14

Bits 31:16 Reserved, must be kept at reset value.
Bits 15:0 ODRy: Port output data bit (y = 0..15)
  These bits can be read and written by software

Again this is a register offset from the GPIOC base register and we set the pin that we are interested in writing, one of pin 6, 7, 8, or pin 9. Just note that if you want to change the led or enable more we also have to set the Mode for those or nothing will happen.

An example of a led blinking with a delay can be found in led.s.

Building

$ make led.elf

Flashing

Start openocd:

$ make openocd

Start a telnet session:

$ telnet localhost 4444
Open On-Chip Debugger
> reset halt
> flash write_image erase main.hex
> reset run

External LED example

This example similar to the previous examples but instead of using the LED on the discovery board it uses an external LED.

Building

$ make led-ext.elf

Flashing

Start openocd:

$ make openocd

Start a telnet session:

$ telnet localhost 4444
Open On-Chip Debugger
> reset halt
> flash write_image erase led-ext.elf.hex
> reset run