STM32F103C8T6 Maximum IO Output Speed

Question

As per the datasheet of STM32F103C8T6, its GPIO pins has a hardware speed limitation of 50MHz. I need to achieve speed above 10MHz, but even with assembly codes I am getting only 7.99MHz.

It is running at the maximum recommended speed of 72MHz (8MHz Crystal + PLL).

Can anyone guide me ? Please check the following assembly language loop I am using for toggling the GPIO pins.

 asm(".equ GPIOB_ODR, 0x4001080C");
 asm("ldr r6, = GPIOB_ODR");

 asm("loop:");
 asm("mov r1, #0xFFFFFFFF");
 asm("strh r1, [r6]");
 asm("mov r1, #0x00000000");
 asm("strh r1, [r6]");
 asm("b loop");

Answer 1

You have a couple of better options for driving GPIO pins:

• Timers. On this part, you have seven timers, most of which can be configured to set/reset/toggle pins on certain events (such as compare match and overflow).

• DMA. Set up a DMA transfer to the GPIO bit-set/reset register and you can toggle bits at bus speed. Or you can hook it up to a timer to set bits at whatever rate you want.

• Creative uses of other peripherals. USART and SPI/I2S seem like particularly likely candidates.

Answer 2

Your method:

:loop
MOV R1, #0xFFFFFFFF  ; 1 cycle
STRH R1, [R6]        ; 2 cycles
MOV R1, #0x00000000  ; 1 cycle
STRH R1, [R6]        ; 2 cycles
b loop               ; 3 cycles

The fastest the core can write registers:

MOV R3, #0xFFFFFFFF
MOV R4, #0x00000000
:loop
STRH R3, [R6]
STRH R4, [R6]
b loop

Note: GPIO is APB2, thus 36 MHz bus. You can never exceed 36 MHz.

A timer however can oscillate the pin at sysclk/2. (often 72/2 Mhz)

To get 10 MHz at an output you'd need to clock the core at a multiple of 10 with the PLL, since you can't divide the clock by 7.2 at the timers.

The datasheet number maximum Fmax(IO)out of 50 MHz is the hardware limit. The GPIO hardware cannot achieve a slope fast enough to go higher.
You can configure the slew rate of outputs, for example, it would be unnecessary to blink a led with slopes of 10 ns. But it would be necessary for high speed external memory.

Answer 3

MOV R3, #0xFFFFFFFF
MOV R4, #0x00000000
:loop
STRH R3, [R6]
STRH R4, [R6]
STRH R3, [R6]
STRH R4, [R6]
STRH R3, [R6]
STRH R4, [R6]
STRH R3, [R6]
STRH R4, [R6]
STRH R3, [R6]
STRH R4, [R6]
STRH R3, [R6]
STRH R4, [R6]
STRH R3, [R6]
STRH R4, [R6]
STRH R3, [R6]
STRH R4, [R6]
STRH R3, [R6]
STRH R4, [R6]
STRH R3, [R6]
STRH R4, [R6]
STRH R3, [R6]
STRH R4, [R6]
STRH R3, [R6]
STRH R4, [R6]
STRH R3, [R6]
STRH R4, [R6]
b loop

do this to redude delay of loop