Digital embedded electronics

J.-M Friedt

FEMTO-ST/time & frequency department

jmfriedt@femto-st.fr

slides at jmfriedt.free.fr

November 25, 2021

1 / 14

List of presentations:

7 lessons introducing baremetal programming of the STM32:

1. Digital electronics and circuit designL3/bachelor: analog aspects, power supply and consumption, datasheet analysisOverview of the various peripherals (RS232, SPI, timer, ADC)data format (size/encoding), masks, architecture

Reminders on Atmega32U4 (Makefile, compilation, masks ...)

2. First steps with the STM32, peripheral addresses, internal architecture

3. gcc operation and optimizations:preprocess–compiler–assembler–linker, C to assembly language translation, pointers

4. libraries and softwware architecture (separating algorithm/hardware), simulators & stubslibopencm3, newlib & stubs, resources needed to execute libraries

5. Communication bus parallel/serial, synchronous/asynchrone

6. arithmetics on embedded systemsintegers v.s floating point number, converting an algorithm expressed with floats to integers, timers

7. interrupts and analog data acquisition, sampling rateinterrupt vectors, clock management on STM32, ADC

2 / 14

Communication protocolsParallel protocol:▶ one bit/wire: extensive use of hardware resources (parallel bus)▶ inductive coupling between adjacent signal at high data rates ⇒ isolation (differential pairs in

LVDS, ground between signals in SCSI)

DJIdronecamera

Serial protocols: bits are sequentially transmitted on 1 or 2 wire(s).Asynchronous v.s synchronous:▶ asynchronous: no clock sharing (historically, over telephone lines)▶ synchronous: clock shared between speakers ⇒ higher data rates 3 / 14

Asynchronous protocol (RS232)1▶ No common clock ⇒ data rate defined by agreement between speakers (baudrate)▶ Beginning: idle state (1) → start bit (0)▶ N data bits, N ∈ {7, 8}, LSB first▶ optional parity bit (E(ven), O(dd), N(one))▶ 1 or 2 stop bit(s) (1)▶ Exercise: what is the duration of a single byte transmitted at 115200 bauds in 8N1?

t0 1T

ST

AR

T

1 0 0 0

2T 3T 4T 5T 6T 7T 8T

lsb msb

9T

ST

OP

1 0 01

T=1000/baud ms1gtvhacker, Hack All The Things: 20 Devices in 45 Minutes, DEF CON 22 (2014) at

https://www.youtube.com/watch?v=h5PRvBpLuJs 4 / 14

RS232 sentence analysis

What is the content of this message?

5 / 14

RS232 sentence analysis

1. identify baudrate (shortest symbol in frame)

2. do not forget start (“s”) and stop (“S”) bits

3. resynchronization on falling edge of start bit

4. possibly parity and 2 stop bits (not used here)

6 / 14

Synchronous protocols (I2C, SPI)Hardware access: degrees of freedom

▶ clock idle state, sampling edge: defined in the I2C protocol, tunable in SPI (CPHA, CPOL)

▶ bit order: I2C : MSB first = SPI: LSB or MSB first

▶ SPI slave selected with CS# signal, I2C with address of receiver defined by hardware settings

Example of XE1203F 2: protocol close to SPI1 0 R/W# A4-A0 D7-D0

2https://datasheet.digchip.com/531/531-00053-0-XE1203.pdf7 / 14

Example: SPIXE1203F on SPI2 3 (PB12 .. PB15), manual setting of CS##i n c l u d e <l i bopencm3 /stm32/ s p i . h>

s t a t i c vo i d c l o c k s e t u p ( vo i d ){r c c c l o c k s e t u p i n h s e 8mhz ou t 7 2mhz ( ) ; // STM32F103r c c p e r i p h c l o c k e n a b l e (RCC GPIOB) ; // SPI2 = PORTBr c c p e r i p h c l o c k e n a b l e (RCC AFIO) ;r c c p e r i p h c l o c k e n a b l e ( RCC SPI2 ) ;

}

s t a t i c vo i d s p i s e t u p ( vo i d ) { // SS=PB12 , SCK=PB13 , MISO=PB14 and MOSI=PB15 ∗/gp i o s e t mode (GPIOB ,GPIO MODE OUTPUT 50 MHZ ,GPIO CNF OUTPUT ALTFN PUSHPULL , GPIO13|GPIO15 ) ;gp i o s e t mode (GPIOB , GPIO MODE OUTPUT 2 MHZ, GPIO CNF OUTPUT PUSHPULL , GPIO12 ) ;gp i o s e t mode (GPIOB , GPIO MODE INPUT , GPIO CNF INPUT FLOAT , GPIO14 ) ;s p i r e s e t ( SPI2 ) ;s p i i n i t m a s t e r ( SPI2 , SPI CR1 BAUDRATE FPCLK DIV 128 , SPI CR1 CPOL CLK TO 0 WHEN IDLE ,

SPI CR1 CPHA CLK TRANSITION 2 , SPI CR1 DFF 8BIT , SPI CR1 MSBFIRST ) ;s p i e n a b l e ( SPI2 ) ;

}

i n t s p i t x r x ( cha r c ){ s p i s e n d ( SPI2 , c ) ;

r e t u r n ( s p i r e a d ( SPI2 ) ) ; // a lways read a f t e r w r i t e to avo i d a v e r f l ow}

i n t main ( ){ [ . . . ]g p i o c l e a r (GPIOB , GPIO12 ) ;s p i t x r x (0 x80+(addr&0x1 f ) ) ; // w r i t e#s p i t x r x ( 0x55 ) ;s p i t x r x ( 0 x f f ) ;g p i o s e t (GPIOB , GPIO12 ) ;

}

3RM0008 at https://www.st.com/resource/en/reference_manual/cd00171190-stm32f101xx-stm32f102xx-stm32f103xx-stm32f105xx-and-stm32f107xx-advanced-arm-based-32-bit-mcus-stmicroelectronics.

pdf8 / 14

Reading Writing

9 / 14

STM32 asynchronous port initialization

https://www.st.com/resource/en/datasheet/

stm32f410cb.pdf

1. activate peripheral clock

2. set pin alternative function

3. configure peripheral (8N1, 115200 bauds)

#i n c l u d e <l i bopencm3 /stm32/ gp io . h>#i n c l u d e <l i bopencm3 /stm32/ u s a r t . h>

r c c p e r i p h c l o c k e n a b l e (RCC USART1) ;gp io mode se tup (GPIOA , GPIO MODE AF , GPIO PUPD NONE , GPIO9) ; //TXgp io mode se tup (GPIOA , GPIO MODE AF , GPIO PUPD NONE , GPIO10 ) ; //RXg p i o s e t a f (GPIOA , GPIO AF7 , GPIO9) ;g p i o s e t a f (GPIOA , GPIO AF7 , GPIO10 ) ;

u s a r t s e t b a u d r a t e (USART1, 115200) ;u s a r t s e t d a t a b i t s (USART1, 8) ;u s a r t s e t s t o p b i t s (USART1, USART STOPBITS 1) ;u s a r t s e t mode (USART1, USART MODE TX RX) ;u s a r t s e t p a r i t y (USART1, USART PARITY NONE) ;u s a r t s e t f l o w c o n t r o l (USART1, USART FLOWCONTROL NONE) ;u s a r t e n a b l e (USART1) ;

https://www.st.com/resource/en/reference_manual/dm00180366.pdf section 24 10 / 14

STM32 synchronous port initialization

https://www.st.com/resource/en/datasheet/

stm32f410cb.pdf

1. activate peripheral clock

2. set pin alternative function

3. set datarate (synchronous communication)

4. clock idle state and data sampling edge

#i n c l u d e <l i bopencm3 /stm32/ gp io . h>#i n c l u d e <l i bopencm3 /stm32/ s p i . h>r c c p e r i p h c l o c k e n a b l e ( RCC SPI1 ) ;gp io mode se tup (GPIOA ,GPIO MODE AF , GPIO PUPD PULLUP , GPIO5|GPIO6|GPIO7) ;g p i o s e t a f (GPIOA , GPIO AF5 , GPIO5|GPIO6|GPIO7) ; // A5=SCK 6=MISO 7=MOSIs p i d i s a b l e ( SPI1 ) ;s p i i n i t m a s t e r ( SPI1 ,

SPI CR1 BAUDRATE FPCLK DIV 256 , SPI CR1 CPOL CLK TO 1 WHEN IDLE , // c l k=1 when i d l eSPI CR1 CPHA CLK TRANSITION 1 , // f a l l i n g edge => T r a n s i t i o n 1SPI CR1 DFF 8BIT , // 8 b i t sSPI CR1 MSBFIRST ) ; // MSB f i r s t

s p i e n a b l e s o f twa r e s l a v e manag emen t ( SPI1 ) ;s p i s e t n s s h i g h ( SPI1 ) ;s p i e n a b l e ( SPI1 ) ;

https://www.st.com/resource/en/reference_manual/dm00180366.pdf section 25 11 / 14

RS232 communication4 (32U4)#i n c l u d e "avr_mcu_section.h"

AVR MCU(F CPU , "atmega32" ) ;AVR MCU VCD FILE( "trace_file.vcd" , 1000) ;con s t s t r u c t a v r mmcu vcd t r a c e t myt race [ ] MMCU = {

{ AVR MCU VCD SYMBOL( "UDR1" ) , . what = ( vo i d ∗)&UDR1, } , };

to declare the register being monitored (UDR1), then#de f i n e UART BAUDRATE (57600)//#d e f i n e UART BAUDRATE (9600)

vo i d i n i t u a r t ( vo i d ){uns i gned s h o r t baud ;UCSR1A = (1<<UDRE1) ; // impo r t a n t l y U2X1 = 0UCSR1B = (1 << RXEN1) | (1 << TXEN1) ; // enab l e r e c e i v e r and t r a n sm i t t e rUCSR1C = (1<<UCSZ11)|(1<<UCSZ10) ; // 8N1 // UCSR1D = 0 ; // no r t c / c t sbaud = ( ( ( ( F CPU / ( UART BAUDRATE ∗ 16UL) ) ) − 1) ) ;UBRR1H = ( uns i gned char ) ( baud>>8); UBRR1L = ( uns i gned char ) baud ;

}

vo i d u a r t t r a n sm i t ( uns i gned char data ){wh i l e ( ! ( UCSR1A & (1<<UDRE1) ) ) ; // wa i t wh i l e r e g i s t e r i s f r e eUDR1 = data ; // l oad data i n the r e g i s t e r

}

vo i d s end by t e ( uns i gned char c ){uns i gned char tmp ;tmp=c>>4; i f ( tmp<10) u a r t t r a n sm i t ( tmp+’0’ ) ; e l s e u a r t t r a n sm i t ( tmp+’A’−10) ;tmp=(c&0x0 f ) ; i f ( tmp<10) u a r t t r a n sm i t ( tmp+’0’ ) ; e l s e u a r t t r a n sm i t ( tmp+’A’−10) ;

}

57600 bauds:13 ms

xx 46 39 44 38 30TimeUDR1[7:0]=44

0,188 ms

9600 bauds:10 ms 20 ms

xx 46 39 44 38 30TimeUDR1[7:0]=39

1,234 ms

4J.-M Friedt, Developper sur microcontroleur sans microcontroleur : les emulateurs, GNU/Linux Magazine FranceHS 103 (Juillet-Aout 2019) 12 / 14

Laboratory session1. Configure SPI communication port2. Send a message on the SPI bus3. Check on the oscilloscope the frames transmitted over MOSI after having identified the appropriate

pin on the connector + consistency with CS# signal?4. Observe the impact of changing CPHA and CPOL on the clock signal with respect to MOSI5. Observe RS232 sentence and check the

consistency with transmitted messages (manualdecoding)

6. Write to a register of the XE1203 (SPI2) andread back the value to check it has beenproperly stored

7. CS# is manually handled by toggling PB12(Make sure to manually reset the microcontrollerafter transferring the program to flash memory)

{c}SENSeOR2011

U$1

HM

C470LP

3

U1

R1

R2

R3

C1

C2

C3

C4 C5C6

C7

C8

C11

Q3

C15 L

2

C16

C17C18 C19

C20

C21

C22

L3

L4

C25

L5

L6

C26

C27

L7

R5

C28

C29

C30

U$4

U$5

RE

SE

T

R8

C33

C34

C36 DLOAD

C10

C12

C13

D1

R4

R6

IC4

C39

X4

C9

C14

C23

C24

C31

C32

C35

C37 C38

Q4

Q5

Q_32K

_S

MD

R7

Xe1203

IC5

X2

Q1

R9

AD694

R10

X1 C40

TLV

R11

R12

R16R17

AD

M809

R13

R14

R15

D2

L1

STM32F10XRXT6

5600

5600

NC

13p

13p

13p

13p

100n

100n

100n

100n

100n

39MHz

5.6

p 22n

47p

4.7p

22p

6.8p

1p

68p

1u

22n

22n

6.8p

33n 33n

1.5p

1.5p 100n

1.2

k220p

3.3

n

1p

SAW3.3

SA

W3.3

RE

SE

T

0

100n

47u

NC

100p 100p

100p

1k

1K

FT232RL

100n

MINI-USB-SHIELD-UX60-MB-5ST

1.5

p1.5

p1.5

p1.5

p1.5

p1.5

p1.5

p

1.5

p

100nQ_8MHZ_SMD

5600

2n2222

5600

100n

5K

15k

4k710k

15k

NC

NC

BB

141

SCLK MOSIMISO CS#

UART1

13 / 14

14 / 14