ST10F276 datasheet(61/229 Pages) STMICROELECTRONICS

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ST10F276

Bootstrap loader

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Error induced by the polling

The code used for the polling is the following:

WaitCom:

JNB

P4.5,CAN_Boot

; if SOF detected on CAN, then go to

CAN

; loader

P3.11,WaitCom

; Wait for start bit at RxD0

BSET T6R

; Start Timer T6

....

CAN_Boot:

BSET PWMCON0.0

; Start PWM Timer0

; (resolution is 1 CPU clk cycle)

JMPR cc_UC,WaitRecessiveBit

WaitDominantBit:

P4.5,WaitDominantBit; wait for end of stuff bit

WaitRecessiveBit:

JNB

P4.5,WaitRecessiveBit; wait for 1st dominant bit = Stuff

bit

CMPI1R1,#5

; Test if 5th stuff bit detected

JMPR cc_NE,WaitDominantBit; No, go back to count more

BCLR PWMCON.0

; Stop timer

; here the 5th stuff bit is detected:

; PT0 = 29 Bit_Time (25D and 4R)

Therefore the maximum error at the detection of the communication on CAN pin is:

(1 not taken + 1 taken jumps) + 1 taken jump + 1 bit set: (6) + 6 CPU clock cycles

The error at the detection for the 5th recessive bit is:

(1 taken jump) + 1 not taken jump + 1 compare + 1 bit clear: (4) + 6 CPU cycles

In the worst case, the induced error is 6 CPU clock cycles, so the polling could induce an

error of 6 timer ticks.

Error induced by the baud rate calculation

The content of the timer PT0 counter corresponds to 29 bit times, resulting in the following

equation:

PT0 = 58 x (BRP + 1) X (1 + Tseg1 + Tseg2)

where BRP, Tseg1 and Tseg2 are the field of the CAN Bit Timing register.

The CAN protocol specification recommends to implement a bit time composed of at least 8

time quanta (tq). This recommendation is applied here. Moreover, the maximum bit time

length is 25 tq. To ensure precision, the aim is to have the smallest Bit Rate Prescaler (BRP)

and the maximum number of tq in a bit time.

This gives the following ranges for PT0 according to BRP:

≤ 1 + Tseg1 + Tseg2 ≤ 25

464 x (1 + BRP)

≤ PT0 ≤ 1450 x (1 + BRP)

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