CY3220LINBUS-RD Cypress Semiconductor Corp, CY3220LINBUS-RD Datasheet - Page 31

• Manufacturer Part Number: CY3220LINBUS-RD
• Description: KIT REF DESIGN LIN BUS
• Manufacturer: Cypress Semiconductor Corp
• Series: PSoC®r

Specifications of CY3220LINBUS-RD

• Main Purpose: Interface, LIN
• Embedded: Yes, MCU, 8-Bit
• Utilized Ic / Part: CY8C27143, CY8C27443
• Processor To Be Evaluated: CY8C27143-24PXI and CY8C27443-24PXI
• Interface Type: RS-232
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Contains lead / RoHS non-compliant
• Other names: 428-1926

LIN Bus 2.0 Reference Design
The overall CPU overhead for a frame is calculated by add-
ing all the time components for a frame and then finding the
fraction of the total frame time. Remember, this method only
gives the overhead over a complete frame. The overhead at
different instances of the frame may be different.
3.8.2
The following calculations are based on a baud rate of 19.2
kbps and CPU speed of 24 MHz. For lower baud rates, the
CPU overhead is less.
Example 1: A frame of 1 byte being transmitted.
Total time for break/synch: This is the sum of the time taken
in the synchro break ISR and inside the bit time counter ISR.
Time taken in synchro break ISR = 36 µS.
Time taken inside the bit time counter ISR = 14 * 2.63 µS =
36.82 µS.
Transmission: 3 bytes sent are synch byte, data byte and
checksum. Total time is time taken by the TX ISR and the
TX bit time counter ISR.
Time taken by TX ISR = 3 * 2.42 µS = 7.26 µS.
Time taken by bit time ISR = 30 * 2.7 µS = 81 µS.
Time taken in bit time ISR at frame complete = 41.21 µS.
Total time taken by ISRs = 202 µS.
Total bits in frame = 54.
Total frame time = 1.4 * 54 * 1/19.2K = 3.93 mS.
Overall CPU overhead = 202 µS / 3.93 mS = 5.14%.
For calculation purposes, the worst case frame length of 1
byte was used. For an 8-byte frame, the overhead is
reduced to 4.5%.
Example 2: A frame of 1 byte being received.
Total time for break/synch: This is the sum of the time taken
in the synchro break ISR and inside the bit time counter ISR.
Time taken in synchro break ISR = 36 µS.
Time taken inside the bit time counter ISR = 14 * 2.63 µS =
36.82 µS.
Transmission: 1 synch byte, data byte and checksum. Total
time is time taken by the TX ISR and the TX bit time counter
ISR.
Time taken by TX ISR = 1 * 2.42 µS = 2.42 µS.
Time taken by bit time ISR = 10 * 2.7 µS = 27 µS.
Reception: 1 data byte.
Time taken by RX ISR = 1 * 3.96 µS = 3.96 µS.
Time taken by RX bit time counter ISR = 4 * 1.13 µS = 4.52.
October 25, 2006
Calculation of CPU Overhead Over
a Frame
Cypress Semiconductor – Rev. **
Frame reception complete during checksum byte = 69.96
µS. (The RX ISR time during checksum byte is different.)
Total time = 180.68 µS.
Total bits in frame = 54.
Total frame time = 1.4 * 54 * 1/19.2K = 3.93 mS.
Overall CPU Overhead = 180.68 µS / 3.93 mS = 4.59%.
For calculation purposes, the worst case frame length of 1
byte was used. For an 8-byte frame, the overhead will come
down to 2.5%.
3.8.3
This is the maximum latency the LIN node can cause in an
application. Using the above table, the maximum time taken
inside the ISR is in the RX ISR when a slave response was
received and this value is 74 µS. Take this value into consid-
eration when the interrupts of the main application are
designed/analyzed.
Maximum Interrupt Latency
3. Master Design IP
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