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Frequently asked questions

Industrial UART Interrupt Arbitration FAQ

Q What is interrupt arbitration?
A Interrupt arbitration is the process used to determine that an interrupt request should be presented to the host.
Q What is the function of I2A (Intelligent Interrupt Arbitration) technology?
A The I2A technology improves system performance by minimizing the processor time for servicing interrupt requests because the I2A technology can identify the highest priority interrupt request and channel it first to the processor.
Q Which UART products have the I2A technology?
A Our UARTs with I2A are:
  • SC28L202 (dual UART)
  • SC28L194A (quad UART)
  • SC28L198A (octal UART)
Q What is the function of IMR (Interrupt Mask Register) on the devices without the I2A technology?
A
To select which bits in the ISR cause the interrupt (INTRN) output.
If both the ISR bit and the corresponding IMR bit are set, the interrupt output will be asserted (low).
If the ISR bit is set but the corresponding IMR bit is not set, the ISR bit has no effect on the interrupt output.
Q What is the function of IMR on the devices with the I2A technology?
A
To select which bits in the ISR enter the interrupt arbitration process.
If both the ISR bit and the corresponding IMR bit are set, the interrupt source of the ISR bit is entered the arbitration process. If the interrupt source value exceeds the threshold value, the interrupt (IRQN) output will be asserted (low).
If the ISR bit is set but the corresponding IMR bit is not set, the ISR bit has no effect on the interrupt output.
Q How does the interrupt arbitration work?
A The arbitration is done at the rate of the X1 clock and works as follows:
  1. The IMR allows the interrupt source to enter the arbitration process.
  2. The interrupt source value is compared with the threshold value. The threshold value can be programmed in the ICR.
  3. If the source value exceeds the threshold value, the interrupt (IRQN) output will be asserted (low).
  4. The host either asserts the IACKN or writes at 0x2A to update the CIR value with the interrupt source value.
Here is an example:
The source values are: 0xAD (Receiver B, 5 bytes, without error) and 0x6C (Receiver A, 3 bytes, without error).
If the threshold value in ICR is programmed to 0x40, receiver B interrupts followed by receiver A.
If the threshold value in ICR is programmed to 0x80, receiver B interrupts but receiver A does not (because 0x6C is less than 0x80).
Q Why is the CIR value 0x00 on read?
A The CIR is 0x00 if update CIR is asserted when there is no interrupt because no arbitration value exceeds the threshold value.
Q If the IACKN signal is not used, how do you connect it?
A The IACKN is an active low input signal. If it is not used, a pull up resistor (10K ohm) is required.

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