SC28L202 Philips Semiconductors, SC28L202 Datasheet - Page 21

• Manufacturer Part Number: SC28L202
• Description: Dual universal asynchronous receiver/transmitter DUART
• Manufacturer: Philips Semiconductors

Philips Semiconductors
Address Recognition and change of state interrupts become active
when the associated events occur and the arbitration value
generated thereby exceeds the threshold value programmed in the
ICR (Interrupt Control Register).
The transmitter and receiver functions have additional controls to
modify the condition upon which the initiation of interrupt ”bidding”
begins: the TxINT and RxINT fields of the MR0 and MR2 registers.
These fields can be used to start bidding or arbitration when the
RxFIFO is not empty, 50% full, 75% full or 100% full. For the
transmitter it is not full, 50% empty, 75% empty and empty.
Example: To increase the probability of transferring the contents of a
nearly full RxFIFO, do not allow it to start bidding until 50% or 75%
full. This will prevent its relatively high priority from winning the
arbitration process at low fill levels. A high threshold level could
accomplish the same thing, but may also mask out low priority
interrupt sources that must be serviced. Note that for fast channels
and/or long interrupt latency times using this feature should be used
with caution since it reduces the time the host CPU has to respond
to the interrupt request before receiver overrun occurs.
Setting interrupt priorities
The bid or interrupt number presented to the interrupt arbiter is
composed of character counts, channel codes, fixed and
programmable bit fields. The interrupt values are generated for
various interrupt sources as shown in Table 1. The value
represented by the bits 11 to 4 in Table 1 are compared against the
value represented by the “Threshold. The “Threshold”, bits 10 to 0 of
the ICR (Interrupt Control Register), is aligned such that bit 0 of the
threshold is compared to bit 1 of the interrupt value generated by
any of the sources. Whenever the value of the interrupt source is
greater than the threshold the interrupt will be generated.
The channel number arbitrates only against other channels. The
threshold is not used for the channel arbitration. This results in
Table 1. Interrupt Values
2000 Feb 10
Type
Receiver w/o error
Receiver w/ error
Receiver Watch–dog
Transmitter
Change of Break
Rx Loop Back Error
Change of State
Xon/Xoff
Counter timer
Address Recognition
No interrupt
Threshold
Dual UART
Bit 11:4
RxFIFO filled Byte Count
RxFIFO filled Byte Count
RxFIFO filled Byte Count
TxFIFO empty Byte Count
Programmed Field
Programmed Field
Programmed Field
Programmed Field
Programmed Field
Programmed Field
0
Bits 7:0 of Interrupt Control Register
(ICR)
15
channel B having the highest arbitration number. The decreasing
order is B to A. If all other parts of an arbitration cycle are equal then
the channel number will determine which channel will dominate in
the arbitration process.
Note several characteristics of Table 1 in bits 4:1. These bits contain
the identification of the bidding source as indicated below:
The codes form bits 4:1 drive part of the interrupt vector modification
and the Global Interrupt Type Register. The codes are unique to
each source type and identify them completely. The channel
numbering progresses from ”A” to ”B” as the binary numbers 0 to 1
and identify the interrupting channel uniquely. As the channels
arbitrate ”B” will have the highest bidding value and ”A” the lowest
1
1
0
0
1
1
0
0
0000 No interrupt source active
x001 Receiver without error
x101 Receiver with error (errors are: parity, framing and
x010 Transmitter
1110 Change of Break
0110 Change of State on I/O Ports
0111 Xon/Xoff Event
1011 Address Recognition
x100 Receiver watch dog
1000 Counter timer
1111 Rx Loop Back Error
overrun. Break is not considered an error
Bit 3
0
1
1
0
1
1
1
1
0
0
0
0
Bit 2
0
0
0
1
1
1
1
1
0
1
0
0
Bit 1
1
1
0
0
0
1
0
1
0
1
0
0
Bit 0
Channel No.
Channel No.
Channel No.
Channel No.
Channel No.
Channel No.
Port 0 or 1
Channel No.
Counter 0 or 1
Channel No.
0
Objective specification
SC28L202