Intel® PXA26x Processor Family Developer’s Manual 10-21
Universal Asynchronous Receiver/Transmitter

10.4.2.11 Scratchpad Register (SPR)

The read/write SPR has no effect on the UART . It is intend ed as a s cratch pad regi ster for use b y the
programmer. It is included for 16550 compatibility.
10.4.3 FIFO Interrupt Mode Operation
This section describes how to service interrupts in FIFO mode.

10.4.3.1 Receive Interrupt

For a receive interrupt to occur, the receive FIFO and receive interr upts must be enabled. IIR[IID]
changes to show that receive data is available when the FIFO reaches its trigger level. IIR[ II D]
changes to show the next waiting interrupt when the FIFO drops below the trigger level. A change
in IIR[IID] triggers an interrupt to the core. Software reads II R [I ID] to determine the cause of the
interrupt.
The receiver line status interrupt (IIR = 0xC6) has the highest priority. The received data available
interrupt (IIR = 0xC4) is lower. The line status int errup t occurs only w hen the char acte r at the fr ont
of the FIFO has errors.
The data ready bit (DR in the LSR) is set when a character is transferred from the shift register to
the receive FIFO. The DR bit is cleare d when the FIFO is empty.

10.4.3.2 Character Timeout Indication Interrupt

A character timeout indication interrupt occurs when the receive FIFO and character tim e out
indication interrupt are enabled and all of the following conditions exist:
At least one character is in the FIFO.
The most recently received character was received more than four continuous character times
ago. If two stop bits are programmed, the second is included in this interval.
The most recent FIFO read was performed more than four continuous character times ago.
Table 10-16. Scratch Pad Register SPR
Base+0x1C Scratch Pad Register UART
Bit 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Reserved
SP
Reset 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Read/Write
Bits Name Description
31:8 — Reserved
7:0 SP SCRATCH PAD:
No effect on UART functionality