
Intelligent Two-Channel LAN and WAN Communications Controller — CD2231
Datasheet
125
8.5.1.5
Interrupt Stack Register (STK)
This register is a 4-bit-deep by 2-bit-wide stack that contains the internal interrupt nesting history.
The stack is pushed from bits 7 and 0 toward the center during an interrupt acknowledge cycle, and
popped from the center during a write to an end of interrupt register.
Bits 7, 0
CLvl [0:1]These bits provide the currently active interrupt level.
Bits 6, 1
MLvl [0:1]These bits hold a previously active interrupt now nested.
Bits 5, 2
TLvl [0:1]These bits hold the oldest interrupt now nested two bits deep.
Bits 4:3
Reserved – always returns ‘0’ when read.
8.5.2
Receive Interrupt Registers
8.5.2.1
Receive Priority Interrupt Level Register (RPILR)
This register must be initialized by the host to contain the codes that are presented on the address
bus by the host system to indicate which of the three CD2231 interrupt types (modem, transmit, or
receive) is being acknowledged when IACKIN* is asserted. The CD2231 compares bits 0–6 in this
register with A[0–6] to determine if the acknowledge level is correct. The value programmed in the
MSB of the register has no effect on the IACK cycle.
RPILR must contain the code used to acknowledge receive interrupts.
Register Name: STK
Register Description: Interrupt Stack
Default Value: x’00
Access: Byte Read only
Intel Hex Address: x’E0
Motorola Hex Address: x’E2
Bit 7
Bit 6Bit 5Bit 4Bit 3Bit 2Bit 1Bit 0
CLvl [1]
MLvl [1]
TLvl [1]
0
TLvl [0]
MLvl [0]
CLvl [0]
CLvl [1]
CLvl [0]
0
No interrupt active; CAR provides the current channel number
01
Currently in a modem interrupt service, MIR provides the
current channel number.
10
Currently in a transmit interrupt service, TIR provides the
current channel number.
1
Currently in a receive interrupt service, RIR provides the
current channel number.
Register Name: RPILR
Register Description: Receive Priority Interrupt Match
Default Value: x’00
Access: Byte Read/Write
Intel Hex Address: x’E3
Motorola Hex Address: x’E1
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
User-assigned priority match value