AM8530H AMD [Advanced Micro Devices], AM8530H Datasheet - Page 39

• Manufacturer Part Number: AM8530H
• Description: Serial Communications Controller
• Manufacturer: AMD [Advanced Micro Devices]

AMD
The interrupt protocol is diagrammed in Figure 3–5. In the quiescent state (i.e. no inter-
rupts pending or under service) each SCC on the daisy chain passes its IEI input through
to its IEO output. An interrupt source that requires servicing requests an interrupt by pull-
ing the INT pin Low if the following conditions exist: 1) interrupt source is enabled (i.e., IE
and MIE bits are set to ‘1’), 2) interrupt source is not already under service (i.e., internal
IUS bit set to ‘0’), 3) no higher priority interrupt is under service (i.e., internal IUS bit set to
‘1’), and 4) an interrupt acknowledge cycle is not currently being executed (i.e., INTACK
is High).
When the processor responds with an Interrupt Acknowledge cycle all SCCs that have
enabled interrupt sources with an interrupt pending or already under service, hold their
IEO outputs lines Low. When RD goes Low, only the highest priority SCC with an inter-
rupt pending will have a high IEI input; this is the interrupt being acknowledged, and that
source’s internal IUS bit will be set to ‘1’.
When servicing of the SCC has completed, the Reset Highest IUS Command in WR0
must be issued to unlock the daisy chain, reset the IUS bit, and enable lower-priority in-
terrupt requests.
3.5.2
In this mode, INTACK does not have to be generated, and the INTACK input pin must be
tied High. This allows a simpler hardware design that does not have to meet the Interrupt
Acknowledge timing (AC timing parameter #38,TdlAi(RD)). Soon after the SCC’s INT pin
goes active, an external interrupt controller will jump to the interrupt routine. In the inter-
rupt routine, the code must read RR2 from Channel B to read the vector including status.
When the vector is read from Channel B, it always includes the status regardless of the
VIS bit in WR9 (D0). The status given will decode the highest priority interrupt pending at
the time RR2 is read. Note that the vector is not latched in RR2 so that the next read of
RR2 could produce a different vector if another interrupt occurs; however, accessing RR2
disables it from change during the read operation to prevent an error if a higher interrupt
occurs exactly during the read operation.
Once RR2 is read, the interrupt routine must decode the interrupt pending, and clear the
condition. For example, writing a character to the Transmit Buffer will clear the Transmit
Buffer Empty IP. Removing the interrupt condition clears the IP bit and deactivates
INT, but only if there are no other IP bits set. When the interrupt IP is cleared, RR2
can be read again. This allows the interrupt routine to clear all IPs with one interrupt re-
quest to the processor.
3.5.3
In this mode of operation, the processor must respond to the activation of INT by activat-
ing INTACK. After enough time has elapsed to allow the daisy chain to settle (AC timing
parameter #38,TdlAi(RD)), the SCC sets the IUS bit for the highest priority IP. If the No
Vector bit in WR9 (D1) is reset to ‘0’, the SCC will then place the interrupt vector on the
data bus during the read strobe.
To speed the interrupt response time, the SCC can also modify 3 bits in the vector to indi-
cate status. If it is programmed to include status information in the vector, this status may
be encoded and placed in either bits 1–3 or in bits 4–6 as programmed by the Status
High/Status Low bit in WR9. To include status, the VIS bit in WR9 (D0) must be set to ‘1’.
The service routine must then clear the interrupting condition. For example, writing a
character to the Transmit Buffer will clear the Transmit Buffer empty IP. After the inter-
rupting condition is cleared, the routine can read RR3 to determine if any other IP bits are
set and clear them. At the end of the interrupt routine, a Reset IUS command must then
be issued via WR0 to unlock the daisy chain and enable lower-priority interrupt requests.
This is the only way, short of a software or hardware reset, that an IUS bit may be reset.
3–8
Interrupt Without Acknowledge
Interrupt With Acknowledge With Vector
I/O Programming Functional Description