SC68C94 Datasheet, PDF (32/33 Pages) NXP Semiconductors – Quad universal asynchronous receiver/transmitter QUART

Philips Semiconductors
Quad universal asynchronous receiver/transmitter (QUART)
Product specification
SC68C94
Note that interrupt threshold value in the ICR is 6 bits long. This
value is aligned with the bid arbitration logic such that it bids only
through the most significant 6 bits. The result of this is that the
channel value does not ’bid’. However the logic is such that other
parts of the bid being equal the condition of the highest channel will
be captured in CIR. The increasing order of the channels is A, B, C,
D. Thus channel D is the ”strongest” of the four.
It could be that the giving the highest strength to channel D may,
from time to time, not be what would be most desired. Further it
may be desired to alter the authority of a channel’s bid. This may
be done by setting the Rx and/or Tx interrupt bits in MR0 and MR1
to values different than zero. This will have the effect of not allowing
the associated receiver or transmitter to bid until its FIFO reaches a
particular fill level. Although this compromises the idea of the
bidding interrupt scheme, it is entirely safe to use. In fact it is setting
of MR0 and MR1 interrupt bits to zero that causes the receiver to
stop bidding when it is empty and causes the transmitter to stop
bidding when it is full. Altering the MR0 and MR1 interrupt bits only
changes the level at which the Rx & Tx bidding is stopped.
See the “Interrupt Note on 68C94” which refers to the use of the MR
registers in controlling the Rx and Tx bidding.
In normal operation the character of an interrupt will be controlled by
the above registers in conjunction with the IMR (Interrupt Mask
Register (one for each DUART)) . The function of the IMR will be to
enable bidding of any particular source. Recall that the QUART has
18 functions which may generate an interrupt.
The format of the interrupt vector is controlled by the ICR[1:0] bits.
The formats are shown in Table 7. The purpose of the vector
modification is to allow the interrupting source (either channel or
type and channel) to direct the processor to appropriate service
routine. We have found that some users wish to use extremely tight
loops for the service routines and find the addition of several tests of
status bytes to be very ’expensive’ in processor time.
Table 10. Configuration of Interrupt Vector for the QUART
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
Interrupt vector for →
ICR[1:0]=00
INTERRUPT VECTOR FORMATS (Controlled by ICR[1:0])
IVR[7:0]
Full interrupt vector
Interrupt vector for →
ICR[1:0]=01
IVR[7:2]
Interrupt vector for →
ICR[1:0]=10
IVR[7:5]
Interrupt vector 6 MSBs
ICR[4:2]
Interrupt vector for →
ICR[1:0]=11 (Inhibit)
Interrupt vector 3 MSBs
Interrupt type
Inhibit vector output. (Set bus to FFh)
CURRENT INTERRUPT REGISTER FORMAT CIR[7:0]
Rx or Tx byte count
Interrupt type: R/Tx CT COS BRK
INTERRUPT CONTROL REGISTER FORMAT ICR[0:7]
Interrupt threshold ICR[7:2]
BIT 1
BIT 0
ICR[1:0]
Channel number
ICR[1:0]
Channel number
Channel number
Interrupt vector format
ICR[1:0]
NOTE ON QUART INTERFACE TO ITS
CONTROLLING PROCESSOR
The QUART, has been designed to interface in either the
synchronous interrupt environment (without DACKN) or the
asynchronous interrupt environment (with DACKN). The 80xxx
devices of Intel design are usually operated in a synchronous
interrupt mode while those of Motorola design, 68xxx devices,
operate in an asynchronous interrupt mode.
Note: Synchronous and asynchronous interrupt modes are not
in any way associated with synchronous or asynchronous data
transmission.
The QUART has been designed with the pins required to service
either interface. In general then it is probable that in any application
some of the interface pins will not be used. This note discusses
what is required for the ”text book” connections of the two methods.
It should be noted that features of either method are not mutually
exclusive.
The interface pins are all active low. (at V_SS or ground) The pins
used for normal reading and writing to the QUART (the generation of
a bus cycle) are CEN (Chip Enable), RDN (Read Enable), WRN
(Write Enable). The pins used in the interrupt service are IRQN
(Interrupt Request), IACKN (Interrupt Acknowledge). The pin used
for data transfer is DACKN (Data Acknowledge). IRQN and DACKN
are open drain outputs.
DACKN signaling can be enabled or disabled via writing to address
27h or 26h respectively. Note that if DACKN is enabled that writing
to the QUART will occur on the falling edge of DACKN. The use of
hardware reset (required at power up) enables DACKN.
The Asynchronous Interface
Those familiar with 68xxx I/O will note the use of the two pins RDN
and WRN to be in conflict with 68xxx devices use of the one R/WN
pin. The R/WN must be inverted such that the R/WN may drive the
WRN input while the inversion of R/WN drives the RDN input. It is
good practice to condition the inversion of R/WN such that RDN will
not become active on the termination of a write to the QUART while
CEN is still asserted. These short periods of read could upset FIFO
pointers in the chip.
During a read of the QUART DACKN signals that valid data is on the
data bus. During a write to the QUART DACKN signals that data
placed on the bus by the control processor has been written to the
1995 May 1
32