sc26c94 NXP Semiconductors, sc26c94 Datasheet - Page 18

• Manufacturer Part Number: sc26c94
• Description: Quad Universal Asynchronous Receiver/transmitter Quart
• Manufacturer: NXP Semiconductors

Philips Semiconductors
ISR[1] – Receiver Ready or FIFO Full Channel a
See the description of ISR[5]. The channel ‘a’ receiver operation is
the same as channel ‘b’.
ISR[0] – Transmitter Ready Channel a
See the description of ISR[4]. Channel “a” transmitter operates in
the same manner as channel “b.”
IMR – Interrupt Mask Register
The programming of this register selects which interrupt sources will
be allowed to enter the interrupt arbitration process. This register is
logically ANDED with the interrupt status register. Its function is to
allow the interrupt source it represents to join the bidding process if
the corresponding IMR and ISR bits are both 1. It has no effect on
the value in the ISR. It does not mask the reading of the ISR.
CTUR and CTLR – Counter/Timer Registers
The CTUR and CTLR hold the eight MSBs and eight LSBs,
respectively, of the value to be used by the counter/timer in either
the counter or timer modes of operation. The minimum value which
may be loaded into the CTUR/CTLR registers is H‘0002’. Note that
these registers are write-only and cannot be read by the CPU.
In the timer (programmable divider) mode, the C/T generates a
square wave with a period of twice the value (in clock periods) of
the CTUR and CTLR. If the value in CTUR or CTLR is changed, the
current half-period will not be affected, but subsequent half-periods
will be. The C/T will not be running until it receives an initial ‘Start
Counter’ command (read address at A5–A0 0Eh for C/T ab or read
address 1Eh for C/T cd ). After this, while in timer mode, the C/T will
run continuously. Receipt of a subsequent start counter command
causes the C/T to terminate the current timing cycle and to begin a
new cycle using the values in the CTUR and CTLR.
The counter ready status bit (ISR[3]) is set once each cycle of the
square wave. The bit is reset by a “Stop Counter” command (read
address at A5–A0 0Fh for C/T ab or read address 1Fh for C/T cd).
The command, however, does not stop the C/T. It only resets the
ISR[3] bit; the C/T continues to run. The ISR[3] bit will set again as
the counter passes through 0. The generated square wave is output
on an I/O pin if it is programmed to be the C/T output.
In the counter mode, the C/T counts down the number of pulses
loaded in CTUR and CTLR by the CPU. Counting begins upon
receipt of a start counter command. Upon reaching the terminal
count H‘0000’, the counter ready interrupt bit (ISR[3]) is set. The
counter rolls over to 65535 and continues counting until stopped by
1995 May 1
Quad universal asynchronous receiver/transmitter (QUART)
18
the CPU. If I/O is programmed to be the output of the C/T, the output
remains High until the terminal count is reached, at which time it
goes Low. The output returns to the High state and ISR[3] is cleared
when the counter is stopped by a stop counter command. The CPU
may change the values of CTUR and CTLR at any time, but the new
count becomes effective only on the next start counter command. If
new values have not been loaded, the previous values are
preserved and used for the next count cycle.
In the counter mode, the current value of the upper and lower eight
bits of the counter (CTU, CTL) may be read by the CPU. It is
recommended that the counter be stopped when reading to prevent
potential problems which may occur if a carry from the lower eight
bits to the upper eight bits occurs between the times that both
halves of the counter is read. However, note that a subsequent start
counter command will cause the counter to begin a new count cycle
using the values in CTUR and CTLR.
I/O LOGIC
The QUART has four I/O pins for each channel. These pins may be
individually programmed as an input or output under control of the
I/OPCR (I/O Port Control Register). Functions which may use the
I/O pins as inputs (Rx or Tx external clock, for example) are always
sensitive to the signal on the I/O pin regardless of it being
programmed as an input or an output. For example if I/O1a was
programmed to output the RxC1X clock and the Counter/Timer was
programmed to use I/O pin as its clock input the result would be the
Counter/Timer being clocked by the RxC1X clock.
The 16 I/O ports are accessed and/or controlled by five (5) registers:
IPR, ACR, I/OPCR, IPCR, OPR. They are shown in Table 8 of this
document. Each UART has four pins. Two of these pins have
“Change of State Detectors” (COS). These detectors set
whenever the pin to which they are attached changes state. (1 to 0
or 0 to 1) The “Change of State Detectors” are enabled via the
ACR. When enabled the COS devices may generate interrupts via
the IMR and IPCR registers. Note that when the COS interrupt is
enabled that any one or more of the four COS bits in the IPCR will
enable the COS bidding. Each of the channel’s four I/O lines are
configured as inputs on reset.
The Change of State detectors sample the I/O pins at the rate of the
38.4KHz clock. A change on the pin will be required to be stable for
at least 26.04 s and as much as 52.08 s for the COS detectors to
confirm a change. Note that changes in the X1/clock frequency will
effect this stability requirement.
COS detectors are reset by a read of the IPCR.
Product specification
SC26C94