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F UNCTIONAL
4
DE SCRIPTI ON
Section 4
Functional
RCP-GPIO
During the first half of the bus cycle, port C presents the lower address
byte (A0-A2). This information is presented to U38 and remains stable un-
til the beginning of the next bus cycle when KBD_SEL is driven high by
the processor. During the last half of the bus cycle port C presents data
during write cycles and accepts data during read cycles.
Clock
The master system clock is provided by oscillator U6 pin 8. SYSCLK is
available to the processor (U35 pin 7). The frequency of SYSCLK is
7.3728 MHz. This value was chosen to provide an appropriate frequency
for the baud rate generator inside the 68HC11.
Reset
As with all microprocessors, the 68HC11 requires initialization during
power-up. The 68HC11 requires that the RESET pin (U35 pin 17) be held
low for 4064 cycles of E clock (2.2 mS @ 1.8432 MHz E clock). In addition
the RESET pin must be held low while VDD is below legal limits to protect
the internal EPROM register contents. A Maxim MAX690 chip (U34) per-
forms the reset function for the 68HC11. The MAX690 monitors the supply
voltage and asserts RESET (U34 pin 7) whenever VCC falls below 4.5
VDC. The RESET signal is guaranteed to be asserted for a minimum of
50 mS after VCC rises above 4.75 VDC. This is more than adequate to
meet the 2.2 mS requirement of the 68HC11.
RS485 Communications
Communication between the panel and the system controller is accom-
plished by the 68HC11 internal serial communications interface (SCI). The
SCI is an asynchronous receiver/transmitter, sometimes referred to as a
UART. The RS485 standard is used for the electrical interface between
control panels and the system controller. A 75ALS176 (U37) chip is used
convert between RS485 and the levels required by the SCI. Transmit data
(TXD) is presented by the SCI on U35 pin 21. This signal drives the input
to the RS485 transceiver on U37 pin 4. Data received from the system
controller is converted to the appropriate levels by the RS485 transceiver
and is presented on U37 pin 1. This received data (RXD) signal is then fed
to the SCI receiver at U35 pin 20. Since the RS485 interface requires the
transmitter to be tri-stated when not is use, a third signal is required to en-
able/disable the RS485 transmitter.
4.2 GPIO Interface Card Continued:
Microprocessor Continued:
