Z86C21 Datasheet, PDF (10/35 Pages) Zilog, Inc. – 8K ROM Z8 CMOS MICROCONTROLLER

PIN FUNCTIONS (Continued)
Z86C21 MCU
WITH 8K ROM
Port 3 (P37-P30). Port 3 is an 8-bit, CMOS compatible four-
fixed-input and four-fixed-output port. These eight I/O lines
have four-fixed input (P33-P30) and four fixed output
(P37-P34) ports. Port 3, when used as serial I/O, is pro-
grammed as serial in and serial out, respectively (Figure 8
and Table 4) Port 3 pins have Auto Latches only.
Port 3 is configured under software control to provide the
following control functions: handshake for Ports 0 and 2
(/DAV and RDY); four external interrupt request signals
(IRQ3-IRQ0); timer input and output signals (T_IN and T_OUT),
and Data Memory Select (/DM).
UART Operation. Port 3 lines P30 and P37, are be pro-
grammed as serial I/O lines for full-duplex serial asynchro-
nous receiver/transmitter operation. The bit rate is con-
trolled by the Counter/Timer0.
The Z86C21 automatically adds a start bit and two stop bits
to transmitted data (Figure 9). Odd parity is also available
as an option. Eight data bits are always transmitted,
regardless of parity selection. If parity is enabled, the
eighth bit is the odd parity bit. An interrupt request (IRQ4)
is generated on all transmitted characters.
Received data must have a start bit, eight data bits and at
least one stop bit. If parity is on, bit 7 of the received data
is replaced by a parity error flag. Received characters
generate the IRQ3 interrupt request.
Z86C21
MCU
Port 3
(I/O or Control)
Out
In
\10
PAD
Port 3 Output Configuration
R ≈ 500 KΩ
Port 3 Input Configuration
Figure 8. Port 3 Configuration
PAD
Auto Latch