Z8 Encore! XP® 4K Series
Product Specification
97
asserts at least one UART bit period and no greater than two UART bit periods before the
Start bit is transmitted. This allows a setup time to enable the transceiver. The Driver
Enable signal deasserts one system clock period after the final Stop bit is transmitted. This
one system clock delay allows both time for data to clear the transceiver before disabling
it, as well as the ability to determine if another character follows the current character. In
the event of back to back characters (new data must be written to the Transmit Data Regis-
ter before the previous character is completely transmitted) the DE signal is not deasserted
between characters. The Depol bit in the UART Control Register 1 sets the polarity of the
Driver Enable signal.
1
DE
0
Data Field
Stop Bit
Idle State
of Line
lsb
msb
Bit7
1
0
Start
Bit0
Bit1
Bit2
Bit3
Bit4
Bit5
Bit6
Parity
1
Figure 14.UART Driver Enable Signal Timing (shown with 1 Stop Bit and Parity)
The Driver Enable to Start bit setup time is calculated as follows:
1
2
⎛
⎝
⎞
⎠
⎛
⎝
⎞
⎠
-------------------------------------
-------------------------------------
≤ DE to Start Bit Setup Time (s) ≤
Baud Rate (Hz)
Baud Rate (Hz)
UART Interrupts
The UART features separate interrupts for the transmitter and the receiver. In addition,
when the UART primary functionality is disabled, the Baud Rate Generator can also func-
tion as a basic timer with interrupt capability.
Transmitter Interrupts
The transmitter generates a single interrupt when the Transmit Data Register Empty bit
(TDRE) is set to 1. This indicates that the transmitter is ready to accept new data for trans-
mission. The TDRE interrupt occurs after the Transmit shift register has shifted the first
bit of data out. The Transmit Data register can now be written with the next character to
send. This action provides 7 bit periods of latency to load the Transmit Data register
before the Transmit shift register completes shifting the current character. Writing to the
UART Transmit Data register clears the TDRE bit to 0.
PS022815-0206
UART
ZILOG [ ZILOG, INC. ]
