NXP Semiconductors P89LPC9321 UM10310 - Manual (Page 63)

UM10310 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2010. All rights reserved.
User manual Rev. 2 — 1 November 2010 63 of 139
NXP Semiconductors
UM10310
P89LPC9321 User manual
10.10 More about UART Mode 0
In Mode 0, a write to SBUF will initiate a transmission. At the end of the transmission, TI
(SCON.1) is set, which must be cleared in software. Double buffering must be disabled in
this mode.
Reception is initiated by clearing RI (SCON.0). Synchronous serial transfer occurs and RI
will be set again at the end of the transfer. When RI is cleared, the reception of the next
character will begin. Refer to
Figure 27 .
Table 59. Serial Port Status register (SSTAT - address BAh) bit description
Bit Symbol Description
0 STINT Status Interrupt Enable. When set = 1, FE, BR, or OE can cause an interrupt. The
interrupt used (vector address 0023h) is shared with RI (CIDIS = 1) or the
combined TI/RI (CIDIS = 0). When cleared = 0, FE, BR, OE cannot cause an
interrupt. (Note: FE, BR, or OE is often accompanied by a RI, which will generate
an interrupt regardless of the state of STINT). Note that BR can cause a break
detect reset if EBRR (AUXR1.6) is set to logic 1.
1 OE Overrun Error flag is set if a new character is received in the receiver buffer while it
is still full (before the software has read the previous character from the buffer),
i.e., when bit 8 of a new byte is received while RI in SCON is still set. Cleared by
software.
2 BR Break Detect flag. A break is detected when any 11 consecutive bits are sensed
low. Cleared by software.
3 FE Framing error flag is set when the receiver fails to see a valid STOP bit at the end
of the frame. Cleared by software.
4 DBISEL Double buffering transmit interrupt select. Used only if double buffering is enabled.
This bit controls the number of interrupts that can occur when double buffering is
enabled. When set, one transmit interrupt is generated after each character written
to SBUF, and there is also one more transmit interrupt generated at the beginning
(INTLO = 0) or the end (INTLO = 1) of the STOP bit of the last character sent (i.e.,
no more data in buffer). This last interrupt can be used to indicate that all transmit
operations are over. When cleared = 0, only one transmit interrupt is generated per
character written to SBUF. Must be logic 0 when double buffering is disabled. Note
that except for the first character written (when buffer is empty), the location of the
transmit interrupt is determined by INTLO. When the first character is written, the
transmit interrupt is generated immediately after SBUF is written.
5 CIDIS Combined Interrupt Disable. When set = 1, Rx and Tx interrupts are separate.
When cleared = 0, the UART uses a combined Tx/Rx interrupt (like a conventional
80C51 UART). This bit is reset to logic 0 to select combined interrupts.
6 INTLO Transmit interrupt position. When cleared = 0, the Tx interrupt is issued at the
beginning of the stop bit. When set = 1, the Tx interrupt is issued at end of the stop
bit. Must be logic 0 for mode 0. Note that in the case of single buffering, if the Tx
interrupt occurs at the end of a STOP bit, a gap may exist before the next start bit.
7 DBMOD Double buffering mode. When set = 1 enables double buffering. Must be logic 0 for
UART mode 0. In order to be compatible with existing 80C51 devices, this bit is
reset to logic 0 to disable double buffering.