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XR68C192 Datasheet(PDF) 9 Page - Exar Corporation |
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XR68C192 Datasheet(HTML) 9 Page - Exar Corporation |
9 / 33 page XR68C92/192 9 Rev. 1.33 and is controlled in a similar way by the channel B registers. OP2 - OP7: The other outputs (OP2 - OP7) are configured via the OPCR. Please see the description under the OPCR register for the details. CRYSTAL INPUTS (XTAL1 & XTAL2) If a crystal is used, it is connected between XTAL1 and XTAL2, in which case a capacitor of approximately 22 to 47 pF should be connected from each of these pins to ground (see Figure 1). If an external CMOS-level clock is used, the pin XTAL2 must be left open. RESET The XR68C92/192 can be reset by asserting the -RESET signal or by programming the appropriate internal registers. A hardware reset (assertion of -RESET) clears the following registers: • Status Registers A and B (SRA and SRB) • Interrupt Mask Register (IMR) • Interrupt Status Register (ISR) • Output Port Configuration Register (OPCR) RESET also performs the following operations: • Initializes the interrupt vector register (IVR) to 0x0F. • Places the outputs OP0 through OP7 in the high state • Places the counter/timer in counter mode • Places channels A and B in the inactive state with the transmitter serial-data outputs (TXA and TXB) in the mark (high) state. Reset commands can be programmed through the command registers to reset the receiver, transmitter, error status, or break-change interrupts for each chan- nel. TRANSMITTER The transmitter converts the parallel data from the CPU to a serial bit stream on the transmitter output pin (TXA, TXB). It automatically sends a start bit followed by the programmed number of data bits, an optional parity bit, and the programmed number of stop bits. The least-significant bit is sent first. Data is shifted out the transmit serial data output pin (TXA, TXB) on the falling edge of the programmed clock source (XTAL1, IP3 or IP5: see CSR bits 3:0). After the transmission of the stop bits, and a new character is not available in the transmit FIFO, the transmitter serial data output (TXA, TXB) remains high. Transmission resumes when the CPU loads a new character into the transmit FIFO. If the transmitter receives a disable command (CRA, CRB bits 3:2), it will continue operating until the char- acter in the transmit shift register is completely sent out. Other characters in the FIFO are neither sent nor discarded, but will be sent when the transmitter is re- enabled. TX RTS Control: Users can program the transmitter to automatically negate the request-to-send (RTS) out- put (alternate function of OP0 and OP1 for channels A and B respectively) on completion of a message trans- mission (using MR2A, MR2B bit-5). If the transmitter is programmed to operate with RTS control, the RTS output must be manually asserted before each mes- sage is transmitted. Also, the transmitter needs to be disabled after all the required data are loaded into the FIFO. Then, the RTS output will be automatically negated when the transmit-shift register and the TX FIFO are both empty. In automatic RTS mode, no more characters can be written to the FIFO after the trans- mitter is disabled. If auto clear-to-send (CTS) control is enabled (using MR2A, MR2B bit-4), the CTS input (alternate function of IP0 and IP1 for channels A and B respectively) must be asserted (low) in order for the character to be transmitted. If it gets negated (high) in the middle of a transmission, the character in the shift register is transmitted and the transmit data output (TXA, TXB) then remains in the marking state until CTSA, CTSB gets asserted again. The transmitter can also be forced to send a continu- ous low (space) condition by issuing the start-break command (see CRA, CRB bits 7:4). The state of CTS is ignored by the transmitter when it is set to send a break. NOTE: The terms assertion and negation will be used extensively to avoid confusion when dealing with a mixture of “active low” and “active high” signals. The term assert or assertion indicates that a signal is active or true, independent of whether that level is repre- sented by a high or low voltage. The term negate or negation indicates that a signal is inactive or false. |
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