XR16M654 Datasheet, PDF (15/58 Pages) Exar Corporation – 1.62V TO 3.63V QUAD UART WITH 64-BYTE FIFO

XR16M654/654D
REV. 1.0.0
1.62V TO 3.63V QUAD UART WITH 64-BYTE FIFO
operation by loading or unloading the FIFO in a block sequence determined by the programmed trigger level.
The following table show their behavior. Also see Figure 21 through 26.
TABLE 5: TXRDY# AND RXRDY# OUTPUTS IN FIFO AND DMA MODE FOR CHANNELS A-D
PINS
FCR BIT-0=0
(FIFO DISABLED)
FCR BIT-0=1 (FIFO ENABLED)
FCR BIT-3 = 0
(DMA MODE DISABLED)
FCR BIT-3 = 1
(DMA MODE ENABLED)
RXRDY# LOW = 1 byte
HIGH = no data
LOW = at least 1 byte in FIFO
HIGH = FIFO empty
HIGH to LOW transition when FIFO reaches the
trigger level, or timeout occurs
LOW to HIGH transition when FIFO empties
TXRDY# LOW = THR empty LOW = FIFO empty
LOW = FIFO has at least 1 empty location
HIGH = byte in THR HIGH = at least 1 byte in FIFO HIGH = FIFO is full
2.7 Crystal Oscillator or External Clock Input
The M654 includes an on-chip oscillator (XTAL1 and XTAL2) to produce a clock for both UART sections in the
device. The CPU data bus does not require this clock for bus operation. The crystal oscillator provides a
system clock to the Baud Rate Generators (BRG) section found in each of the UART. XTAL1 is the input to the
oscillator or external clock buffer input with XTAL2 pin being the output. For programming details, see
“Section 2.8, Programmable Baud Rate Generator with Fractional Divisor” on page 15.
FIGURE 6. TYPICAL CRYSTAL CONNECTIONS
R=300K to 400K
XTAL1 14.7456 XTAL2
MHz
C1
22-47pF
C2
22-47pF
The on-chip oscillator is designed to use an industry standard microprocessor crystal (parallel resonant,
fundamental frequency with 10-22 pF capacitance load, ESR of 20-120 ohms and 100ppm frequency
tolerance) connected externally between the XTAL1 and XTAL2 pins. Typical oscillator connections are shown
in Figure 6. Alternatively, an external clock can be connected to the XTAL1 pin to clock the internal baud rate
generator for standard or custom rates. For further reading on oscillator circuit please see application note
DAN108 on EXAR’s web site.
2.8 Programmable Baud Rate Generator with Fractional Divisor
Each UART has its own Baud Rate Generator (BRG) with a prescaler for the transmitter and receiver. The
prescaler is controlled by a software bit in the MCR register. The MCR register bit-7 sets the prescaler to divide
the input crystal or external clock by 1 or 4. The output of the prescaler clocks to the BRG. The BRG further
divides this clock by a programmable divisor between 1 and (2¹⁶ - 0.0625) in increments of 0.0625 (1/16) to
obtain a 16X or 8X or 4X sampling clock of the serial data rate. The sampling clock is used by the transmitter
for data bit shifting and receiver for data sampling. The BRG divisor (DLL, DLM and DLD registers) defaults to
the value of ’1’ (DLL = 0x01, DLM = 0x00 and DLD = 0x00) upon reset. Therefore, the BRG must be
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