ST16654 datasheet(21/49 Pages) EXAR

21 / 49 page

ST16C654/654D

5-85

Rev. 4.10

DMA Operation

The 654 FIFO trigger level provides additional flexibil-

ity to the user for block mode operation. LSR bits 5-6

provide an indication when the transmitter is empty or

has an empty location(s). The user can optionally

operate the transmit and receive FIFOs in the DMA

mode (FCR bit-3). When the transmit and receive

FIFOs are enabled and the DMA mode is deactivated

(DMA Mode “0”), the 654 activates the interrupt output

pin for each data transmit or receive operation. When

DMA mode is activated (DMA Mode “1”), the user

takes the advantage of block mode operation by

loading or unloading the FIFO in a block sequence

determined by the preset trigger level. In this mode,

the 654 sets the interrupt output pin when characters

in the transmit FIFOs are below the transmit trigger

level, or the characters in the receive FIFOs are above

the receive trigger level.

Sleep Mode

The 654 is designed to operate with low power con-

sumption. A special sleep mode is included to further

reduce power consumption when the chip is not being

used. With EFR bit-4 and IER bit-4 enabled (set to a

logic 1), the 654 enters the sleep mode but resumes

normal operation when a start bit is detected, a change

of state on any of the modem input pins RX, -RI, -CTS,

-DSR, -CD, or transmit data is provided by the user. If

the sleep mode is enabled and the 654 is awakened by

one of the conditions described above, it will return to

the sleep mode automatically after the last character

is transmitted or read by the user. In any case, the

sleep mode will not be entered while an interrupt(s) is

pending. The 654 will stay in the sleep mode of

operation until it is disabled by setting IER bit-4 to a

logic 0.

Loopback Mode

The internal loopback capability allows onboard diag-

nostics. In the loopback mode the normal modem

interface pins are disconnected and reconfigured for

loopback internally. MCR register bits 0-3 are used for

controlling loopback diagnostic testing. In the

loopback mode OP1 and OP2 in the MCR register

(bits 3/2) control the modem -RI and -CD inputs

respectively. MCR signals -DTR and -RTS (bits 0-1)

are used to control the modem -CTS and -DSR inputs

respectively. The transmitter output (TX) and the

receiver input (RX) are disconnected from their asso-

ciated interface pins, and instead are connected to-

gether internally (See Figure 12). The -CTS, -DSR, -

CD, and -RI are disconnected from their normal

modem control inputs pins, and instead are connected

internally to -DTR, -RTS, -OP1 and -OP2. Loopback

test data is entered into the transmit holding register

via the user data bus interface, D0-D7. The transmit

UART serializes the data and passes the serial data to

the receive UART via the internal loopback connec-

tion. The receive UART converts the serial data back

into parallel data that is then made available at the

user data interface, D0-D7. The user optionally com-

pares the received data to the initial transmitted data

for verifying error free operation of the UART TX/RX

circuits.

In this mode, the receiver and transmitter interrupts

are fully operational. The Modem Control Interrupts

are also operational. However, the interrupts can only

be read using lower four bits of the Modem Control

Status Register bits 4-7. The interrupts are still con-

trolled by the IER.

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