Operation Of Asynchronous Transfer - Epson 0C88832 Technical Manual

5 PERIPHERAL CIRCUITS AND THEIR OPERATION (Serial Interface)

5.7.7 Operation of asynchronous transfer

Asynchronous transfer is a mode that transfers by
adding a start bit and a stop bit to the front and the
back of each piece of serial converted data. In this
mode, there is no need to use a clock that is fully
synchronized clock on the transmit side and the
receive side, but rather transmission is done while
adopting the synchronization at the start/stop bits
that have attached before and after each piece of
data. The RS-232C interface functions can be easily
realized by selecting this transfer mode.
This interface has separate transmit and receive
shift registers and is designed to permit full duplex
transmission to be done simultaneously for trans-
mitting and receiving.
For transfer data in the asynchronous 7-bit mode,
either 7 bits data (no parity) or 7 bits data + parity
bit can be selected. In the asynchronous 8-bit mode,
either 8 bits data (no parity) or 8 bits data + parity
bit can be selected.
Parity can be even or odd, and parity checking of
received data and adding a party bit to transmitting
data will be done automatically. Thereafter, it is not
necessary to be conscious of parity itself in the
program.
The start bit and stop bit are respectively fixed at
one bit and data is transmitted and received by
placing the LSB (bit 0) at the front.
Sampling
clock
7bit data s1 D0 D1 D2 D3 D4 D5 D6
7bit data
s1 D0 D1 D2 D3 D4 D5 D6 p
+parity
s1 D0 D1 D2 D3 D4 D5 D6 D7
8bit data
8bit data
s1 D0 D1 D2 D3 D4 D5 D6 D7
+parity
s1 : Start bit (Low level, 1 bit)
s2 : Stop bit (High level, 1 bit)
p : Parity bit
Fig. 5.7.7.1 Transfer data configuration
for asynchronous system
Here following, we will explain the control se-
quence and operation for initialization and trans-
mitting /receiving in case of asynchronous data
transfer. See "5.7.8 Interrupt function" for the serial
interface interrupts.
56
Initialization of serial interface
The below initialization must be done in cases of
asynchronous system transfer.
(1) Setting of transmitting/receiving disable
To set the serial interface into a status in which
both transmitting and receiving are disabled, "0"
must be written to both the transmit enable
register TXEN and the receive enable register
RXEN. Fix these two registers to a disable status
until data transfer actually begins.
(2) Port selection
Because serial interface input/output terminals
SIN and SOUT are set as I/O port terminals P10
and P11 at initial reset, "1" must be written to
the serial interface enable register ESIF in order
to set these terminals for serial interface use.
SCLK and SRDY terminals set in the clock
synchronous mode are not used in the asynchro-
nous mode. These terminals function as I/O
port terminals P12 and P13.
(3) Setting of transfer mode
Select the asynchronous mode by writing the
data as indicated below to the two bits of the
mode selection registers SMD0 and SMD1.
7-bit mode:
8-bit mode:
(4) Parity bit selection
When checking and adding parity bits, write "1"
into the parity enable register EPR to set to "with
parity check". As a result of this setting, in the
s2
asynchronous 7-bit mode, it has a 7 bits data +
parity bit configuration and in the asynchronous
s2
8-bit mode it has an 8 bits data + parity bit
configuration.In this case, parity checking for
s2
receiving and adding a party bit for transmitting
p s2 is done automatically in hardware. Moreover,
when "with parity check" has been selected,
"odd" or "even" parity must be further selected in
the parity mode selection register PMD.
When "0" is written to the PMD register to select
"without parity check" in the asynchronous 7-bit
mode, data configuration is set to 7 bits data (no
parity) and in the asynchronous 8-bit mode (no
parity) it is set to 8 bits data (no parity) and parity
checking and parity bit adding will not be done.
(5) Clock source selection
Select the clock source by writing data to the
two bits of the clock source selection registers
SCS0 and SCS1. (See Table 5.7.4.1.)
Since all the registers mentioned in (2)–(5) are
assigned to the same address, it's possible to set
them all with one instruction.
EPSON
SMD0 = "0", SMD1 = "1"
SMD0 = "1", SMD1 = "1"
E0C88832/88862 TECHNICAL MANUAL