Calculator User Manual

Overview of the Asynchronous Serial Port
10-2
10.1 Overview of the Asynchronous Serial Port
The on-chip asynchronous serial port (ASP) provides easy serial data commu-
nication between host CPUs and the ’C2xx or between two ’C2xx devices. The
asynchronous mode of data communication is often referred to as UART (uni-
versal asynchronous receive and transmit). For transmissions, data written to
a transmit register is converted from an 8-bit parallel form to a 10- or 11-bit seri-
al form (the eight bits preceded by one start bit and followed by one or two stop
bits). Each of the ten or eleven bits is transmitted sequentially (LSB first) to a
transmit pin. For receptions, data is received one bit at a time (LSB first) at a
receive pin (one start bit, eight data bits, and one or two stop bits). The received
bits are converted from serial form to parallel form and stored in the lower eight
bits of a 16-bit receive register. Errors in data transfers are indicated by flags
and/or interrupts.
The maximum rate for transmissions and receptions is determined by the rate
of the internal baud clock, which operates at a fraction of the rate of CLKOUT1.
The exact fraction is determined by the value in the 16-bit programmable
baud-rate divisor register (BRD). For receptions, you may enable (through
software) the auto-baud detection logic, which allows the ASP to lock to the
incoming data rate.