The Monitor Interface consists of two signals implementing an RS-232 protocol compatible, full duplex serial port for exchange of commands and status. The following configuration is used:
• Baud: 9600
• Settings: 8-N-1
• Flow Control: None
• Terminal Setup: VT100
° TX Newline: CR (Terminal transmits CR [0x0D] as end of line)
°
RX Newline: CR+LF (Terminal receives CR [0x0D] as end of line,
and expands to CR+LF [0x0D, 0x0A])
° Local Echo: NO
Any external device connected to the Monitor Interface must support this configuration. This Figure shows the switching behavior, and is representative of both transmit and receive.
Transmit and receive timing is derived from a 16x bit rate enable signal which is created inside the system-level example design using a counter. The behavior of this counter is to start counting from zero, up to and including a terminal count (a condition detected and used to synchronously reset the counter). The terminal count output is also supplied to transmit and receive processes as a time base.
From a compatibility perspective, the advantages of 9600 baud are that it is a standard bit rate, and it is also realizable with a broad range of input clock frequencies. It is used in the system-level design example for these reasons.
From a practical perspective, the disadvantage of such a low bit rate is communication performance (both data rate and latency). This performance can throttle the controller. For this reason, changing to a higher bit rate is strongly encouraged. A wide variety of other bit rates are possible, including standard bit rates: 115200, 230400, 460800, and 921600 baud.
In the MON shim system-level example design module, the parameter V_ENABLETIME sets the communication bit rate. The value for V_ENABLETIME is calculated using:
Equation 3-1
V_ENABLETIME= round to integer
A rounding error as great as ±0.5 can result from the computation of V_ENABLETIME. This error produces a bit rate that is slightly different than the nominal bit rate. A difference of 2% between RS-232 devices is considered acceptable, which suggests a bit rate tolerance of ±1% for each device.
Example : The input clock is 66 MHz, and the desired bit rate is 115200 baud.
Equation 3-2
V_ENABLETIME= round to integer
=35
The actual bit rate that results is approximately 114583 baud, which deviates -0.54% from the nominal bit rate of 115200 baud. This is acceptable because the difference is within ±1%.
When exploring bit rates, if the difference from nominal exceeds the ±1% tolerance, select another combination of bit rate and input clock frequency that yields less error. No additional switching characteristics are specified.
Electrically, the I/O pins used by the Monitor Interface use LVCMOS signaling, which is suitable for interfacing with other devices. No specific I/O mode is required. When full electrical compatibility with RS-232 is desired, an external level translator must be used.
TIP: For details about customizing the MON shim that attaches to the Monitor Interface, see MON Shim Customizations .