xr

DATA COMMUNICATIONS APPLICATION NOTE

DAN180

8

are actually present at these locations by checking the values returned by the reads. The sequence of steps

that this BIOS performs to validate the existence of a serial port (UART) is given below:

1) Write 0x80 to LCR register; // Enable access to baud rate registers DLL & DLM in the shadow register page.

2) Write 0x55 to DLL;

3) Write 0x00 to DLM;

4) Read DLL; // Expect a value of 0x55. If data is correct, continue

5) Write 0x00 to DLL;

6) Write 0x00 to DLM; // DLL becomes DREV and DLM becomes DVID

7) Read DLL;

// Expect a value of 0x00. Correct data validates the presence of a serial port.

// "A2 YYWW" device fails this step because the value read is 0x01.

8) Write 0x00 to LCR;

// Return to base register page.

Here is the sequence of steps to read the Device Revision and Device ID registers:

a) Write 0x80 to LCR; // Enable access to baud rate registers DLL and DLM in the shadow register page.

b) Write 0x00 to DLL;

c) Write 0x00 to DLM;

d) Read DLL;

// This gives a value of 0x01 for device revision

e) Read DLM;

// This gives a value of 0x02 indicating that the device is ST16C2550

Writing a value of 0x00 to both DLL and DLM registers is not a valid setting since the resulting baud rate is un-

determined. The Device ID and Device Revision registers should not interfere in normal operation.

This problem can be resolved by modifying the Phoenix BIOS by writing alternate values to DLL and DLM in

steps 5) and 6) above. This will detect the two serial ports of the "A2" device correctly. For example, change

the steps 5), 6) and 7) above as follows:

5) Write 0xAA to DLL;

6) Write 0x55 to DLM; // DLL and DLM stay as DLL and DLM

7) Read DLL;

// Expect a value of 0xAA. Correct data validates the presence of a serial port.

// "A2 YYWW" device will pass this step correctly.