apnt_130+++Using the XDATA Bank Switching Location

Application Note Using the Bank Switching XDATA Location APNT_130 Page 1 of 4 Revision date: 5-Nov-99 Author: Andrew Ayre (aayre@keil.com), Keil Software, Inc. ( 972-735-8052 OVERVIEW When you use an XDATA memory location to perform the bank switching in your 8051 target hardware, it is not possible for the unused bits of the location to be used as general purpose I/O. This is because all eight bits are written when performing the bank switching, as it is not possible to selectively write to particular bits in external memory. The result is corruption of data on any unused bits. It may be desirable to utilize the unused bits of the memory location in order to reduce the amount of hardware required and thereby reduce product size and cost. This application note describes a modified L51_BANK.A51 file that allows the unused bits of an XDATA memory location to be used for general purpose I/O. In addition, C code examples are given to show how the location can be safely read from and written to in any bank without causing problems for the bank switching hardware. CONFIGURING THE BANK SWITCHING The modified L51_BANK.A51 file is available from the Keil web site at http://www.keil.com/download/c51/apnt_130.zip. Simply replace your existing L51_BANK file with this modified version. The default configuration of the file is such that when assembled it produces identical code to the old L51_BANK.A51. In order to enable the new functionality the ?B_XDATAIO variable must be set to 1 (in addition to setting ?B_MODE to 1). The ?B_FIRSTBIT variable allows you to set where the Least Significant Bit (LSB) of the bank switching signals is placed. For example, if you set ?B_FIRSTBIT to 0 and set the maximum number of banks to 4 then bits 0 and 1 would be used for bank switching. Bits 2 to 7 could then be used for general purpose I/O. If you set ?B_FIRSTBIT to 3 and set the maximum number of banks to 16 then bits 3 to 6 would be used for bank switching. Bits 0 to 2 and bit 7 could then be used for I/O. Note that the bits used for bank switching must be consecutively ordered. It is not possible to use just bits 1 and 3 for bank switching, for example. Other variables that may required changing are ?B_XDATAPORT which sets the address of the XDATA memory location, ?B_RTX which indicates if RTX51 Full is being used or not and ?B_NBANKS which sets the maximum number of banks. Application Note Using the Bank Switching XDATA Location APNT_130 Page 2 of 4 Revision date: 5-Nov-99 READING FROM AND WRITING TO THE XDATA LOCATION Assumptions The code examples in this section make several assumptions. Modify your code accordingly for your particular hardware arrangement. § The maximum number of banks (?B_NBANKS) is 16, therefore four bits are used for bank switching. § The first bit (?B_FIRSTBIT) is bit 0, therefore bits 0 to 3 are used for bank switching. § The address of the XDATA memory location (?B_XDATAPORT) is FFFFH. Defining a Variable In order to read and write to the location a variable has to be defined. The following code will define a variable called bank at FFFFH: xdata unsigned char bank _at_ 0xFFFF; To access the variable in other C files use the extern keyword. For example: extern xdata unsigned char bank; Reading the XDATA Location The following code reads the location and masks off the bits used for bank switching. The result can then be used to determine which bits were set and unset: unsigned char result; result = bank & 0x0F; Writing the XDATA Location The folllowing code sets bits 4 and 7 bank = (bank & 0x0F) | 0x90; The location is read, the bank switching bits are masked off, bits 4 and 7 are set and the result is written back. The following code clears bits 4 and 7: bank = bank & 0x6F; Application Note Using the Bank Switching XDATA Location APNT_130 Page 3 of 4 Revision date: 5-Nov-99 NOTE It is vital when reading, modifying, and writing to the XDATA location that the bank switching bits are NOT modified. This will cause your hardware to switch banks! INTERRUPTS It is vital that the I/O pins at the xdata location are not modified during a bank switch. The result will be corruption of the logic levels on those pins. In order to ensure this will not happen, interrupts are disabled when performing a bank switch and ?B_XDATAIO = 1. TIME COST In order to add this extra functionality, additional assembly instructions were added to the bank switching code. The result is an increase in the time required to switch banks, and could adversely affect the performance of an application. Some of this time may be recouped if the number of bank switches is optimized. Possible ways of achieving this are to move functions to the bank they are most frequently called from, or duplicating the most used functions in the banks they are called from. The following table gives the time cost per bank switch in processor cycles. The test conditions were: ?B_MODE = 1 ?B_NBANKS = 16 ?B_RTX = 0 ?B_RTX = 1 ?B_XDATAIO = 0 (cycles) ?B_XDATAIO = 1 (cycles) % increase ?B_XDATAIO = 0 (cycles) ?B_XDATAIO = 1 (cycles) % increase Function Call 34 47 38.2% 42 47 11.9% Function Return 14 26 85.7% 21 26 23.8% Copyright © 1999 Keil Software, Inc. All rights reserved. In the USA: In Europe: Keil Software, Inc. Keil Elektronik GmbH 16990 Dallas Parkway, Suite 120 Bretonischer Ring 15 Dallas, TX 75248-1903 D-85630 Grasbrunn b. Munchen USA Germany Sales: 800-348-8051 Phone: (49) (089) 45 60 40 - 0 Phone: 972-735-8052 FAX: (49) (089) 46 81 62 FAX: 972-735-8055 E-mail: sales.us@keil.com Internet: http://www.keil.com/ E-mail: sales.intl@keil.com support.us@keil.com support.intl@keil.com Application Note Using the Bank Switching XDATA Location APNT_130 Page 4 of 4 Revision date: 5-Nov-99 LIMITATIONS Because R1 is corrupted in the process of switching banks with ?B_XDATAIO = 1 a generic pointer may not be passed in registers to a function in another code bank. The reason is because the LSB is stored in R1. In order to avoid this problem you may specifying the NOREGPARAMS for the function in question. Please refer to page 49 in the C51 Compiler User’s Guide (01.97) for more information. Alternatively ensure that calling the function does not result in a bank switch.