Tutorial: Background about Dallas 8051 Derivatives NOTE: This document was contributed to 8052.com by Dallas Semiconductor, a manufacturer of 8051-compatible derivative chips. This document is copyrighted by Dallas Semiconductor is used by permission. Introduction When a semiconductor company chooses to make and sell microcontrollers it faces a fundamental choice - whether to use an existing architecture or to develop a new one. Developing a new architecture has its advantages. The designers are able to use every new engineering tactic available to improve performance of the hardware and the efficiency of software (most notable of these are C compilers). The other alternative is for a semiconductor company to use an existing architecture and make whatever improvements are possible. This requires more ingenuity on the part of the designers as they are not free to implement any change - only those that maintain the compatibility of the architecture. The main benefactor of this approach is the customer. Familiarity with a microcontroller helps companies to develop products quicker but the over riding reason to use an existing microcontroller architecture is to re-use software (the Intellectual Property of a company). Dallas Semiconductor is a company which has chosen to follow the latter development path - Evolution Not Revolution. Dallas Semiconductor began making microcontrollers in 1987. Since then key policies adopted in the development program are: Maintain 100% software compatibility with the standard 8051 instruction set. Maintain 100% compatibility with the pin-out of existing standard devices where appropriate. Improve the architecture to achieve a significant performance advantage. Add additional features to the microcontrollers The latest microcontroller from Dallas Semiconductor follows in the footsteps of their earlier parts and complies with these policies. The DS87C550 is 100% software compatible with the standard 8051 instruction set. It is also pin compatible with existing parts produced by other manufacturers. These two features alone are not enough for Dallas Semiconductor - many additional features have been added to enable customers to do more and reduce their time to market at the same time. The DS87C550 uses the Dallas Semiconductor high speed core. This core executes all 8015 instructions up to 3 times faster than a standard 8051. Additionally the clock speed is up at 33MHz giving the DS87C550 a maximum instruction rate of 8.25 million instruction per second making the DS87C550 one of the quickest 8-bit microcontrollers on the market today. Not bad when you consider all of the new architectures that have been introduced since its inception. Even those starting with a clean sheet have not been able to match this level of performance. Dealing With The Real World The DS87C550 is the first microcontroller from Dallas Semiconductor that has an integrated A/D converter to measure those 'Real World' analog signals. Dallas Semiconductor has also added a number of features to the A/D converter to make the DS87C550 a better solution. The A/D converter is an 8 channel 10-bit converter. The A/D convert has been specially designed to keep pace with the high performance architecture of the DS87C550 making quicker than those found on many other microcontrollers. Additionally the converter has a window comparator associated with it. This can interrupt the microcontroller if the analogue signal varies outside a specified range or alternatively, if it moves within a specified range. Fig 1 shows this feature configured in the mode where an interrupt is generated when the signal goes outside a specific range. This range is set by the user with two comparison registers. This feature offers significant reduction in processing overhead if the system requires action only when the signal moves outside (or inside) a specific range. This is a common requirement for control systems. Moving Data The data pointers of the DS87C550 have been significantly improved. The standard 8051 has only 1 data pointer and Dallas Semiconductor added a second to its microcontrollers. The DS87C550 takes this improvement further with a number of additional features that are introduced with this part. It is now possible to decrement the data pointers. (This is not possible on a standard 8051.) This feature has long been requested and can significantly speed up data transfer routines (see example below). After a data pointer has been called into use by an instruction the DS87C550 can be set up to automatically switch to the other data pointer. This would be used when moving data from a source to a destination. In this case one data pointer would contain the source address and the second the destination address. Once the source pointer is used the microcontroller automatically switches to use the destination pointer and data can be written immediately to the desired location. This use of the destination pointer causes the microcontroller to switch back to the source pointer again. This operation saves an instruction for every read and every write during a block data move. These extended capabilities of the data pointers allow you to write: Cycles Loop: MOVX A,@DPTR 8 MOVX @DPTR,A 8 INC DPTR 12 INC DPTR 12 DJNZ Counter, Loop 12 Total 52 cycles The automatic switching between data pointers means that instructions to switch data pointers are eliminated. If these were not available then a similar operation would be: Loop: MOVX A,@DPTR 8 INC DPS 8 MOVX @DPTR,A 8 INC DPTR 12 INC DPS 8 INC DPTR 12 DJNZ Counter, Loop 12 Total 68 cycles You do not need to be the world's best programmer to see that the second method carries an overhead of two extra instructions per loop. The second example takes an extra 16 cycles which makes it 30% slower. It seems too good to be true that this simple modification can make block data moves some 30% quicker. CE Compliance It has become well known that reducing the frequency of any signals to the devices in a system can be a big advantage when it is time to reduce EMC emissions for CE compliance. The DS87C550 has a simple but effective means of achieving this. The DS87C550 contains a phase locked loop to optionally multiply the frequency by 2 or 4. This multiplication takes place before the clock to the microcontroller is generated. Fig. 2 shows how this is implemented. An important point to note is that the device itself does not operate any faster if the frequency multiplier is used. This means that a 33MHz device can only have a maximum crystal frequency of 16.5MHz if the X2 option is used and 8.25MHz if the X4 option is used. Using the X2 or X4 options reduces the frequencies that are present off the chip removing the high frequencies over long signal lengths which contribute so much to EMC emissions. An additional benefit is that lower frequency crystals are generally more readily available and therefore often for a lower cost. This is just another factor that can make a feature rich microcontroller result in a lower overall system cost. Staying In Control As well as the ability to deal with real world analogue signals the microcontroller must be able react to what it measures. This is achieved with PWM outputs. As well as maintaining compatibility with traditional 8051 microcontrollers already on the market Dallas Semiconductor has considerably improved the PWM function. First of all the number of channels of PWM has been increased to 4 x 8-bit channels (this class of microcontroller typically has only 2 x 8-bit channels). Secondly, the DS87C550 can cascade the 8-bit channels of PWM together to form channels of 16-bit PWM. All of the possible options are available so the DS87C550 can be configured as: 4 x 8-bit PWM channels 2 x 16-bit PWM channels 1 x 16-bit channel and 2 x 8-bit PWM channels Finally, as to be expected from the higher performance Dallas microcontrollers, the PWM channels are able to run at a higher frequency than many other parts available on the market. The clock source for the PWM channels is derived from the System Clock which can be as high as 33MHz. Catastrophic Failure In many applications, system reliability is paramount. The most common way of achieving this is to use a Watchdog Timer. This is an excellent device which is available on all of the Dallas Semiconductor range of microcontrollers. However, the traditional watchdog timers relies on a clock which is derived from the system clock. If the clock fails the watchdog also fails. Since crystal failure is a relatively common failure mode the DS87C550 has implemented an Oscillator Fail Detect circuit to cope with this situation. The Oscillator Fail Detect circuit simply checks that the clock frequency is above a certain threshold. If the frequency ever falls below that level the device is held in reset until the frequency once again rises above the threshold. In the reset state the pins are all forced to know states which the designer can use to implement a 'Fail-safe' state. Summary Dallas Semiconductor has set itself the target of providing customers with solutions to their microcontroller requirements without forcing them to use a brand new architecture and rewrite any code that they already have. This has been achieved by maintaining 100% software compatibility with the standard 8051 architecture and implementing devices that are pin compatible with the pin-out of existing standard devices. To provide cost effective solutions to their customers additional features have been added to the Dallas Semiconductor microcontrollers which contribute to a lower overall system cost. The future holds many exciting developments as Dallas Semiconductor continues to push back the limits of current microcontrollers. A significant improvement to solve the twin problems of address space limitations and mathematical performance will be announced early in the third quarter. Later in the year Dallas Semiconductor will announce a ground-breaking development which will re-define the capabilities of 8-bit microcontrollers. (C) Copyright 1997 - 2008 by Vault Information Services LLC. All Rights Reserved. Information provided "as-is" without warranty. Please see details. Contact us for usage and copy permission.