Links to all my PIC tips, tricks and 'mini-project' notes
Whilst the mid-range PIC's can tackle many complex and otherwise almost impossible applications with ease, the challenge is to minimise cost by using the cheapest baseline PIC 'whenever possible'. Baseline PIC's can be had for less than 50p each = I purchased many 16F5x chips for between 40 and 50p each (mainly from CPC as 'remaindered' stock in their 'Bargain bin' section).
The even cheaper to use 12F675 (it has an internal OSC) can be found for as little as 20p (in Qty 10pcs, eBay), as can many other PIC's for less than £1 each. These PIC's are so cheap that you will soon start using them 'for everything' (especially as the PIC can often be used in place of a higher cost 'single function' digital chip - such as divider, ADC, PWM generator etc.) !
Buying the PIC in a 'TSOP' package is (sometimes) cheaper than the DIL/DIP package version = and whilst this costs you 10-20p extra for a mini-PCB TSOP-DIP 'converter', if you use a 'bigger' PCB than the PIC TSOP really needs you can mount other devices (resistors, caps, even osc. crystals) on the same board - and make use of the extra 'pin holes' to wire this up to the rest of your circuit
Below is a mix of programming tips and tricks, common circuit tricks and all the 'mini-projects' I've used the PIC for
I hope these details proves as useful to you as it does to me !
Below, click on the '+' to expand 'in place' (includes diagrams/images) or click the title URL (to view/download the text only version).
(+) 0004 Multi byte ADD - (24bit)
(+) 0005 new PIC 33 instruction set - (macros)
(+) 0006 Binary multiply methods
(+) 0007 8x8 - (multiply)
(+) 0008 8x16 - (multiply)
(+) 0011 Bi color LED driving
(+) 0012 One pin dual LED and button detect
(+) 0013 Input only multi button detect
(+) 001a One pin controls motor Fwd off Reverse
(+) 001c One pin controls 3 relays
(+) 0020 I2C bit banging
(+) 0021 I2C code
(+) 0021 Serial link - (9600 baud)
(+) 0028 RS422 RS485 drive with one PIC pin
(+) 0030 D to A conversion - (R2R DAC)
(-) 0031 Ternary DAC - (R3R)
Ternary logic Binary gives you 2^n states, ternary 3^n. This makes a real difference very quickly as 3 binary bits (2^3) is only 8 states, whist 3 ternary bits (3^3) gets you 27 ! To get a 64 state DAC we need 6 binary bits, whilst 4 ternary 'tets' gives us 81, and 5 ternary gives 243 v's 8 binary bits for 255. With most PIC projects, i/o pins are in sort supply - so if we could build a 'ternary DAC' we would only need 5 i/o pins to get almost as many states (243) as a full byte using 8 i/o pins (256) ! Building a ternary DAC Ternary logic needs 3 'states' = and PIC i/o pins have 3 distinct states !! These are:- Output Lo (typ 0.6v) Output Hi (typ 4.3v) and Output 'nothing' (high impedance, or 'tri-state') when the pin is set to input mode To convert the 'nothing' into a 'half'** level voltage, a pair of 'low value' resistors (low value compared to the DAC 'chain' resistors) are used as a 'voltage divider' between +5v and Gnd. This holds the pin half way between '1' and '0' when it's switched to 'input' mode. If Pin Lo is 0.6 and Hi is 4.3v, then Vhalf = (0.6+4.3/)/2 = 2.45v (to within 2%), however that's not quite the end of the story, since at high current (20mA) loads the pin output voltage may not reach 4.3v (nor go as low as 0.6v) and (as usual) actual device operation is likely to differ from the data sheet (which shows only the min Hi and max Lo). Typical circuit Unfortunately there seems to be no data on 'real world' PIC pin o/p voltages V's current, so I started with 'data sheet' values (using LTspice) and then measured actual performance with a 'breadboard' of the design When the PIC pin is used as an output, the divider resistors will dominate the current draw. If we want to keep within 20mA**, Rdivider = 5/.02 = 250 ohms, or nearest E24 (5%) = 270 ohms (although we are using 5% resistors that's the 'absolute value' accuracy - in any 'random' batch you should be able to find 2 resistors making a 'matched pair' (i.e. correct ratio) to within the accuracy of your multimeter (typ 4 digits i.e. .1%) and that's all we need). **The PIC has both a single pin current limit (20 or 25mA) and a 'total PORT' limit (typically 100mA). So you can build a 5 pin DAC at 20mA per pin on one PORT, but a 6 pin DAC (using 6 pins on the same PORT) means reducing the current demand on each to 100/6 = 16mA .. Next consider the R-nR 'ladder'. For ternary 'chain' the ratios need to be 3R/4R (rather than R/2R). Since the 'chain' resistors must be high enough to avoid impacting the 'divider' resistor effect, the 'obvious' choice is 75k and 100k Sensing the 'chain' voltage The high values of the chain resistors means little current is available to 'sense' the voltage. The obvious approach is to use a high-input impedance Op-Amp in 'voltage follower' mode as a buffer. Using a 100k per step (i.e per pin) 'chain', a 4 pin ternary DAC means a 400k 'chain'. A typical Op-Amp (LM741) has a typical input resistance of 2M Ohm, which is not insignificant compared to 400k Reducing the 'chain' resistor values to (7k5 and) 10k gives us a 40k chain and means Op Amp input resistance can be ignored, however this means the effect of the voltage divider resistors (at 25mA these can't be less than about 133R each) becomes significant (i.e. will impact the accuracy of the result). Again, the only way to design 'for real' is the simulate the circuit (eg using LTspice) and then breadboard it.
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