I am trying to write a function in C that will shift out the individual bits of a byte based on a clock signal. So far I have come up with this...
void ShiftOutByte (char Data)
{
int Mask = 1;
int Bit = 0;
while(Bit < 8)
{
while(ClkPin == LOW);
DataPin = Data && Mask;
Mask = Mask * 2;
Bit++;
}
}
where DataPin represents the port pin that I want to shift data out on and ClkPin is the clock port pin.
I want the device to shift out 8 bits, starting on the LSB of the byte. For some reason my output pin stays high all the time. I am certain that the port pins are configured properly so it is purely a logical issue.
You want to use &, not &&. && is the logical and, whereas & is the bitwise and.
Your solution is almost there, but there a few problems:
When clock goes high, you will pump out data in a burst before the clock has a chance to go low again, so you need to pause while clock is high before you check for it being low (unless the hardware pauses execution until clock goes low). This can be done either at the end of the loop, or at the beginning. I chose the beginning in the sample below, because it allows you to return from the function while clock is still high, and do some processing while it is still high.
You used the logical and (&&) instead of bitwise and (&) as others have pointed out
I am not familiar with your architecture, so I can't say if DataPin can only accept 0 or 1. But that is also another point where things may go wrong. Data & Mask will return a bit that is shifted left (1, 2, 4, 8, ...) depending on the bit position.
So, in summary, I would do something like:
void ShiftOutByte (unsigned char Data)
{
int Bit;
for(Bit=0; Bit < 8; ++Bit)
{
while(ClkPin == HIGH);
while(ClkPin == LOW);
DataPin = Data & 1;
Data >>= 1;
}
}
Note: I have used a for loop instead of the while ... Bit++ pattern for clarity of purpose.
You have a typo, that has replaced one operator with another that has a similar function. That is, && is guaranteed to produce the answer 1 if both of its operands are logically true. Since you are almost certainly testing with Data not zero, then this is the case for all eight iterations of your loop.
This may be masking a more subtle issue. I don't know what your target architecture is, but if DataPin is effectively a one-bit wide register, then you probably need to be careful to not try to assign values other than 0 or 1 to it. One way to achieve that is to write DataPin = !!(Data & Mask);. Another is to shift the data the other direction and use a fixed mask of 1.
Related
Take macro:
GPIOxMODE(gpio,mode,port) ( GPIO##gpio->MODER = ((GPIO##gpio->MODER & ~((uint32_t)GPIO2BITMASK << (port*2))) | (mode << (port * 2))) )
Assuming that the reset value of the register is 0xFFFF.FFFF, I want to set a 2 bit width to an arbitrary value. This was written for an STM32
MCU that has 15 pins per port. GPIO2BITMASK is defined as 0x3. Is there a better way for clearing and setting a random 2 bits in anywhere in the
32-bit wide register.
Valid range for port 0 - 15
Valid range for mode 0 - 3
The method I came up with is to bit shift the mask, invert it, logically AND it with the existing register value, logically OR the result with a bit shifted new value.
I am looking to combine the mask and new value to reduce the number of logical operations bit shift operations. The goal is also keep the process generic enough so that I can use for bit operations of 1,2,3 or 4 bit widths.
Is there a better way?
In the long and sort of it, is there a better way is really an opened question. I am looking specifically for a method that will reduce the number of logical operations and bit shift operations, while being a simple one lined statement.
The answer is NO.
You MUST do reset/set to ensure that the bit field you are writing to has the desired value.
The answers received can be better (in a matter of opinion/preference/philosophy/practice) in that they aren't necessary a macros and have have parameter checking. Also pit falls of this style have been pointed out in both the comments and responses.
This kind of macros should be avoided as a plaque for many reasons:
They are not debuggable
They are hard to find error prone
and many other reasons
The same result you can archive using inline functions. The resulting code will be the same effective
static inline __attribute__((always_inline)) void GPIOMODE(GPIO_TypeDef *gpio, unsigned mode, unsigned pin)
{
gpio -> MODER &= ~(GPIO_MODER_MODE0_Msk << (pin * 2));
gpio -> MODER |= mode << (pin * 2);
}
but if you love macros
#define GPIOxMODE(gpio,mode,port) {volatile uint32_t *mdr = &GPIO##gpio->MODER; *mdr &= ~(GPIO_MODER_MODE0_Msk << (port*2)); *mdr |= mode << (port * 2);}
I am looking to combine the mask and new value to reduce the number of
logical operations bit shift operations.
you cant. You need to reset and then set the bits.
The method I came up with is to bit shift the mask, invert it,
logically AND it with the existing register value, logically OR the
result with a bit shifted new value.
That or an equivalent is the way to do it.
I am looking to combine the mask and new value to reduce the number of
logical operations bit shift operations. The goal is also keep the
process generic enough so that I can use for bit operations of 1,2,3
or 4 bit widths.
Is there a better way?
You must accomplish two basic objectives:
ensure that the bits that should be off in the affected range are in fact off, and
ensure that the bits that should be on in the affected range are in fact on.
In the general case, those require two separate operations: a bitwise AND to force bits off, and a bitwise OR (or XOR, if the bits are first cleared) to turn the wanted bits on. There may be ways to shortcut for specific cases of original and target values, but if you want something general-purpose, as you say, then your options are limited.
Personally, though, I think I would be inclined to build it from multiple pieces, separating the GPIO selection from the actual computation. At minimum, you can separate out a generic macro for setting a range of bits:
#define SETBITS32(x,bits,offset,mask) ((((uint32_t)(x)) & ~(((uint32_t)(mask)) << (offset))) | (((uint32_t)(bits)) << (offset)))
#define GPIOxMODE(gpio,mode,port) (GPIO##gpio->MODER = SETBITS32(GPIO##gpio->MODER, mode, port * 2, GPIO2BITMASK)
But do note that there appears to be no good way to avoid such a macro evaluating some of its arguments more than once. It might therefore be safer to write SETBITS32 as a function instead. The compiler will probably inline such a function in any case, but you can maximize the likelihood of that by declaring it static and inline:
static inline uint32_t SETBITS32(uint32_t x, uint32_t bits, unsigned offset, uint32_t mask) {
return x & ~(mask << offset) | (bits << offset);
}
That's easier to read, too, though it, like the macro, does assume that bits has no set bits outside the mask region.
Of course there are other, similar formulations. For instance, if you do not need to support discontinuous bit ranges, you might specify a bit count instead of a bit mask. This alternative does that, protects against the user providing bits outside the specified range, and also has some parameter validation:
static inline uint32_t set_bitrange_32(uint32_t x, uint32_t bits, unsigned width,
unsigned offset) {
if (width + offset > 32) {
// error: invalid parameters
return x;
} else if (width == 0) {
return x;
}
uint32_t mask = ~(uint32_t)0 >> (32 - width);
return x & ~(mask << offset) | ((bits & mask) << offset);
}
I'm trying to learn a bit about emulation and I'm trying to think of how I can decode opcodes. Each opcode is a short data type, 16 bits. I'd like to be able to compare only specific sets of 4 bits. For example: there are multiple opcodes that start with 00, such as 0x00E0.
I'd like to be able to compare each of these values in either bit or hexidecimal form. I was thinking maybe something along the lines of bit shifting to bump of everything else off so that the bits I don't care about would zero out. That may cause issues for the center bits and will require additional steps. What kind of solutions do you guys use for a problem like this?
Use a bit mask, which has the bits set that you care about. Then use the & operator to zero out everything that you don't care about. For instance, say we want to compare the lowest four bits in a and b:
uint16 mask = 0x000f;
if ((a & mask) == (b & mask)) {
// lowest 4 bits are equal
}
This is simple bit manipulation. You can mask the relevant bits with
int x = opcode & 0x00f0;
and compare the resulting value
if (x == 0x00e0) {
/* do something */
}
you can easily create the mask of "nbits" and and shift "pos" number of bits and do comparision
uint32_t mask = ~((~0) << nbits);
if( (num(mask << pos)) == 0x00e0 ) {
/* Do something */
}
I am programming a PIC18F94K20 to work in conjunction with a MCP7941X I2C RTCC ship and a 24AA128 I2C CMOS Serial EEPROM device. Currently I have code which successfully intialises the seconds/days/etc values of the RTCC and starts the timer, toggling a LED upon the turnover of every second.
I am attempting to augment the code to read back the correct data for these values, however I am running into trouble when I try to account for the various 'extra' bits in the values. The memory map may help elucidate my problem somewhat:
Taking, for example, the hours column, or the 02h address. Bit 6 is set as 1 to toggle 12 hour time, adding 01000000 to the hours bit. I can read back the entire contents of the byte at this address, but I want to employ an if statement to detect whether 12 or 24 hour time is in place, and adjust accordingly. I'm not worried about the 10-hour bits, as I can calculate that easily enough with a BCD conversion loop (I think).
I earlier used the bitwise OR operator in C to augment the original hours data to 24. I initialised the hours in this particular case to 0x11, and set the 12 hour control bit which is 0x64. When setting the time:
WriteI2C(0x11|0x64);
which as you can see uses the bitwise OR.
When reading back the hours, how can I incorporate operators into my code to separate the superfluous bits from the actual time bits? I tried doing something like this:
current_seconds = ReadI2C();
current_seconds = ST & current_seconds;
but that completely ruins everything. It compiles, but the device gets 'stuck' on this sequence.
How do I separate the ST / AMPM / VBATEN bits from the actual data I need, and what would a good method be of implementing for loops for the various circumstances they present (e.g. reading back 12 hour time if bit 6 = 0 and 24 hour time if bit6 = 1, and so on).
I'm a bit of a C novice and this is my first foray into electronics so I really appreciate any help. Thanks.
To remove (zero) a bit, you can AND the value with a mask having all other bits set, i.e., the complement of the bits that you wish to zero, e.g.:
value_without_bit_6 = value & ~(1<<6);
To isolate a bit within an integer, you can AND the value with a mask having only those bits set. For checking flags this is all you need to do, e.g.,
if (value & (1<<6)) {
// bit 6 is set
} else {
// bit 6 is not set
}
To read the value of a small integer offset within a larger one, first isolate the bits, and then shift them right by the index of the lowest bit (to get the least significant bit into correct position), e.g.:
value_in_bits_4_and_5 = (value & ((1<<4)|(1<<5))) >> 4;
For more readable code, you should use constants or #defined macros to represent the various bit masks you need, e.g.:
#define BIT_VBAT_EN (1<<3)
if (value & BIT_VBAT_EN) {
// VBAT is enabled
}
Another way to do this is to use bitfields to define the organisation of bits, e.g.:
typedef union {
struct {
unsigned ones:4;
unsigned tens:3;
unsigned st:1;
} seconds;
uint8_t byte;
} seconds_register_t;
seconds_register_t sr;
sr.byte = READ_ADDRESS(0x00);
unsigned int seconds = sr.seconds.ones + sr.seconds.tens * 10;
A potential problem with bitfields is that the code generated by the compiler may be unpredictably large or inefficient, which is sometimes a concern with microcontrollers, but obviously it's nicer to read and write. (Another problem often cited is that the organisation of bit fields, e.g., endianness, is largely unspecified by the C standard and thus not guaranteed portable across compilers and platforms. However, it is my opinion that low-level development for microcontrollers tends to be inherently non-portable, so if you find the right bit layout I wouldn't consider using bitfields “wrong”, especially for hobbyist projects.)
Yet you can accomplish similarly readable syntax with macros; it's just the macro itself that is less readable:
#define GET_SECONDS(r) ( ((r) & 0x0F) + (((r) & 0x70) >> 4) * 10 )
uint8_t sr = READ_ADDRESS(0x00);
unsigned int seconds = GET_SECONDS(sr);
Regarding the bit masking itself, you are going to want to make a model of that memory map in your microcontroller. The simplest, cudest way to do that is to #define a number of bit masks, like this:
#define REG1_ST 0x80u
#define REG1_10_SECONDS 0x70u
#define REG1_SECONDS 0x0Fu
#define REG2_10_MINUTES 0x70u
...
And then when reading each byte, mask out the data you are interested in. For example:
bool st = (data & REG1_ST) != 0;
uint8_t ten_seconds = (data & REG1_10_SECONDS) >> 4;
uint8_t seconds = (data & REG1_SECONDS);
The important part is to minimize the amount of "magic numbers" in the source code.
Writing data:
reg1 = 0;
reg1 |= st ? REG1_ST : 0;
reg1 |= (ten_seconds << 4) & REG1_10_SECONDS;
reg1 |= seconds & REG1_SECONDS;
Please note that I left out the I2C communication of this.
Our OS professor mentioned that for assigning a process id to a new process, the kernel incrementally searches for the first zero bit in a array of size equivalent to the maximum number of processes(~32,768 by default), where an allocated process id has 1 stored in it.
As far as I know, there is no bit data type in C. Obviously, there's something I'm missing here.
Is there any such special construct from which we can build up a bit array? How is this done exactly?
More importantly, what are the operations that can be performed on such an array?
Bit arrays are simply byte arrays where you use bitwise operators to read the individual bits.
Suppose you have a 1-byte char variable. This contains 8 bits. You can test if the lowest bit is true by performing a bitwise AND operation with the value 1, e.g.
char a = /*something*/;
if (a & 1) {
/* lowest bit is true */
}
Notice that this is a single ampersand. It is completely different from the logical AND operator &&. This works because a & 1 will "mask out" all bits except the first, and so a & 1 will be nonzero if and only if the lowest bit of a is 1. Similarly, you can check if the second lowest bit is true by ANDing it with 2, and the third by ANDing with 4, etc, for continuing powers of two.
So a 32,768-element bit array would be represented as a 4096-element byte array, where the first byte holds bits 0-7, the second byte holds bits 8-15, etc. To perform the check, the code would select the byte from the array containing the bit that it wanted to check, and then use a bitwise operation to read the bit value from the byte.
As far as what the operations are, like any other data type, you can read values and write values. I explained how to read values above, and I'll explain how to write values below, but if you're really interested in understanding bitwise operations, read the link I provided in the first sentence.
How you write a bit depends on if you want to write a 0 or a 1. To write a 1-bit into a byte a, you perform the opposite of an AND operation: an OR operation, e.g.
char a = /*something*/;
a = a | 1; /* or a |= 1 */
After this, the lowest bit of a will be set to 1 whether it was set before or not. Again, you could write this into the second position by replacing 1 with 2, or into the third with 4, and so on for powers of two.
Finally, to write a zero bit, you AND with the inverse of the position you want to write to, e.g.
char a = /*something*/;
a = a & ~1; /* or a &= ~1 */
Now, the lowest bit of a is set to 0, regardless of its previous value. This works because ~1 will have all bits other than the lowest set to 1, and the lowest set to zero. This "masks out" the lowest bit to zero, and leaves the remaining bits of a alone.
A struct can assign members bit-sizes, but that's the extent of a "bit-type" in 'C'.
struct int_sized_struct {
int foo:4;
int bar:4;
int baz:24;
};
The rest of it is done with bitwise operations. For example. searching that PID bitmap can be done with:
extern uint32_t *process_bitmap;
uint32_t *p = process_bitmap;
uint32_t bit_offset = 0;
uint32_t bit_test;
/* Scan pid bitmap 32 entries per cycle. */
while ((*p & 0xffffffff) == 0xffffffff) {
p++;
}
/* Scan the 32-bit int block that has an open slot for the open PID */
bit_test = 0x80000000;
while ((*p & bit_test) == bit_test) {
bit_test >>= 1;
bit_offset++;
}
pid = (p - process_bitmap)*8 + bit_offset;
This is roughly 32x faster than doing a simple for loop scanning an array with one byte per PID. (Actually, greater than 32x since more of the bitmap is will stay in CPU cache.)
see http://graphics.stanford.edu/~seander/bithacks.html
No bit type in C, but bit manipulation is fairly straight forward. Some processors have bit specific instructions which the code below would nicely optimize for, even without that should be pretty fast. May or may not be faster using an array of 32 bit words instead of bytes. Inlining instead of functions would also help performance.
If you have the memory to burn just use a whole byte to store one bit (or whole 32 bit number, etc) greatly improve performance at the cost of memory used.
unsigned char data[SIZE];
unsigned char get_bit ( unsigned int offset )
{
//TODO: limit check offset
if(data[offset>>3]&(1<<(offset&7))) return(1);
else return(0);
}
void set_bit ( unsigned int offset, unsigned char bit )
{
//TODO: limit check offset
if(bit) data[offset>>3]|=1<<(offset&7);
else data[offset>>3]&=~(1<<(offset&7));
}
I need to count from 0 to 10 and store those values in binary format in ADCON0(5:2). How do I point at bit 5 of this register? Bit 5 is named ADCON0bits.CHS3. If I store a 4 bit variable to ADCON0bits.CHS3, will bits 1 - 3 be written to bits 4 - 2 of the register?
Also, are there any 4 bit data types that I could use?
This is all on a PIC microcontroller.
Edit: I need to store 4 bits in the register like so:
unsigned char count = 10 //max value
[X][X][1][0][1][0][X][X]
This is in line with what was assumed below, but I figured I would clear up my question a bit.
When you say you are writing bits 1-3 of your count into positions 4-2 of your register, do you explicitly mean you are reversing the order of the bits? In this answer I will presume that that was not what you meant.
You can express a bit field explicitly as a struct.
Presuming that you are dealing with a 16 bit register, your struct could look something like this:
struct adcon {
unsigned char someflag : 2;
unsigned char count : 4;
unsigned char other_bits : 2;
};
With each struct member, you specify the number of bits. Then you can operate on the appropriate bits in the register by casting the register to the struct type, and operating on the members of the struct.
(adcon) ADCON0.count = count;
Edit: fixed up the code based on feedback, thanks.
Writing to a bit variable stores the truth value of that variable to the bit. For example, writing:
ADCON0bits.CHS3 = 3;
will set that bit to 1.
If bit5 refers to the bit masked by 0x20 (00100000) and you need to store the 4 bit number in bits masked 0x3c (00111100) then you can use bit shifts and bitwise operations:
// First clear bits 1-5:
ADCON0 &= ~0x3c;
// Now set the bits to correct value:
ADCON0 |= (count << 2); // <-- remember to shift 2 bits to the left
update: As mentioned by Ian in the comments. This sets ADCON0 to an intermediate value before updating. In this case it is OK since it is only selecting the A/D channel and not actually executing the conversion. But in general it's better to do:
unsigned char temp_adcon;
temp_adcon = ADCON0 & ~0x3c;
ADCON0 = temp_adcon | (count << 2);
See the answers for this SO question.
Note that you are doing a read-modify-write operation. You have to be careful of race conditions when doing this. Race conditions may be caused by:
The hardware itself changing bits in the register (e.g. A/D converter operation completes and sets flags). The design of the hardware should provide a means for you to avoid this problem—there are several possible solutions—read the manual for the micro/peripheral to find out.
Your own interrupt routine(s) also writing to the register. If so, when your main (non-interrupt) code writes to the register, it should be done within an "interrupts disabled" context.
I'm not sure about the exact register ADCON0, but often you can read the register, mask the 4 bits and insert your count and then use that value to write back to the register.
Just in case, masking is performed with an AND operation and inserting is an OR operation with the count shift over 2 bits in your case.