Testing Conditions on Port Bytes - ignoring certain bits?
Being new to testing byte and bit conditions, I feel like there has to be a better way to test for conditions/values on a byte (read from a port).
For example, I'm reading 8 bits from PORTK. I need to test for a pattern of high bits. (Say, byte value of 207 - bits 7, 6, 3, 2, 1, 0 all high.) Bits 4 and 5 I don't care about. They CAN be high, or can be low. (Using datasheet terminology, they're "X - Don't Care".)
I could do an if, and test if the value equals 207, 223, 239, or 255, but that seems messy and like there would be an easier (and cleaner to read) way.
Edit to add: I've looked at Bit Operators, and .. something is telling me I should/could be using one of those. But I'm having a difficult time walking through which one would help me.
Thanks!
-Mike
arduino-mega bit
asked Dec 12 '18 at 13:24
Coyttl
184
add a comment |
Being new to testing byte and bit conditions, I feel like there has to be a better way to test for conditions/values on a byte (read from a port).
For example, I'm reading 8 bits from PORTK. I need to test for a pattern of high bits. (Say, byte value of 207 - bits 7, 6, 3, 2, 1, 0 all high.) Bits 4 and 5 I don't care about. They CAN be high, or can be low. (Using datasheet terminology, they're "X - Don't Care".)
I could do an if, and test if the value equals 207, 223, 239, or 255, but that seems messy and like there would be an easier (and cleaner to read) way.
Edit to add: I've looked at Bit Operators, and .. something is telling me I should/could be using one of those. But I'm having a difficult time walking through which one would help me.
Thanks!
-Mike
arduino-mega bit
asked Dec 12 '18 at 13:24
Coyttl
184
add a comment |
Being new to testing byte and bit conditions, I feel like there has to be a better way to test for conditions/values on a byte (read from a port).
For example, I'm reading 8 bits from PORTK. I need to test for a pattern of high bits. (Say, byte value of 207 - bits 7, 6, 3, 2, 1, 0 all high.) Bits 4 and 5 I don't care about. They CAN be high, or can be low. (Using datasheet terminology, they're "X - Don't Care".)
I could do an if, and test if the value equals 207, 223, 239, or 255, but that seems messy and like there would be an easier (and cleaner to read) way.
Edit to add: I've looked at Bit Operators, and .. something is telling me I should/could be using one of those. But I'm having a difficult time walking through which one would help me.
Thanks!
-Mike
arduino-mega bit
asked Dec 12 '18 at 13:24
Coyttl
184
Being new to testing byte and bit conditions, I feel like there has to be a better way to test for conditions/values on a byte (read from a port).
For example, I'm reading 8 bits from PORTK. I need to test for a pattern of high bits. (Say, byte value of 207 - bits 7, 6, 3, 2, 1, 0 all high.) Bits 4 and 5 I don't care about. They CAN be high, or can be low. (Using datasheet terminology, they're "X - Don't Care".)
I could do an if, and test if the value equals 207, 223, 239, or 255, but that seems messy and like there would be an easier (and cleaner to read) way.
Edit to add: I've looked at Bit Operators, and .. something is telling me I should/could be using one of those. But I'm having a difficult time walking through which one would help me.
Thanks!
-Mike
arduino-mega bit
arduino-mega bit
asked Dec 12 '18 at 13:24
Coyttl
184
asked Dec 12 '18 at 13:24
Coyttl
184
asked Dec 12 '18 at 13:24
Coyttl
184
asked Dec 12 '18 at 13:24
Coyttl
184
asked Dec 12 '18 at 13:24
Coyttl
184
184
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
You need to use boolean arithmetic. Use the AND operator (&) to combine the incoming value AND the "mask" value (the value of the bits you do care about) and see if it equals the mask bit:
byte wanted = 0b11001110; // 207
byte incoming = 223; // for example 11011111
if ((wanted & incoming) == wanted) {
// whatever
}
The AND operator compares each bit of the bytes in turn. If they are BOTH 1 then the result is 1. Otherwise the result is 0. So the above gives:
wanted: 11001110
incoming: 11011111
AND: 11001110
And the result matches "wanted". If you had some other value for incoming it could look like this:
wanted: 11001110
incoming: 01011011
AND: 01001010
And you can see that the result doesn't match.
answered Dec 12 '18 at 13:34
Majenko♦
66.4k42974
AH! Okay, that makes sense - I would want my 'mask' value to have bits set to '1' I care about, and '0' I don't. This would effectively force the 'don't care' bits to 0.
– Coyttl
Dec 12 '18 at 16:17
1
That is correct.
– Majenko♦
Dec 12 '18 at 17:48
add a comment |
There are ways to mask off the don't care bits.
Say your example, 0b11xx1111.
Then
incomingByte = Serial.read();
incomingByte = incomingByte || 0b00110000; // make 4, 5 high
if (incomingByte == 0b11111111){
// got all 1s, do something
}
else {
// some 0s received, do something else
}
edited Dec 12 '18 at 17:35
Juraj
6,6582925
answered Dec 12 '18 at 15:38
CrossRoads
1,0167
add a comment |
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2 Answers
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2 Answers
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votes
You need to use boolean arithmetic. Use the AND operator (&) to combine the incoming value AND the "mask" value (the value of the bits you do care about) and see if it equals the mask bit:
byte wanted = 0b11001110; // 207
byte incoming = 223; // for example 11011111
if ((wanted & incoming) == wanted) {
// whatever
}
The AND operator compares each bit of the bytes in turn. If they are BOTH 1 then the result is 1. Otherwise the result is 0. So the above gives:
wanted: 11001110
incoming: 11011111
AND: 11001110
And the result matches "wanted". If you had some other value for incoming it could look like this:
wanted: 11001110
incoming: 01011011
AND: 01001010
And you can see that the result doesn't match.
answered Dec 12 '18 at 13:34
Majenko♦
66.4k42974
AH! Okay, that makes sense - I would want my 'mask' value to have bits set to '1' I care about, and '0' I don't. This would effectively force the 'don't care' bits to 0.
– Coyttl
Dec 12 '18 at 16:17
1
That is correct.
– Majenko♦
Dec 12 '18 at 17:48
add a comment |
You need to use boolean arithmetic. Use the AND operator (&) to combine the incoming value AND the "mask" value (the value of the bits you do care about) and see if it equals the mask bit:
byte wanted = 0b11001110; // 207
byte incoming = 223; // for example 11011111
if ((wanted & incoming) == wanted) {
// whatever
}
The AND operator compares each bit of the bytes in turn. If they are BOTH 1 then the result is 1. Otherwise the result is 0. So the above gives:
wanted: 11001110
incoming: 11011111
AND: 11001110
And the result matches "wanted". If you had some other value for incoming it could look like this:
wanted: 11001110
incoming: 01011011
AND: 01001010
And you can see that the result doesn't match.
answered Dec 12 '18 at 13:34
Majenko♦
66.4k42974
AH! Okay, that makes sense - I would want my 'mask' value to have bits set to '1' I care about, and '0' I don't. This would effectively force the 'don't care' bits to 0.
– Coyttl
Dec 12 '18 at 16:17
1
That is correct.
– Majenko♦
Dec 12 '18 at 17:48
add a comment |
You need to use boolean arithmetic. Use the AND operator (&) to combine the incoming value AND the "mask" value (the value of the bits you do care about) and see if it equals the mask bit:
byte wanted = 0b11001110; // 207
byte incoming = 223; // for example 11011111
if ((wanted & incoming) == wanted) {
// whatever
}
The AND operator compares each bit of the bytes in turn. If they are BOTH 1 then the result is 1. Otherwise the result is 0. So the above gives:
wanted: 11001110
incoming: 11011111
AND: 11001110
And the result matches "wanted". If you had some other value for incoming it could look like this:
wanted: 11001110
incoming: 01011011
AND: 01001010
And you can see that the result doesn't match.
answered Dec 12 '18 at 13:34
Majenko♦
66.4k42974
You need to use boolean arithmetic. Use the AND operator (&) to combine the incoming value AND the "mask" value (the value of the bits you do care about) and see if it equals the mask bit:
byte wanted = 0b11001110; // 207
byte incoming = 223; // for example 11011111
if ((wanted & incoming) == wanted) {
// whatever
}
The AND operator compares each bit of the bytes in turn. If they are BOTH 1 then the result is 1. Otherwise the result is 0. So the above gives:
wanted: 11001110
incoming: 11011111
AND: 11001110
And the result matches "wanted". If you had some other value for incoming it could look like this:
wanted: 11001110
incoming: 01011011
AND: 01001010
And you can see that the result doesn't match.
answered Dec 12 '18 at 13:34
Majenko♦
66.4k42974
answered Dec 12 '18 at 13:34
Majenko♦
66.4k42974
answered Dec 12 '18 at 13:34
Majenko♦
66.4k42974
answered Dec 12 '18 at 13:34
Majenko♦
66.4k42974
66.4k42974
AH! Okay, that makes sense - I would want my 'mask' value to have bits set to '1' I care about, and '0' I don't. This would effectively force the 'don't care' bits to 0.
– Coyttl
Dec 12 '18 at 16:17
1
That is correct.
– Majenko♦
Dec 12 '18 at 17:48
add a comment |
AH! Okay, that makes sense - I would want my 'mask' value to have bits set to '1' I care about, and '0' I don't. This would effectively force the 'don't care' bits to 0.
– Coyttl
Dec 12 '18 at 16:17
1
That is correct.
– Majenko♦
Dec 12 '18 at 17:48
AH! Okay, that makes sense - I would want my 'mask' value to have bits set to '1' I care about, and '0' I don't. This would effectively force the 'don't care' bits to 0.
– Coyttl
Dec 12 '18 at 16:17
AH! Okay, that makes sense - I would want my 'mask' value to have bits set to '1' I care about, and '0' I don't. This would effectively force the 'don't care' bits to 0.
– Coyttl
Dec 12 '18 at 16:17
1
1
That is correct.
– Majenko♦
Dec 12 '18 at 17:48
That is correct.
– Majenko♦
Dec 12 '18 at 17:48
add a comment |
There are ways to mask off the don't care bits.
Say your example, 0b11xx1111.
Then
incomingByte = Serial.read();
incomingByte = incomingByte || 0b00110000; // make 4, 5 high
if (incomingByte == 0b11111111){
// got all 1s, do something
}
else {
// some 0s received, do something else
}
edited Dec 12 '18 at 17:35
Juraj
6,6582925
answered Dec 12 '18 at 15:38
CrossRoads
1,0167
add a comment |
There are ways to mask off the don't care bits.
Say your example, 0b11xx1111.
Then
incomingByte = Serial.read();
incomingByte = incomingByte || 0b00110000; // make 4, 5 high
if (incomingByte == 0b11111111){
// got all 1s, do something
}
else {
// some 0s received, do something else
}
edited Dec 12 '18 at 17:35
Juraj
6,6582925
answered Dec 12 '18 at 15:38
CrossRoads
1,0167
add a comment |
There are ways to mask off the don't care bits.
Say your example, 0b11xx1111.
Then
incomingByte = Serial.read();
incomingByte = incomingByte || 0b00110000; // make 4, 5 high
if (incomingByte == 0b11111111){
// got all 1s, do something
}
else {
// some 0s received, do something else
}
edited Dec 12 '18 at 17:35
Juraj
6,6582925
answered Dec 12 '18 at 15:38
CrossRoads
1,0167
There are ways to mask off the don't care bits.
Say your example, 0b11xx1111.
Then
incomingByte = Serial.read();
incomingByte = incomingByte || 0b00110000; // make 4, 5 high
if (incomingByte == 0b11111111){
// got all 1s, do something
}
else {
// some 0s received, do something else
}
edited Dec 12 '18 at 17:35
Juraj
6,6582925
answered Dec 12 '18 at 15:38
CrossRoads
1,0167
edited Dec 12 '18 at 17:35
Juraj
6,6582925
edited Dec 12 '18 at 17:35
Juraj
6,6582925
edited Dec 12 '18 at 17:35
Juraj
6,6582925
6,6582925
answered Dec 12 '18 at 15:38
CrossRoads
1,0167
answered Dec 12 '18 at 15:38
CrossRoads
1,0167
answered Dec 12 '18 at 15:38
CrossRoads
1,0167
1,0167
add a comment |
add a comment |
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