Datasheet

Section 2 CPU
Rev. 6.00 Mar. 18, 2010 Page 99 of 982
REJ09B0054-0600
The BSET, BCLR, BNOT, BST, and BIST instructions perform their operations in the following
order.
1. Read the data in byte units
2. Perform the bit manipulation operation according to the instruction on the data read
3. Write the data back in byte units
Example: Using the BCLR instruction to clear only bit 4 in the port 1 P1DDR register.
The P1DDR register consists of 8 write-only bits and sets the I/O direction of the port 1 pins.
Reading this register is invalid. When read, the values returned are undefined.
Here we present an example in which P14 is specified to be an input port using the BCLR
instruction. Currently, P17 to P14 are set to be output pins and P13 to P10 are set to be input pins.
At this point, the value of P1DDR is H'F0.
P17 P16 P15 P14 P13 P12 P11 P10
I/O Output Output Output Output Input Input Input Input
P1DDR 1 1 1 1 0 0 0 0
To switch P14 from the Output pin to the input pin function, the value of P1DDR bit 4 must be
changed from 1 to 0 (H'F0 → H'E0). Here we assume that the BCLR instruction is used to clear
P1DDR bit 4.
BCLR #4,@P1DDR
However if a bit manipulation instruction of the type shown above is used on P1DDR, which is a
write-only register, the following problem may occur.
Although the first thing that happens is that data is read from P1DDR in byte units, the value read
at this time is undefined. An undefined value is a value that is either 0 or 1 in the register but reads
out as an arbitrary value whose relationship to the actual value is unknown. Since the P1DDR bits
are all write-only bits, every bit reads out as an undefined value. Although the actual value of
P1DDR at this point is H'F0, assume that bit 3 becomes a 1 here, and the value read out is H'F8.
P17 P16 P15 P14 P13 P12 P11 P10
I/O Output Output Output Output Input Input Input Input
P1DDR 1 1 1 1 0 0 0 0
Read value 1 1 1 1 1 0 0 0