BAUD RATE ERROR DETECTION CIRCUIT AND BAUD RATE ERROR DETECTION METHOD
A baud rate error detection circuit has an edge detector, a start bit sampling circuit and an abnormal waveform detection circuit. The edge detector receives sync-field used for adjusting a baud rate of serial communication, and generates an edge detection signal in response to an edge included in the sync-field. The start bit sampling circuit measures a bit width of a start bit of the sync-field based on the edge detection signal and an internal clock signal, and generates an expected value signal indicating the bit width of the start bit as an expected value. The abnormal waveform detection circuit measures an inter-edge width after the start bit based on the edge detection signal and the internal clock signal, and generates an abnormal waveform detection signal if an error between the inter-edge width and the expected value indicated by the expected value signal exceeds a predetermined allowable error range.
1 . A baud rate error detection circuit comprising:
an edge detector configured to receive sync-field used for adjusting a baud rate of serial communication, and to generate an edge detection signal in response to an edge included in said sync-field;
a start bit sampling circuit configured to measure a bit width of a start bit of said sync-field based on said edge detection signal and an internal clock signal, and to generate an expected value signal that indicates said measured bit width of said start bit as an expected value; and
an abnormal waveform detection circuit configured to measure, based on said edge detection signal and said internal clock signal, an inter-edge width that is a width between edges after said start bit, and to generate an abnormal waveform detection signal if an error between said inter-edge width and said expected value indicated by said expected value signal exceeds a predetermined allowable error range.
2 . The baud rate error detection circuit according to claim 1 ,
wherein said start bit sampling circuit performs count operation in synchronization with said internal clock signal and obtains as said expected value a count value corresponding said bit width of said start bit,
wherein said abnormal waveform detection circuit comprises:
a counter configured to perform count operation in synchronization with said internal clock signal to generate a count value signal indicating a count value; and
a determination circuit configured to reset said count value of said counter to an initial value in response to said edge detection signal,
wherein said inter-edge width corresponds to a difference between said initial value and said count value indicated by said count value signal at a time when said determination circuit receives said edge detection signal, and
wherein said determination circuit refers to said count value signal and generates said abnormal waveform detection signal when said error between said inter-edge width and said expected value exceeds said predetermined allowable error range.
3 . The baud rate error detection circuit according to claim 2 ,
wherein said counter is a down counter that performs down-count operation in synchronization with said internal clock signal,
said initial value is said expected value indicated by said expected value signal, and
said error between said inter-edge width and said expected value is said count value indicated by said count value signal at a time when said determination circuit receives said edge detection signal.
4 . The baud rate error detection circuit according to claim 3 ,
wherein said determination circuit generates said abnormal waveform detection signal also when said count value indicated by said count value signal falls below a lower limit of said predetermined allowable error range.
5 . The baud rate error detection circuit according to claim 1 , further comprising:
a baud rate correction circuit configured to correct said baud rate based on said error between said inter-edge width and said expected value; and
a baud rate correction value register configured to retain a baud rate correction value obtained by said baud rate correction circuit.
6 . The baud rate error detection circuit according to claim 5 ,
wherein said predetermined allowable error range is updated by a CPU based on said baud rate correction value.
7 . The baud rate error detection circuit according to claim 1 , further comprising:
an edge counter configured to count, based on said edge detection signal, a number of edges detected by said edge detector,
wherein said edge counter activates said abnormal waveform detection circuit upon end of said start bit and deactivates said abnormal waveform detection circuit upon end of said sync-field.
8 . A baud rate error detection method comprising:
receiving sync-field used for adjusting a baud rate of serial communication;
generating an edge detection signal in response to an edge included in said sync-field;
measuring a bit width of a start bit of said sync-field based on said edge detection signal and an internal clock signal to generate an expected value signal that indicates said measured bit width of said start bit as an expected value;
measuring, based on said edge detection signal and said internal clock signal, an inter-edge width that is a width between edges after said start bit; and
generating an abnormal waveform detection signal, if an error between said inter-edge width and said expected value indicated by said expected value signal exceeds a predetermined allowable error range.