AN3258

HSI calibration

Doc ID 17825 Rev 1

5/16

1

HSI calibration

1.1

HSI trimming bits

the device specific datasheet) which is sufficient for many applications. For a given voltage

and temperature condition, the HSI oscillator frequency can be calibrated to ±1 % or ±0.5 %

by using calibration bits as described below and in Figure 1.

automatically loaded into the internal calibration register. The HSI frequency can be fine-

tuned by writing the calibration bits present in the HSI clock calibration trimming register

(CLK_HSITRIMR). The maximum number of calibration bits present is either three or four

depending on the device. The calibration bits provide an additional trimming value which is

added to the factory calibration value to fine-tune the HSI output frequency. The additional

trimming value, written in the trimming bits, is interpreted as a signed value with two’s

complement representation (see Table 1). In the case of three trimming bits, this additional

trimming value can vary from -4 (100b) to 3 (011b) and in the case of four trimming bits, this

additional trimming value varies from -7 (1001b) to 7 (0111b). Thus, the additional trimming

value can be added or subtracted to the factory calibration value. An increase in the

trimming value causes a decrease in the HSI frequency. The frequency change per step is

±1 % or ±0.5 % depending on the device and number of trimming bits used (three or four).

Some devices can use either three or four trimming bits which are programmable via the

HSITRIM option byte.

Figure 1.

STM8A/S HSI trimming principle

RC OSCILLATOR

DECODER

CLK_HSITRIM

HSI Clock Output

+

INIT_TRIM