Semtech 2006
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12-6
XE8802 Sensing Machine Data Acquisition MCU
with ZoomingADC and LCD driver
1/f2
1/f1
Figure 12-2. CPU output clock timing.
The timing of the CPU clock (Figure 12-2) depends on the selection of the CpuSel bit in the RegSysClock register
and is given in Table 12-11. fmax is the frequency of fastest clock present in the circuit. Note that the tolerance on
the 32 kHz depends on the selected clock source (see clock block documentation).
CpuSel
f1
f2
0
fmax/4
fmax
1
fmax
32 kHz
Table 12-11. CPU clock timing parameters.
Pins PB[5] and PB[4] can be used for S1 and S0 of the USRT (see USRT documentation) when the UsrtEnable bit
is set in RegUsrtCtrl. The PB[5] and PB[4] then become open-drain. This overrides the values contained in
PBOpen(5:4), PBOut(5:4) and PBDir(5:4). If there is no external pull-up resistor on these pins, internal pull-ups
should be selected by setting PBPullup(5:4). When S0 is an output, the pin PB[4] takes the value of UsrtS0 in
RegUsrtS0. When S1 is an output, the pin PB[5] takes the value of UsrtS1 in RegUsrtS1.
Pins PB[6] and PB[7] can be used by the UART (see UART documentation). When UartEnTx in RegUartCtrl is set
to 1, PB[6] is used as output signal Tx. When UartEnRx in RegUartCtrl is set to 1, PB[7] is used as input signal
Rx. This overrides the values contained in PBOut(7:6) and PBDir(7:6).
12.7
Port B digital capabilities
12.7.1
Port B digital configuration
The direction of each bit within Port B (input only or input/output) can be individually set using the RegPBDir
register. If PBDir[x] = 1, both the input and output buffer are active on the corresponding Port B. If PBDir[x] = 0,
the corresponding Port B pin is an input only and the output buffer is in high impedance. After reset (nresetpconf)
Port B is in input only mode (PBDir[x] are reset to 0).
The input values of Port B are available in RegPBIn (read only). Reading is always direct - there is no debounce
function in Port B. In case of possible noise on input signals, a software debouncer with polling or an external
hardware filter have to be realized. The input buffer is also active when the port is defined as output and the
effective value on the pin can be read back.
Data stored in RegPBOut are outputted at Port B if PBDir[x] is 1. The default values after reset is low (0).
When a pin is in output mode (PBDir[x] is set to 1), the output can be a conventional CMOS (Push-Pull) or a N-
channel Open-drain, driving the output only low. By default, after reset (nresetpconf) the PBOpen[x] in
RegPBOpen is cleared to 0 (push-pull). If PBOpen[x] in RegPBOpen is set to 1 then the internal P transistor in
the output buffer is electrically removed and the output can only be driven low (PBOut[x]=0). When PBOut[x]=1,
the pin is high Impedance. The internal pull-up or an external pull-up resistor can be used to drive to pin high.
Note: Because the P transistor actually exists (this is not a real Open-drain output) the pull-up range is limited to
VDD + 0.2V (avoid forward bias the P transistor / diode).
Each bit can be set individually for pull-up or not using register RegPBPullup. Input is pulled up when its
corresponding bit in this register is set to 1. Default status after (nresetpconf) is 1, which means with pull up. To
Not
Recommended
for
New
Designs