diff --git a/include/utl/com/spi.h b/include/utl/com/spi.h
index 6b9c2e9..43d87ea 100644
--- a/include/utl/com/spi.h
+++ b/include/utl/com/spi.h
@@ -78,7 +78,7 @@ namespace utl {
       spi_i () = default;              //!< Allow constructor from derived only
       ~spi_i () = default;             //!< Allow destructor from derived only
       spi_i (const type&) = delete;    //!< No copies
-      spi_i& operator= (const spi_i&) = delete;
+      type& operator= (const type&) = delete;
    //!@}
 
    /*!
diff --git a/include/utl/com/spi_bb.h b/include/utl/com/spi_bb.h
index 2b94793..ceecd2e 100644
--- a/include/utl/com/spi_bb.h
+++ b/include/utl/com/spi_bb.h
@@ -74,12 +74,18 @@ namespace utl {
    private:
    //! \name SPI implementation specific functions
    //!@{
-      template <spi::cpol C =CPOL> constexpr bool clkHigh () { return !static_cast<bool>(C); }
-      template <spi::cpol C =CPOL> constexpr bool clkLow ()  { return static_cast<bool>(C); }
       template <spi::bitOrder B =BitOrder> constexpr
          use_if_t <(B == spi::bitOrder::LSB_First), void> shift (byte_t& b) { b <<=1; }
       template <spi::bitOrder B =BitOrder> constexpr
          use_if_t <(B == spi::bitOrder::MSB_First), void> shift (byte_t& b) { b >>=1; }
+      template <spi::cpol C =CPOL> static constexpr bool clkHigh () {
+         return !static_cast<bool>(C);
+      }
+      template <spi::cpol C =CPOL> static constexpr bool clkLow () {
+         return static_cast<bool>(C);
+      }
+      static constexpr bool clkH_ {clkHigh()};
+      static constexpr bool clkL_ {clkLow()};
    //!}
 
    /*!
@@ -110,9 +116,13 @@ namespace utl {
       use_if_t <(C == spi::cpha::LOW), byte_t> _tx_data_impl (byte_t out);
       template <spi::cpha C =CPHA>
       use_if_t <(C == spi::cpha::HIGH), byte_t> _tx_data_impl (byte_t out);
+   //!@}
 
+   //! Data members
+   //! @{
+   private:
       uint32_t nsec_;      //!< half period of SPI bus
-   //!@}
+   //! @}
    };
 
    /*!
@@ -127,14 +137,14 @@ namespace utl {
       use_if_t <(C == spi::cpha::LOW), byte_t>
       spi_bb_i<impl_t, CPOL, CPHA, BitOrder>::_tx_data_impl (byte_t out) {
          byte_t in {};
-         SCLK (clkLow());
+         SCLK (clkL_);
          for (uint8_t bit {static_cast<uint8_t>(BitOrder)} ; bit ; shift (bit)) {
             MOSI (out & bit);    // Out at preceding clock trailing edge
             delay (nsec_);       // Half cycle delay
-            SCLK (clkHigh());    // Leading edge
+            SCLK (clkH_);        // Leading edge
             in |= (MISO ()) ? bit : 0; // In at leading clock edge
             delay (nsec_);       // Half cycle delay
-            SCLK (clkLow());     // Trailing edge
+            SCLK (clkL_);        // Trailing edge
          }
          return in;
       }
@@ -151,13 +161,13 @@ namespace utl {
       use_if_t <(C == spi::cpha::HIGH), byte_t>
       spi_bb_i<impl_t, CPOL, CPHA, BitOrder>::_tx_data_impl (byte_t out) {
          byte_t in {};
-         SCLK (clkLow());
+         SCLK (clkL_);
          for (uint8_t bit {static_cast<uint8_t>(BitOrder)} ; bit ; shift (bit)) {
             delay (nsec_);       // Half cycle delay
-            SCLK (clkHigh());    // Leading edge
+            SCLK (clkH_);        // Leading edge
             MOSI (out & bit);    // Out at leading clock edge
             delay (nsec_);       // Half cycle delay
-            SCLK (clkLow());     // Trailing edge
+            SCLK (clkL_);        // Trailing edge
             in |= (MISO ()) ? bit : 0; // In at trailing clock edge
          }
          return in;
@@ -196,12 +206,18 @@ namespace utl {
    private:
    //! \name SPI implementation specific functions
    //!@{
-      template <spi::cpol C =CPOL> constexpr bool clkHigh () { return !static_cast<bool>(C); }
-      template <spi::cpol C =CPOL> constexpr bool clkLow ()  { return static_cast<bool>(C); }
       template <spi::bitOrder B =BitOrder> constexpr
          use_if_t <(B == spi::bitOrder::LSB_First), void> shift (byte_t& b) { b <<=1; }
       template <spi::bitOrder B =BitOrder> constexpr
          use_if_t <(B == spi::bitOrder::MSB_First), void> shift (byte_t& b) { b >>=1; }
+      template <spi::cpol C =CPOL> static constexpr bool clkHigh () {
+         return !static_cast<bool>(C);
+      }
+      template <spi::cpol C =CPOL> static constexpr bool clkLow () {
+         return static_cast<bool>(C);
+      }
+      static constexpr bool clkH_ {clkHigh()};
+      static constexpr bool clkL_ {clkLow()};
    //!}
 
 
@@ -228,9 +244,13 @@ namespace utl {
       use_if_t <(C == spi::cpha::LOW), byte_t> _tx_data_impl (byte_t out);
       template <spi::cpha C =CPHA>
       use_if_t <(C == spi::cpha::HIGH), byte_t> _tx_data_impl (byte_t out);
+   //!@}
 
+   //! Data members
+   //! @{
+   private:
       uint32_t nsec_;      //!< half period of SPI bus
-   //!@}
+   //! @}
    };
 
    /*!
@@ -245,14 +265,14 @@ namespace utl {
       use_if_t <(C == spi::cpha::LOW), byte_t>
       spi_bb_i<virtual_tag, CPOL, CPHA, BitOrder>::_tx_data_impl (byte_t out) {
          byte_t in {};
-         SCLK (clkLow());
+         SCLK (clkL_);
          for (uint8_t bit {static_cast<uint8_t>(BitOrder)} ; bit ; shift (bit)) {
             MOSI (out & bit);    // Out at preceding clock trailing edge
             delay (nsec_);       // Half cycle delay
-            SCLK (clkHigh());    // Leading edge
+            SCLK (clkH_);        // Leading edge
             in |= (MISO ()) ? bit : 0; // In at leading clock edge
             delay (nsec_);       // Half cycle delay
-            SCLK (clkLow());     // Trailing edge
+            SCLK (clkL_);        // Trailing edge
          }
          return in;
       }
@@ -269,13 +289,13 @@ namespace utl {
       use_if_t <(C == spi::cpha::HIGH), byte_t>
       spi_bb_i<virtual_tag, CPOL, CPHA, BitOrder>::_tx_data_impl (byte_t out) {
          byte_t in {};
-         SCLK (clkLow());
+         SCLK (clkL_);
          for (uint8_t bit {static_cast<uint8_t>(BitOrder)} ; bit ; shift (bit)) {
             delay (nsec_);       // Half cycle delay
-            SCLK (clkHigh());    // Leading edge
+            SCLK (clkH_);        // Leading edge
             MOSI (out & bit);    // Out at leading clock edge
             delay (nsec_);       // Half cycle delay
-            SCLK (clkLow());     // Trailing edge
+            SCLK (clkL_);        // Trailing edge
             in |= (MISO ()) ? bit : 0; // In at trailing clock edge
          }
          return in;